Bump the version to 0.10.0 (#171)

Co-authored-by: Sergej Jaskiewicz <jaskiewiczs@icloud.com>

.circleci/config.yml

@@ -63,35 +63,35 @@ jobs:
           command: |
             bash <(curl -s https://codecov.io/bash) -D DerivedData
 
-  "Execute compatibility tests on iOS 13.3 (Xcode 11.3.0, Swift 5.1.3)":
+  "Execute compatibility tests on iOS 13.5 (Xcode 11.5.0, Swift 5.2.4)":
     macos:
-      xcode: "11.3.0"
+      xcode: "11.5.0"
     environment:
-      SWIFT_VERSION: "5.1.3"
+      SWIFT_VERSION: "5.2.4"
     steps:
       - checkout
       - run:
           name: Generating Xcode project
           command: make generate-compatibility-xcodeproj
       - run:
-          name: Building for testing on iOS 13.3 with xcodebuild
+          name: Building for testing on iOS 13.5 with xcodebuild
           command: |
             set -o pipefail \
               && xcodebuild build-for-testing \
                    -scheme OpenCombine-Package \
-                   -destination "platform=iOS Simulator,name=iPhone 11,OS=13.3" \
+                   -destination "platform=iOS Simulator,name=iPhone 11,OS=13.5" \
                    -derivedDataPath DerivedData \
               | tee xcodebuild_build-for-testing.log \
               | xcpretty
       - store_artifacts:
           path: xcodebuild_build-for-testing.log
       - run:
-          name: Testing against Combine on iOS 13.3 with xcodebuild
+          name: Testing against Combine on iOS 13.5 with xcodebuild
           command: |
             set -o pipefail \
               && xcodebuild test-without-building \
                    -scheme OpenCombine-Package \
-                   -destination "platform=iOS Simulator,name=iPhone 11,OS=13.3" \
+                   -destination "platform=iOS Simulator,name=iPhone 11,OS=13.5" \
                    -derivedDataPath DerivedData \
               | tee xcodebuild_test-without-building.log \
               | xcpretty --report junit -o build/reports/results.xml
@@ -161,11 +161,11 @@ jobs:
           command: |
             bash <(curl -s https://codecov.io/bash) -D DerivedData
 
-  "Execute tests on Ubuntu 18.04 (Swift 5.1.1)":
+  "Execute tests on Ubuntu 18.04 (Swift 5.2)":
     docker:
-      - image: swift:5.1.1-bionic
+      - image: swift:5.2-bionic
     environment:
-      SWIFT_VERSION: "5.1.1"
+      SWIFT_VERSION: "5.2"
     steps:
       - checkout
       - run:
@@ -259,13 +259,13 @@ workflows:
       - "Execute tests on macOS 10.15.0 (Xcode 11.3.0, Swift 5.1.3)"
   "OpenCombine: execute compatibility tests":
     jobs:
-      - "Execute compatibility tests on iOS 13.3 (Xcode 11.3.0, Swift 5.1.3)"
+      - "Execute compatibility tests on iOS 13.5 (Xcode 11.5.0, Swift 5.2.4)"
   "OpenCombine: execute tests on iOS":
     jobs:
       - "Execute tests on iOS 9.3 (Xcode 10.2.1, Swift 5.0.1)"
   "OpenCombine: execute tests on Linux":
     jobs:
-      - "Execute tests on Ubuntu 18.04 (Swift 5.1.1)"
+      - "Execute tests on Ubuntu 18.04 (Swift 5.2)"
   "OpenCombine: run SwiftLint and Danger":
     jobs:
       - "Run SwiftLint and Danger"

.swiftlint.yml

@@ -23,6 +23,7 @@ disabled_rules:
   - trailing_comma
   - type_body_length
   - opening_brace
+  - untyped_error_in_catch
 
 opt_in_rules:
   - array_init
@@ -65,6 +66,10 @@ opt_in_rules:
   - vertical_whitespace_closing_braces
   - yoda_condition
 
+implicit_return:
+  included: 
+    - closure
+
 line_length:
   warning: 90
   error: 120

OpenCombine.podspec

@@ -1,6 +1,6 @@
 Pod::Spec.new do |spec|
   spec.name          = "OpenCombine"
-  spec.version       = "0.7.0"
+  spec.version       = "0.10.0"
   spec.summary       = "Open source implementation of Apple's Combine framework for processing values over time."
 
   spec.description   = <<-DESC
@@ -24,4 +24,4 @@ Pod::Spec.new do |spec|
   spec.public_header_files = "Sources/COpenCombineHelpers/include/*.h"
 
   spec.libraries = "c++"
-end
+end

OpenCombineDispatch.podspec

@@ -1,6 +1,6 @@
 Pod::Spec.new do |spec|
   spec.name          = "OpenCombineDispatch"
-  spec.version       = "0.7.0"
+  spec.version       = "0.10.0"
   spec.summary       = "OpenCombine + Dispatch interoperability"
 
   spec.description   = <<-DESC
@@ -21,5 +21,5 @@ Pod::Spec.new do |spec|
   spec.tvos.deployment_target    = "9.0"
 
   spec.source_files = "Sources/OpenCombineDispatch/**/*.swift"
-  spec.dependency     "OpenCombine", '>= 0.6'
-end
+  spec.dependency     "OpenCombine", '>= 0.9'
+end

OpenCombineFoundation.podspec

@@ -1,6 +1,6 @@
 Pod::Spec.new do |spec|
     spec.name          = "OpenCombineFoundation"
-    spec.version       = "0.7.0"
+    spec.version       = "0.10.0"
     spec.summary       = "OpenCombine + OpenCombineFoundation interoperability"
   
     spec.description   = <<-DESC
@@ -21,5 +21,5 @@ Pod::Spec.new do |spec|
     spec.tvos.deployment_target    = "9.0"
   
     spec.source_files = "Sources/OpenCombineFoundation/**/*.swift"
-    spec.dependency     "OpenCombine", '~> 0.6'
-  end
+    spec.dependency     "OpenCombine", '>= 0.9'
+  end

README.md

@@ -1,6 +1,6 @@
 # OpenCombine
-[![CircleCI](https://circleci.com/gh/broadwaylamb/OpenCombine/tree/master.svg?style=svg)](https://circleci.com/gh/broadwaylamb/OpenCombine/tree/master)
-[![codecov](https://codecov.io/gh/broadwaylamb/OpenCombine/branch/master/graph/badge.svg)](https://codecov.io/gh/broadwaylamb/OpenCombine)
+[![OpenCombine](https://circleci.com/gh/OpenCombine/OpenCombine.svg?style=svg)](https://circleci.com/gh/OpenCombine/OpenCombine)
+[![codecov](https://codecov.io/gh/OpenCombine/OpenCombine/branch/master/graph/badge.svg)](https://codecov.io/gh/OpenCombine/OpenCombine)
 ![Language](https://img.shields.io/badge/Swift-5.0-orange.svg)
 ![Platform](https://img.shields.io/badge/platform-Linux%20%7C%20macOS%20%7C%20iOS%20%7C%20watchOS%20%7C%20tvOS-lightgrey.svg)
 ![Cocoapods](https://img.shields.io/cocoapods/v/OpenCombine?color=blue)
@@ -13,9 +13,9 @@ The main goal of this project is to provide a compatible, reliable and efficient
 The project is in early development.
 
 ### Installation
-`OpenCombine` contains two public targets: `OpenCombine` and `OpenCombineDispatch` (the third one, `COpenCombineHelpers`, is considered private. Don't import it in your projects).
+`OpenCombine` contains three public targets: `OpenCombine`, `OpenCombineFoundation` and `OpenCombineDispatch` (the fourth one, `COpenCombineHelpers`, is considered private. Don't import it in your projects).
 
-OpenCombine itself does not have any dependencies. Not even Foundation or Dispatch. If you want to use OpenCombine with Dispatch (for example for using `DispatchQueue` as `Scheduler` for operators like `debounce`, `receive(on:)` etc.), you will need to import both `OpenCombine` and `OpenCombineDispatch`.
+OpenCombine itself does not have any dependencies. Not even Foundation or Dispatch. If you want to use OpenCombine with Dispatch (for example for using `DispatchQueue` as `Scheduler` for operators like `debounce`, `receive(on:)` etc.), you will need to import both `OpenCombine` and `OpenCombineDispatch`. The same applies to Foundation: if you want to use, for instance, `NotificationCenter` or `URLSession` publishers, you'll need to also import `OpenCombineFoundation`
 
 ##### Swift Package Manager
 ###### Swift Package
@@ -23,17 +23,19 @@ To add `OpenCombine` to your [SPM](https://swift.org/package-manager/) package,
 
 ```swift
 dependencies: [
-    .package(url: "https://github.com/broadwaylamb/OpenCombine.git", from: "0.7.0")
+    .package(url: "https://github.com/OpenCombine/OpenCombine.git", from: "0.10.0")
 ],
 targets: [
-    .target(name: "MyAwesomePackage", dependencies: ["OpenCombine", "OpenCombineDispatch"])
+    .target(name: "MyAwesomePackage", dependencies: ["OpenCombine",
+                                                     "OpenCombineDispatch",
+                                                     "OpenCombineFoundation"])
 ]
 ```
 
 ###### Xcode
 `OpenCombine` can also be added as a SPM dependency directly in your Xcode project *(requires Xcode 11 upwards)*.
 
-To do so, open Xcode, use **File** → **Swift Packages** → **Add Package Dependency…**, enter the [repository URL](https://github.com/broadwaylamb/OpenCombine.git), choose the latest available version, and activate the checkboxes:
+To do so, open Xcode, use **File** → **Swift Packages** → **Add Package Dependency…**, enter the [repository URL](https://github.com/OpenCombine/OpenCombine.git), choose the latest available version, and activate the checkboxes:
 
 <p align="center">
 <img alt="Select the OpenCombine and OpenCombineDispatch targets" 
@@ -44,15 +46,16 @@ To do so, open Xcode, use **File** → **Swift Packages** → **Add Package Depe
 To add `OpenCombine` to a project using [CocoaPods](https://cocoapods.org/), add `OpenCombine` and `OpenCombineDispatch` to the list of target dependencies in your `Podfile`. 
 
 ```ruby
-pod 'OpenCombine', '~> 0.7'
-pod 'OpenCombineDispatch', '~> 0.7'
+pod 'OpenCombine', '~> 0.10'
+pod 'OpenCombineDispatch', '~> 0.10'
+pod 'OpenCombineFoundation', '~> 0.10'
 ```
 
 ### Contributing
 
 In order to work on this project you will need Xcode 10.2 and Swift 5.0 or later.
 
-Please refer to the [issue #1](https://github.com/broadwaylamb/OpenCombine/issues/1) for the list of operators that remain unimplemented, as well as the [RemainingCombineInterface.swift](https://github.com/broadwaylamb/OpenCombine/blob/master/RemainingCombineInterface.swift) file. The latter contains the generated interface of Apple's Combine from the latest Xcode 11 version. When the functionality is implemented in OpenCombine, it should be removed from the RemainingCombineInterface.swift file.
+Please refer to the [issue #1](https://github.com/OpenCombine/OpenCombine/issues/1) for the list of operators that remain unimplemented, as well as the [RemainingCombineInterface.swift](https://github.com/OpenCombine/OpenCombine/blob/master/RemainingCombineInterface.swift) file. The latter contains the generated interface of Apple's Combine from the latest Xcode 11 version. When the functionality is implemented in OpenCombine, it should be removed from the RemainingCombineInterface.swift file.
 
 You can refer to [this gist](https://gist.github.com/broadwaylamb/c2c8550d76b3ff851c4c1dbf0a872e26) to observe Apple's Combine API changes between different Xcode (beta) versions, or to [this gist](https://gist.github.com/broadwaylamb/82dc2ce4ffbe06527c2c352b8f10910f) to see the relevant contents of the .swiftinterface file for Combine.
 
@@ -64,7 +67,29 @@ $ make test-compatibility
 
 Or enable the `-DOPENCOMBINE_COMPATIBILITY_TEST` compiler flag in Xcode's build settings. Note that on iOS only the latter will work.
 
-> NOTE: Before starting to work on some feature, please consult the [GitHub project](https://github.com/broadwaylamb/OpenCombine/projects/2) to make sure that nobody's already making progress on the same feature! If not, then please create a draft PR to indicate that you're beginning your work.
+> NOTE: Before starting to work on some feature, please consult the [GitHub project](https://github.com/OpenCombine/OpenCombine/projects/2) to make sure that nobody's already making progress on the same feature! If not, then please create a draft PR to indicate that you're beginning your work.
+
+#### Releasing a new version
+
+1. Create a new branch from master and call it `release/<major>.<minor>.<patch>`.
+1. Replace the usages of the old version in `README.md` with the new version (make sure to check the [Swift Package Manager](#swift-package-manager) and [CocoaPods](#cocoapods) sections).
+1. Bump the version in `OpenCombine.podspec`, `OpenCombineDispatch.podspec` and `OpenCombineFoundation.podspec`. In the latter two you will also need to set the `spec.dependency "OpenCombine"` property to the **previous** version. Why? Because otherwise the `pod lib lint` command that we run on our regular CI will fail when validating the `OpenCombineDispatch` and `OpenCombineFoundation` podspecs, since the dependencies are not yet in the trunk. If we set the dependencies to the previous version (which is already in the trunk), everything will be fine. This is purely to make the CI work. The clients will not experience any issues, since the version is specified as `>=`.
+1. Create a pull request to master for the release branch and make sure the CI passes.
+1. Merge the pull request.
+1. In the GitHub web interface on the [releases](https://github.com/OpenCombine/OpenCombine/releases) page, click the **Draft a new release** button.
+1. The **Tag version** and **Release title** fields should be filled with the version number.
+1. The description of the release should be consistent with the previous releases. It is a good practice to divide the description into several sections: additions, bugfixes, known issues etc. Also, be sure to mention the nicknames of the contributors of the new release.
+1. Publish the release.
+1. Switch to the master branch and pull the changes.
+1. Push the release to CocoaPods trunk. For that, execute the following commands:
+
+    ```
+    pod trunk push OpenCombine.podspec --verbose --allow-warnings
+    pod trunk push OpenCombineDispatch.podspec --verbose --allow-warnings
+    pod trunk push OpenCombineFoundation.podspec --verbose --allow-warnings
+    ```
+   
+   Note that you need to be one of the owners of the pod for that.
 
 #### GYB
 
@@ -86,3 +111,16 @@ GYB template files have the `.gyb` extension. Run `make gyb` to generate Swift c
 templates. The generated files are prefixed with `GENERATED-`  and are checked into source control. Those
 files should never be edited directly. Instead, the `.gyb` template should be edited, and after that the files
 should be regenerated using `make gyb`.
+
+#### Debugger Support
+
+The file `opencombine_lldb.py`  defines some `lldb` type summaries for easier debugging. These type summaries improve the way `lldb` and Xcode display some OpenCombine values.
+
+To use `opencombine_lldb.py`, figure out its full path. Let's say the full path is `~/projects/OpenCombine/opencombine_lldb.py`. Then the following statement to your `~/.lldbinit` file:
+
+    command script import ~/projects/OpenCombine/opencombine_lldb.py
+
+Currently, `opencombine_lldb.py` defines type summaries for these types:
+
+- `Subscribers.Demand`
+- That's all for now.

RemainingCombineInterface.swift

@@ -651,49 +651,6 @@ extension Publisher {
     public func merge(with other: Self) -> Publishers.MergeMany<Self>
 }
 
-extension Publishers {
-
-    /// A publisher that “flattens” nested publishers.
-    ///
-    /// Given a publisher that publishes Publishers, the `SwitchToLatest` publisher produces a sequence of events from only the most recent one.
-    /// For example, given the type `Publisher<Publisher<Data, NSError>, Never>`, calling `switchToLatest()` will result in the type `Publisher<Data, NSError>`. The downstream subscriber sees a continuous stream of values even though they may be coming from different upstream publishers.
-    public struct SwitchToLatest<P, Upstream> : Publisher where P : Publisher, P == Upstream.Output, Upstream : Publisher, P.Failure == Upstream.Failure {
-
-        /// The kind of values published by this publisher.
-        public typealias Output = P.Output
-
-        /// The kind of errors this publisher might publish.
-        ///
-        /// Use `Never` if this `Publisher` does not publish errors.
-        public typealias Failure = P.Failure
-
-        /// The publisher from which this publisher receives elements.
-        public let upstream: Upstream
-
-        /// Creates a publisher that “flattens” nested publishers.
-        ///
-        /// - Parameter upstream: The publisher from which this publisher receives elements.
-        public init(upstream: Upstream)
-
-        /// This function is called to attach the specified `Subscriber` to this `Publisher` by `subscribe(_:)`
-        ///
-        /// - SeeAlso: `subscribe(_:)`
-        /// - Parameters:
-        ///     - subscriber: The subscriber to attach to this `Publisher`.
-        ///                   once attached it can begin to receive values.
-        public func receive<S>(subscriber: S) where S : Subscriber, P.Output == S.Input, Upstream.Failure == S.Failure
-    }
-}
-
-extension Publisher where Self.Failure == Self.Output.Failure, Self.Output : Publisher {
-
-    /// Flattens the stream of events from multiple upstream publishers to appear as if they were coming from a single stream of events.
-    ///
-    /// This operator switches the inner publisher as new ones arrive but keeps the outer one constant for downstream subscribers.
-    /// For example, given the type `Publisher<Publisher<Data, NSError>, Never>`, calling `switchToLatest()` will result in the type `Publisher<Data, NSError>`. The downstream subscriber sees a continuous stream of values even though they may be coming from different upstream publishers.
-    public func switchToLatest() -> Publishers.SwitchToLatest<Self.Output, Self>
-}
-
 extension Publishers {
 
     /// A publisher that attempts to recreate its subscription to a failed upstream publisher.
@@ -803,219 +760,6 @@ extension Publisher {
     public func throttle<S>(for interval: S.SchedulerTimeType.Stride, scheduler: S, latest: Bool) -> Publishers.Throttle<Self, S> where S : Scheduler
 }
 
-extension Publishers {
-
-    /// A publisher that publishes elements only after a specified time interval elapses between events.
-    public struct Debounce<Upstream, Context> : Publisher where Upstream : Publisher, Context : Scheduler {
-
-        /// The kind of values published by this publisher.
-        public typealias Output = Upstream.Output
-
-        /// The kind of errors this publisher might publish.
-        ///
-        /// Use `Never` if this `Publisher` does not publish errors.
-        public typealias Failure = Upstream.Failure
-
-        /// The publisher from which this publisher receives elements.
-        public let upstream: Upstream
-
-        /// The amount of time the publisher should wait before publishing an element.
-        public let dueTime: Context.SchedulerTimeType.Stride
-
-        /// The scheduler on which this publisher delivers elements.
-        public let scheduler: Context
-
-        /// Scheduler options that customize this publisher’s delivery of elements.
-        public let options: Context.SchedulerOptions?
-
-        public init(upstream: Upstream, dueTime: Context.SchedulerTimeType.Stride, scheduler: Context, options: Context.SchedulerOptions?)
-
-        /// This function is called to attach the specified `Subscriber` to this `Publisher` by `subscribe(_:)`
-        ///
-        /// - SeeAlso: `subscribe(_:)`
-        /// - Parameters:
-        ///     - subscriber: The subscriber to attach to this `Publisher`.
-        ///                   once attached it can begin to receive values.
-        public func receive<S>(subscriber: S) where S : Subscriber, Upstream.Failure == S.Failure, Upstream.Output == S.Input
-    }
-}
-
-extension Publisher {
-
-    /// Publishes elements only after a specified time interval elapses between events.
-    ///
-    /// Use this operator when you want to wait for a pause in the delivery of events from the upstream publisher. For example, call `debounce` on the publisher from a text field to only receive elements when the user pauses or stops typing. When they start typing again, the `debounce` holds event delivery until the next pause.
-    /// - Parameters:
-    ///   - dueTime: The time the publisher should wait before publishing an element.
-    ///   - scheduler: The scheduler on which this publisher delivers elements
-    ///   - options: Scheduler options that customize this publisher’s delivery of elements.
-    /// - Returns: A publisher that publishes events only after a specified time elapses.
-    public func debounce<S>(for dueTime: S.SchedulerTimeType.Stride, scheduler: S, options: S.SchedulerOptions? = nil) -> Publishers.Debounce<Self, S> where S : Scheduler
-}
-
-extension Publishers {
-
-    public struct Timeout<Upstream, Context> : Publisher where Upstream : Publisher, Context : Scheduler {
-
-        /// The kind of values published by this publisher.
-        public typealias Output = Upstream.Output
-
-        /// The kind of errors this publisher might publish.
-        ///
-        /// Use `Never` if this `Publisher` does not publish errors.
-        public typealias Failure = Upstream.Failure
-
-        public let upstream: Upstream
-
-        public let interval: Context.SchedulerTimeType.Stride
-
-        public let scheduler: Context
-
-        public let options: Context.SchedulerOptions?
-
-        public let customError: (() -> Upstream.Failure)?
-
-        public init(upstream: Upstream, interval: Context.SchedulerTimeType.Stride, scheduler: Context, options: Context.SchedulerOptions?, customError: (() -> Publishers.Timeout<Upstream, Context>.Failure)?)
-
-        /// This function is called to attach the specified `Subscriber` to this `Publisher` by `subscribe(_:)`
-        ///
-        /// - SeeAlso: `subscribe(_:)`
-        /// - Parameters:
-        ///     - subscriber: The subscriber to attach to this `Publisher`.
-        ///                   once attached it can begin to receive values.
-        public func receive<S>(subscriber: S) where S : Subscriber, Upstream.Failure == S.Failure, Upstream.Output == S.Input
-    }
-}
-
-extension Publisher {
-
-    /// Terminates publishing if the upstream publisher exceeds the specified time interval without producing an element.
-    ///
-    /// - Parameters:
-    ///   - interval: The maximum time interval the publisher can go without emitting an element, expressed in the time system of the scheduler.
-    ///   - scheduler: The scheduler to deliver events on.
-    ///   - options: Scheduler options that customize the delivery of elements.
-    ///   - customError: A closure that executes if the publisher times out. The publisher sends the failure returned by this closure to the subscriber as the reason for termination.
-    /// - Returns: A publisher that terminates if the specified interval elapses with no events received from the upstream publisher.
-    public func timeout<S>(_ interval: S.SchedulerTimeType.Stride, scheduler: S, options: S.SchedulerOptions? = nil, customError: (() -> Self.Failure)? = nil) -> Publishers.Timeout<Self, S> where S : Scheduler
-}
-
-extension Publishers {
-
-    /// A strategy for filling a buffer.
-    ///
-    /// * keepFull: A strategy to fill the buffer at subscription time, and keep it full thereafter.
-    /// * byRequest: A strategy that avoids prefetching and instead performs requests on demand.
-    public enum PrefetchStrategy {
-
-        case keepFull
-
-        case byRequest
-
-        /// Returns a Boolean value indicating whether two values are equal.
-        ///
-        /// Equality is the inverse of inequality. For any values `a` and `b`,
-        /// `a == b` implies that `a != b` is `false`.
-        ///
-        /// - Parameters:
-        ///   - lhs: A value to compare.
-        ///   - rhs: Another value to compare.
-        public static func == (a: Publishers.PrefetchStrategy, b: Publishers.PrefetchStrategy) -> Bool
-
-        /// The hash value.
-        ///
-        /// Hash values are not guaranteed to be equal across different executions of
-        /// your program. Do not save hash values to use during a future execution.
-        ///
-        /// - Important: `hashValue` is deprecated as a `Hashable` requirement. To
-        ///   conform to `Hashable`, implement the `hash(into:)` requirement instead.
-        public var hashValue: Int { get }
-
-        /// Hashes the essential components of this value by feeding them into the
-        /// given hasher.
-        ///
-        /// Implement this method to conform to the `Hashable` protocol. The
-        /// components used for hashing must be the same as the components compared
-        /// in your type's `==` operator implementation. Call `hasher.combine(_:)`
-        /// with each of these components.
-        ///
-        /// - Important: Never call `finalize()` on `hasher`. Doing so may become a
-        ///   compile-time error in the future.
-        ///
-        /// - Parameter hasher: The hasher to use when combining the components
-        ///   of this instance.
-        public func hash(into hasher: inout Hasher)
-    }
-
-    /// A strategy for handling exhaustion of a buffer’s capacity.
-    ///
-    /// * dropNewest: When full, discard the newly-received element without buffering it.
-    /// * dropOldest: When full, remove the least recently-received element from the buffer.
-    /// * customError: When full, execute the closure to provide a custom error.
-    public enum BufferingStrategy<Failure> where Failure : Error {
-
-        case dropNewest
-
-        case dropOldest
-
-        case customError(() -> Failure)
-    }
-
-    /// A publisher that buffers elements received from an upstream publisher.
-    public struct Buffer<Upstream> : Publisher where Upstream : Publisher {
-
-        /// The kind of values published by this publisher.
-        public typealias Output = Upstream.Output
-
-        /// The kind of errors this publisher might publish.
-        ///
-        /// Use `Never` if this `Publisher` does not publish errors.
-        public typealias Failure = Upstream.Failure
-
-        /// The publisher from which this publisher receives elements.
-        public let upstream: Upstream
-
-        /// The maximum number of elements to store.
-        public let size: Int
-
-        /// The strategy for initially populating the buffer.
-        public let prefetch: Publishers.PrefetchStrategy
-
-        /// The action to take when the buffer becomes full.
-        public let whenFull: Publishers.BufferingStrategy<Upstream.Failure>
-
-        /// Creates a publisher that buffers elements received from an upstream publisher.
-        /// - Parameter upstream: The publisher from which this publisher receives elements.
-        /// - Parameter size: The maximum number of elements to store.
-        /// - Parameter prefetch: The strategy for initially populating the buffer.
-        /// - Parameter whenFull: The action to take when the buffer becomes full.
-        public init(upstream: Upstream, size: Int, prefetch: Publishers.PrefetchStrategy, whenFull: Publishers.BufferingStrategy<Publishers.Buffer<Upstream>.Failure>)
-
-        /// This function is called to attach the specified `Subscriber` to this `Publisher` by `subscribe(_:)`
-        ///
-        /// - SeeAlso: `subscribe(_:)`
-        /// - Parameters:
-        ///     - subscriber: The subscriber to attach to this `Publisher`.
-        ///                   once attached it can begin to receive values.
-        public func receive<S>(subscriber: S) where S : Subscriber, Upstream.Failure == S.Failure, Upstream.Output == S.Input
-    }
-}
-
-extension Publishers.PrefetchStrategy : Equatable {
-}
-
-extension Publishers.PrefetchStrategy : Hashable {
-}
-
-extension Publisher {
-
-    /// Buffers elements received from an upstream publisher.
-    /// - Parameter size: The maximum number of elements to store.
-    /// - Parameter prefetch: The strategy for initially populating the buffer.
-    /// - Parameter whenFull: The action to take when the buffer becomes full.
-    public func buffer(size: Int, prefetch: Publishers.PrefetchStrategy, whenFull: Publishers.BufferingStrategy<Self.Failure>) -> Publishers.Buffer<Self>
-}
-
 extension Publishers {
 
     /// A publisher created by applying the zip function to two upstream publishers.
@@ -1179,100 +923,6 @@ extension Publisher {
     public func zip<P, Q, R, T>(_ publisher1: P, _ publisher2: Q, _ publisher3: R, _ transform: @escaping (Self.Output, P.Output, Q.Output, R.Output) -> T) -> Publishers.Map<Publishers.Zip4<Self, P, Q, R>, T> where P : Publisher, Q : Publisher, R : Publisher, Self.Failure == P.Failure, P.Failure == Q.Failure, Q.Failure == R.Failure
 }
 
-extension Publishers {
-
-    /// A publisher that handles errors from an upstream publisher by replacing the failed publisher with another publisher.
-    public struct Catch<Upstream, NewPublisher> : Publisher where Upstream : Publisher, NewPublisher : Publisher, Upstream.Output == NewPublisher.Output {
-
-        /// The kind of values published by this publisher.
-        public typealias Output = Upstream.Output
-
-        /// The kind of errors this publisher might publish.
-        ///
-        /// Use `Never` if this `Publisher` does not publish errors.
-        public typealias Failure = NewPublisher.Failure
-
-        /// The publisher that this publisher receives elements from.
-        public let upstream: Upstream
-
-        /// A closure that accepts the upstream failure as input and returns a publisher to replace the upstream publisher.
-        public let handler: (Upstream.Failure) -> NewPublisher
-
-        /// Creates a publisher that handles errors from an upstream publisher by replacing the failed publisher with another publisher.
-        ///
-        /// - Parameters:
-        ///   - upstream: The publisher that this publisher receives elements from.
-        ///   - handler: A closure that accepts the upstream failure as input and returns a publisher to replace the upstream publisher.
-        public init(upstream: Upstream, handler: @escaping (Upstream.Failure) -> NewPublisher)
-
-        /// This function is called to attach the specified `Subscriber` to this `Publisher` by `subscribe(_:)`
-        ///
-        /// - SeeAlso: `subscribe(_:)`
-        /// - Parameters:
-        ///     - subscriber: The subscriber to attach to this `Publisher`.
-        ///                   once attached it can begin to receive values.
-        public func receive<S>(subscriber: S) where S : Subscriber, NewPublisher.Failure == S.Failure, NewPublisher.Output == S.Input
-    }
-
-    /// A publisher that handles errors from an upstream publisher by replacing the failed publisher with another publisher or optionally producing a new error.
-    public struct TryCatch<Upstream, NewPublisher> : Publisher where Upstream : Publisher, NewPublisher : Publisher, Upstream.Output == NewPublisher.Output {
-
-        /// The kind of values published by this publisher.
-        public typealias Output = Upstream.Output
-
-        /// The kind of errors this publisher might publish.
-        ///
-        /// Use `Never` if this `Publisher` does not publish errors.
-        public typealias Failure = Error
-
-        public let upstream: Upstream
-
-        public let handler: (Upstream.Failure) throws -> NewPublisher
-
-        public init(upstream: Upstream, handler: @escaping (Upstream.Failure) throws -> NewPublisher)
-
-        /// This function is called to attach the specified `Subscriber` to this `Publisher` by `subscribe(_:)`
-        ///
-        /// - SeeAlso: `subscribe(_:)`
-        /// - Parameters:
-        ///     - subscriber: The subscriber to attach to this `Publisher`.
-        ///                   once attached it can begin to receive values.
-        public func receive<S>(subscriber: S) where S : Subscriber, NewPublisher.Output == S.Input, S.Failure == Publishers.TryCatch<Upstream, NewPublisher>.Failure
-    }
-}
-
-extension Publisher {
-
-    /// Handles errors from an upstream publisher by replacing it with another publisher.
-    ///
-    /// The following example replaces any error from the upstream publisher and replaces the upstream with a `Just` publisher. This continues the stream by publishing a single value and completing normally.
-    /// ```
-    /// enum SimpleError: Error { case error }
-    /// let errorPublisher = (0..<10).publisher.tryMap { v -> Int in
-    ///     if v < 5 {
-    ///         return v
-    ///     } else {
-    ///         throw SimpleError.error
-    ///     }
-    /// }
-    ///
-    /// let noErrorPublisher = errorPublisher.catch { _ in
-    ///     return Just(100)
-    /// }
-    /// ```
-    /// Backpressure note: This publisher passes through `request` and `cancel` to the upstream. After receiving an error, the publisher sends sends any unfulfilled demand to the new `Publisher`.
-    /// - Parameter handler: A closure that accepts the upstream failure as input and returns a publisher to replace the upstream publisher.
-    /// - Returns: A publisher that handles errors from an upstream publisher by replacing the failed publisher with another publisher.
-    public func `catch`<P>(_ handler: @escaping (Self.Failure) -> P) -> Publishers.Catch<Self, P> where P : Publisher, Self.Output == P.Output
-
-    /// Handles errors from an upstream publisher by either replacing it with another publisher or `throw`ing  a new error.
-    ///
-    /// - Parameter handler: A `throw`ing closure that accepts the upstream failure as input and returns a publisher to replace the upstream publisher or if an error is thrown will send the error downstream.
-    /// - Returns: A publisher that handles errors from an upstream publisher by replacing the failed publisher with another publisher.
-    public func tryCatch<P>(_ handler: @escaping (Self.Failure) throws -> P) -> Publishers.TryCatch<Self, P> where P : Publisher, Self.Output == P.Output
-}
-
-
 extension Publishers.CombineLatest : Equatable where A : Equatable, B : Equatable {
 
     /// Returns a Boolean value that indicates whether two publishers are equivalent.

RemainingFoundationInterface.swift

@@ -37,127 +37,3 @@ extension NSObject.KeyValueObservingPublisher : Combine.Publisher {
   public func receive<S>(subscriber: S) where Value == S.Input, S : Combine.Subscriber, S.Failure == ObjectiveC.NSObject.KeyValueObservingPublisher<Subject, Value>.Failure
 }
 
-extension Timer {
-  public static func publish(every interval: Foundation.TimeInterval, tolerance: Foundation.TimeInterval? = nil, on runLoop: Foundation.RunLoop, in mode: Foundation.RunLoop.Mode, options: Foundation.RunLoop.SchedulerOptions? = nil) -> Foundation.Timer.TimerPublisher
-  final public class TimerPublisher : Combine.ConnectablePublisher {
-    public typealias Output = Foundation.Date
-    public typealias Failure = Swift.Never
-    final public let interval: Foundation.TimeInterval
-    final public let tolerance: Foundation.TimeInterval?
-    final public let runLoop: Foundation.RunLoop
-    final public let mode: Foundation.RunLoop.Mode
-    final public let options: Foundation.RunLoop.SchedulerOptions?
-    public init(interval: Foundation.TimeInterval, tolerance: Foundation.TimeInterval? = nil, runLoop: Foundation.RunLoop, mode: Foundation.RunLoop.Mode, options: Foundation.RunLoop.SchedulerOptions? = nil)
-    final public func receive<S>(subscriber: S) where S : Combine.Subscriber, S.Failure == Foundation.Timer.TimerPublisher.Failure, S.Input == Foundation.Timer.TimerPublisher.Output
-    final public func connect() -> Combine.Cancellable
-    @objc deinit
-  }
-}
-
-extension OperationQueue : Combine.Scheduler {
-  public struct SchedulerTimeType : Swift.Strideable, Swift.Codable, Swift.Hashable {
-    public var date: Foundation.Date
-    public init(_ date: Foundation.Date)
-    public func distance(to other: Foundation.OperationQueue.SchedulerTimeType) -> Foundation.OperationQueue.SchedulerTimeType.Stride
-    public func advanced(by n: Foundation.OperationQueue.SchedulerTimeType.Stride) -> Foundation.OperationQueue.SchedulerTimeType
-    public struct Stride : Swift.ExpressibleByFloatLiteral, Swift.Comparable, Swift.SignedNumeric, Swift.Codable, Combine.SchedulerTimeIntervalConvertible {
-      public typealias FloatLiteralType = Foundation.TimeInterval
-      public typealias IntegerLiteralType = Foundation.TimeInterval
-      public typealias Magnitude = Foundation.TimeInterval
-      public var magnitude: Foundation.TimeInterval
-      public var timeInterval: Foundation.TimeInterval {
-        get
-      }
-      public init(integerLiteral value: Foundation.TimeInterval)
-      public init(floatLiteral value: Foundation.TimeInterval)
-      public init(_ timeInterval: Foundation.TimeInterval)
-      public init?<T>(exactly source: T) where T : Swift.BinaryInteger
-      public static func < (lhs: Foundation.OperationQueue.SchedulerTimeType.Stride, rhs: Foundation.OperationQueue.SchedulerTimeType.Stride) -> Swift.Bool
-      public static func * (lhs: Foundation.OperationQueue.SchedulerTimeType.Stride, rhs: Foundation.OperationQueue.SchedulerTimeType.Stride) -> Foundation.OperationQueue.SchedulerTimeType.Stride
-      public static func + (lhs: Foundation.OperationQueue.SchedulerTimeType.Stride, rhs: Foundation.OperationQueue.SchedulerTimeType.Stride) -> Foundation.OperationQueue.SchedulerTimeType.Stride
-      public static func - (lhs: Foundation.OperationQueue.SchedulerTimeType.Stride, rhs: Foundation.OperationQueue.SchedulerTimeType.Stride) -> Foundation.OperationQueue.SchedulerTimeType.Stride
-      public static func *= (lhs: inout Foundation.OperationQueue.SchedulerTimeType.Stride, rhs: Foundation.OperationQueue.SchedulerTimeType.Stride)
-      public static func += (lhs: inout Foundation.OperationQueue.SchedulerTimeType.Stride, rhs: Foundation.OperationQueue.SchedulerTimeType.Stride)
-      public static func -= (lhs: inout Foundation.OperationQueue.SchedulerTimeType.Stride, rhs: Foundation.OperationQueue.SchedulerTimeType.Stride)
-      public static func seconds(_ s: Swift.Int) -> Foundation.OperationQueue.SchedulerTimeType.Stride
-      public static func seconds(_ s: Swift.Double) -> Foundation.OperationQueue.SchedulerTimeType.Stride
-      public static func milliseconds(_ ms: Swift.Int) -> Foundation.OperationQueue.SchedulerTimeType.Stride
-      public static func microseconds(_ us: Swift.Int) -> Foundation.OperationQueue.SchedulerTimeType.Stride
-      public static func nanoseconds(_ ns: Swift.Int) -> Foundation.OperationQueue.SchedulerTimeType.Stride
-      public init(from decoder: Swift.Decoder) throws
-      public func encode(to encoder: Swift.Encoder) throws
-      public static func == (a: Foundation.OperationQueue.SchedulerTimeType.Stride, b: Foundation.OperationQueue.SchedulerTimeType.Stride) -> Swift.Bool
-    }
-    public init(from decoder: Swift.Decoder) throws
-    public func encode(to encoder: Swift.Encoder) throws
-    public var hashValue: Swift.Int {
-      get
-    }
-    public func hash(into hasher: inout Swift.Hasher)
-  }
-  public struct SchedulerOptions {
-  }
-  public func schedule(options: Foundation.OperationQueue.SchedulerOptions?, _ action: @escaping () -> Swift.Void)
-  public func schedule(after date: Foundation.OperationQueue.SchedulerTimeType, tolerance: Foundation.OperationQueue.SchedulerTimeType.Stride, options: Foundation.OperationQueue.SchedulerOptions?, _ action: @escaping () -> Swift.Void)
-  public func schedule(after date: Foundation.OperationQueue.SchedulerTimeType, interval: Foundation.OperationQueue.SchedulerTimeType.Stride, tolerance: Foundation.OperationQueue.SchedulerTimeType.Stride, options: Foundation.OperationQueue.SchedulerOptions?, _ action: @escaping () -> Swift.Void) -> Combine.Cancellable
-  public var now: Foundation.OperationQueue.SchedulerTimeType {
-    get
-  }
-  public var minimumTolerance: Foundation.OperationQueue.SchedulerTimeType.Stride {
-    get
-  }
-}
-
-extension RunLoop : Combine.Scheduler {
-  public struct SchedulerTimeType : Swift.Strideable, Swift.Codable, Swift.Hashable {
-    public var date: Foundation.Date
-    public init(_ date: Foundation.Date)
-    public func distance(to other: Foundation.RunLoop.SchedulerTimeType) -> Foundation.RunLoop.SchedulerTimeType.Stride
-    public func advanced(by n: Foundation.RunLoop.SchedulerTimeType.Stride) -> Foundation.RunLoop.SchedulerTimeType
-    public struct Stride : Swift.ExpressibleByFloatLiteral, Swift.Comparable, Swift.SignedNumeric, Swift.Codable, Combine.SchedulerTimeIntervalConvertible {
-      public typealias FloatLiteralType = Foundation.TimeInterval
-      public typealias IntegerLiteralType = Foundation.TimeInterval
-      public typealias Magnitude = Foundation.TimeInterval
-      public var magnitude: Foundation.TimeInterval
-      public var timeInterval: Foundation.TimeInterval {
-        get
-      }
-      public init(integerLiteral value: Foundation.TimeInterval)
-      public init(floatLiteral value: Foundation.TimeInterval)
-      public init(_ timeInterval: Foundation.TimeInterval)
-      public init?<T>(exactly source: T) where T : Swift.BinaryInteger
-      public static func < (lhs: Foundation.RunLoop.SchedulerTimeType.Stride, rhs: Foundation.RunLoop.SchedulerTimeType.Stride) -> Swift.Bool
-      public static func * (lhs: Foundation.RunLoop.SchedulerTimeType.Stride, rhs: Foundation.RunLoop.SchedulerTimeType.Stride) -> Foundation.RunLoop.SchedulerTimeType.Stride
-      public static func + (lhs: Foundation.RunLoop.SchedulerTimeType.Stride, rhs: Foundation.RunLoop.SchedulerTimeType.Stride) -> Foundation.RunLoop.SchedulerTimeType.Stride
-      public static func - (lhs: Foundation.RunLoop.SchedulerTimeType.Stride, rhs: Foundation.RunLoop.SchedulerTimeType.Stride) -> Foundation.RunLoop.SchedulerTimeType.Stride
-      public static func *= (lhs: inout Foundation.RunLoop.SchedulerTimeType.Stride, rhs: Foundation.RunLoop.SchedulerTimeType.Stride)
-      public static func += (lhs: inout Foundation.RunLoop.SchedulerTimeType.Stride, rhs: Foundation.RunLoop.SchedulerTimeType.Stride)
-      public static func -= (lhs: inout Foundation.RunLoop.SchedulerTimeType.Stride, rhs: Foundation.RunLoop.SchedulerTimeType.Stride)
-      public static func seconds(_ s: Swift.Int) -> Foundation.RunLoop.SchedulerTimeType.Stride
-      public static func seconds(_ s: Swift.Double) -> Foundation.RunLoop.SchedulerTimeType.Stride
-      public static func milliseconds(_ ms: Swift.Int) -> Foundation.RunLoop.SchedulerTimeType.Stride
-      public static func microseconds(_ us: Swift.Int) -> Foundation.RunLoop.SchedulerTimeType.Stride
-      public static func nanoseconds(_ ns: Swift.Int) -> Foundation.RunLoop.SchedulerTimeType.Stride
-      public init(from decoder: Swift.Decoder) throws
-      public func encode(to encoder: Swift.Encoder) throws
-      public static func == (a: Foundation.RunLoop.SchedulerTimeType.Stride, b: Foundation.RunLoop.SchedulerTimeType.Stride) -> Swift.Bool
-    }
-    public init(from decoder: Swift.Decoder) throws
-    public func encode(to encoder: Swift.Encoder) throws
-    public var hashValue: Swift.Int {
-      get
-    }
-    public func hash(into hasher: inout Swift.Hasher)
-  }
-  public struct SchedulerOptions {
-  }
-  public func schedule(options: Foundation.RunLoop.SchedulerOptions?, _ action: @escaping () -> Swift.Void)
-  public func schedule(after date: Foundation.RunLoop.SchedulerTimeType, tolerance: Foundation.RunLoop.SchedulerTimeType.Stride, options: Foundation.RunLoop.SchedulerOptions?, _ action: @escaping () -> Swift.Void)
-  public func schedule(after date: Foundation.RunLoop.SchedulerTimeType, interval: Foundation.RunLoop.SchedulerTimeType.Stride, tolerance: Foundation.RunLoop.SchedulerTimeType.Stride, options: Foundation.RunLoop.SchedulerOptions?, _ action: @escaping () -> Swift.Void) -> Combine.Cancellable
-  public var now: Foundation.RunLoop.SchedulerTimeType {
-    get
-  }
-  public var minimumTolerance: Foundation.RunLoop.SchedulerTimeType.Stride {
-    get
-  }
-}

Sources/COpenCombineHelpers/COpenCombineHelpers.cpp

@@ -11,6 +11,7 @@
 #include <cstdlib>
 #include <system_error>
 #include <pthread.h>
+#include <signal.h>
 
 #ifdef __APPLE__
 #include <os/lock.h>
@@ -235,4 +236,8 @@ void opencombine_unfair_recursive_lock_dealloc(OpenCombineUnfairRecursiveLock lo
     return delete static_cast<PlatformIndependentMutex*>(lock.opaque);
 }
 
+void opencombine_stop_in_debugger(void) {
+    raise(SIGTRAP);
+}
+
 } // extern "C"

Sources/COpenCombineHelpers/include/COpenCombineHelpers.h

@@ -9,7 +9,6 @@
 #define COPENCOMBINEHELPERS_H
 
 #include <stdint.h>
-#include <signal.h>
 
 #if __has_attribute(swift_name)
 # define OPENCOMBINE_SWIFT_NAME(_name) __attribute__((swift_name(#_name)))
@@ -17,12 +16,6 @@
 # define OPENCOMBINE_SWIFT_NAME(_name)
 #endif
 
-#if __has_attribute(always_inline)
-# define OPENCOMBINE_ALWAYS_INLINE __attribute__((always_inline))
-#else
-# define OPENCOMBINE_ALWAYS_INLINE
-#endif
-
 #ifdef __cplusplus
 extern "C" {
 #endif
@@ -77,12 +70,7 @@ void opencombine_unfair_recursive_lock_dealloc(OpenCombineUnfairRecursiveLock lo
 
 #pragma mark - Breakpoint
 
-OPENCOMBINE_ALWAYS_INLINE
-inline void opencombine_stop_in_debugger(void) OPENCOMBINE_SWIFT_NAME(__stopInDebugger());
-
-void opencombine_stop_in_debugger(void) {
-    raise(SIGTRAP);
-}
+void opencombine_stop_in_debugger(void) OPENCOMBINE_SWIFT_NAME(__stopInDebugger());
 
 #ifdef __cplusplus
 } // extern "C"

Sources/OpenCombine/AnyCancellable.swift

@@ -57,7 +57,7 @@ extension AnyCancellable {
 
     /// Stores this AnyCancellable in the specified set.
     /// Parameters:
-    ///    - collection: The set to store this AnyCancellable.
+    ///    - set: The set to store this AnyCancellable.
     public func store(in set: inout Set<AnyCancellable>) {
         set.insert(self)
     }

Sources/OpenCombine/AnyPublisher.swift

@@ -11,6 +11,7 @@ extension Publisher {
     ///
     /// Use `eraseToAnyPublisher()` to expose an instance of `AnyPublisher` to
     /// the downstream subscriber, rather than this publisher’s actual type.
+    @inlinable
     public func eraseToAnyPublisher() -> AnyPublisher<Output, Failure> {
         return .init(self)
     }

Sources/OpenCombine/Cancellable.swift

@@ -28,7 +28,7 @@ extension Cancellable {
 
     /// Stores this Cancellable in the specified set.
     /// Parameters:
-    ///    - collection: The set to store this Cancellable.
+    ///    - set: The set to store this Cancellable.
     public func store(in set: inout Set<AnyCancellable>) {
         AnyCancellable(self).store(in: &set)
     }

Sources/OpenCombine/CurrentValueSubject.swift

@@ -9,26 +9,29 @@
 /// changes.
 public final class CurrentValueSubject<Output, Failure: Error>: Subject {
 
-    private let _lock = UnfairRecursiveLock.allocate()
+    private let lock = UnfairLock.allocate()
 
-    // TODO: Combine uses bag data structure
-    private var _subscriptions: [Conduit] = []
+    private var active = true
 
-    private var _value: Output
+    private var completion: Subscribers.Completion<Failure>?
 
-    private var _completion: Subscribers.Completion<Failure>?
+    private var downstreams = ConduitList<Output, Failure>.empty
 
-    internal var upstreamSubscriptions: [Subscription] = []
+    private var currentValue: Output
 
-    internal var hasAnyDownstreamDemand = false
+    private var upstreamSubscriptions: [Subscription] = []
 
     /// The value wrapped by this subject, published as a new element whenever it changes.
     public var value: Output {
         get {
-            return _value
+            lock.lock()
+            defer { lock.unlock() }
+            return currentValue
         }
         set {
-            send(newValue)
+            lock.lock()
+            currentValue = newValue
+            sendValueAndConsumeLock(newValue)
         }
     }
 
@@ -36,122 +39,213 @@ public final class CurrentValueSubject<Output, Failure: Error>: Subject {
     ///
     /// - Parameter value: The initial value to publish.
     public init(_ value: Output) {
-        self._value = value
+        self.currentValue = value
     }
 
     deinit {
-        for subscription in _subscriptions {
-            subscription._downstream = nil
+        for subscription in upstreamSubscriptions {
+            subscription.cancel()
         }
-        _lock.deallocate()
+        lock.deallocate()
     }
 
     public func send(subscription: Subscription) {
-        _lock.do {
-            upstreamSubscriptions.append(subscription)
-            subscription.request(.unlimited)
-        }
+        lock.lock()
+        upstreamSubscriptions.append(subscription)
+        lock.unlock()
+        subscription.request(.unlimited)
     }
 
-    public func receive<Subscriber: OpenCombine.Subscriber>(subscriber: Subscriber)
-        where Output == Subscriber.Input, Failure == Subscriber.Failure
+    public func receive<Downstream: Subscriber>(subscriber: Downstream)
+        where Output == Downstream.Input, Failure == Downstream.Failure
     {
-        _lock.do {
-
-            if let completion = _completion {
-                subscriber.receive(subscription: Subscriptions.empty)
-                subscriber.receive(completion: completion)
-                return
-            } else {
-                let subscription = Conduit(parent: self,
-                                           downstream: AnySubscriber(subscriber))
-
-                _subscriptions.append(subscription)
-                subscriber.receive(subscription: subscription)
-            }
+        lock.lock()
+        if active {
+            let conduit = Conduit(parent: self, downstream: subscriber)
+            downstreams.insert(conduit)
+            lock.unlock()
+            subscriber.receive(subscription: conduit)
+        } else {
+            let completion = self.completion!
+            lock.unlock()
+            subscriber.receive(subscription: Subscriptions.empty)
+            subscriber.receive(completion: completion)
         }
     }
 
     public func send(_ input: Output) {
-        _lock.do {
-            _value = input
-            for subscription in _subscriptions where !subscription.isCompleted {
-                if subscription._demand > 0 {
-                    subscription._offer(input)
-                    subscription._demand -= 1
-                } else {
-                    subscription._delivered = false
-                }
-            }
+        lock.lock()
+        sendValueAndConsumeLock(input)
+    }
+
+    private func sendValueAndConsumeLock(_ newValue: Output) {
+#if DEBUG
+        lock.assertOwner()
+#endif
+        guard active else {
+            lock.unlock()
+            return
+        }
+        currentValue = newValue
+        let downstreams = self.downstreams
+        lock.unlock()
+        downstreams.forEach { conduit in
+            conduit.offer(newValue)
         }
     }
 
     public func send(completion: Subscribers.Completion<Failure>) {
-        _completion = completion
-        _lock.do {
-            for subscriber in _subscriptions {
-                subscriber._receive(completion: completion)
-            }
+        lock.lock()
+        guard active else {
+            lock.unlock()
+            return
         }
+        active = false
+        self.completion = completion
+        let downstreams = self.downstreams
+        self.downstreams.removeAll()
+        lock.unlock()
+        downstreams.forEach { conduit in
+            conduit.finish(completion: completion)
+        }
+    }
+
+    private func disassociate(_ conduit: ConduitBase<Output, Failure>) {
+        lock.lock()
+        guard active else {
+            lock.unlock()
+            return
+        }
+        downstreams.remove(conduit)
+        lock.unlock()
     }
 }
 
 extension CurrentValueSubject {
 
-    fileprivate class Conduit: Subscription {
+    private final class Conduit<Downstream: Subscriber>
+        : ConduitBase<Output, Failure>,
+          CustomStringConvertible,
+          CustomReflectable,
+          CustomPlaygroundDisplayConvertible
+        where Downstream.Input == Output, Downstream.Failure == Failure
+    {
 
-        fileprivate var _parent: CurrentValueSubject?
+        fileprivate var parent: CurrentValueSubject?
 
-        fileprivate var _downstream: AnySubscriber<Output, Failure>?
+        fileprivate var downstream: Downstream?
 
-        fileprivate var _demand: Subscribers.Demand = .none
+        fileprivate var demand = Subscribers.Demand.none
 
-        /// Whethere we satisfied the demand
-        fileprivate var _delivered = false
+        private var lock = UnfairLock.allocate()
 
-        var isCompleted: Bool {
-            return _parent == nil
-        }
+        private var downstreamLock = UnfairRecursiveLock.allocate()
 
-        fileprivate func _offer(_ value: Output) {
-            let newDemand = _downstream?.receive(value) ?? .none
-            _demand += newDemand
-            _delivered = true
-        }
+        private var deliveredCurrentValue = false
 
         fileprivate init(parent: CurrentValueSubject,
-                         downstream: AnySubscriber<Output, Failure>) {
-            _parent = parent
-            _downstream = downstream
+                         downstream: Downstream) {
+            self.parent = parent
+            self.downstream = downstream
         }
 
-        fileprivate func _receive(completion: Subscribers.Completion<Failure>) {
-            if !isCompleted {
-                _parent = nil
-                _downstream?.receive(completion: completion)
+        deinit {
+            lock.deallocate()
+            downstreamLock.deallocate()
+        }
+
+        override func offer(_ output: Output) {
+            lock.lock()
+            guard demand > 0, let downstream = self.downstream else {
+                deliveredCurrentValue = false
+                lock.unlock()
+                return
             }
+            demand -= 1
+            deliveredCurrentValue = true
+            lock.unlock()
+            downstreamLock.lock()
+            let newDemand = downstream.receive(output)
+            downstreamLock.unlock()
+            guard newDemand > 0 else { return }
+            lock.lock()
+            demand += newDemand
+            lock.unlock()
         }
 
-        func request(_ demand: Subscribers.Demand) {
-            precondition(demand > 0)
-            _parent?._lock.do {
-                if !_delivered, let value = _parent?.value {
-                    _offer(value)
-                    _demand += demand
-                    _demand -= 1
-                } else {
-                    _demand = demand
-                }
-                _parent?.hasAnyDownstreamDemand = true
+        override func finish(completion: Subscribers.Completion<Failure>) {
+            lock.lock()
+            guard let downstream = self.downstream else {
+                lock.unlock()
+                return
             }
+            self.downstream = nil
+            let parent = self.parent
+            self.parent = nil
+            lock.unlock()
+            parent?.disassociate(self)
+            downstreamLock.lock()
+            downstream.receive(completion: completion)
+            downstreamLock.unlock()
         }
 
-        func cancel() {
-            _parent = nil
+        override func request(_ demand: Subscribers.Demand) {
+            demand.assertNonZero()
+            lock.lock()
+            guard let downstream = self.downstream else {
+                lock.unlock()
+                return
+            }
+            if deliveredCurrentValue {
+                self.demand += demand
+                lock.unlock()
+                return
+            }
+
+            // Hasn't yet delivered the current value
+
+            self.demand += demand
+            deliveredCurrentValue = true
+            if let currentValue = self.parent?.value {
+                self.demand -= 1
+                lock.unlock()
+                downstreamLock.lock()
+                let newDemand = downstream.receive(currentValue)
+                downstreamLock.unlock()
+                guard newDemand > 0 else { return }
+                lock.lock()
+                self.demand += newDemand
+            }
+            lock.unlock()
         }
+
+        override func cancel() {
+            lock.lock()
+            if self.downstream == nil {
+                lock.unlock()
+                return
+            }
+            self.downstream = nil
+            let parent = self.parent
+            self.parent = nil
+            lock.unlock()
+            parent?.disassociate(self)
+        }
+
+        var description: String { return "CurrentValueSubject" }
+
+        var customMirror: Mirror {
+            lock.lock()
+            defer { lock.unlock() }
+            let children: [Mirror.Child] = [
+                ("parent", parent as Any),
+                ("downstream", downstream as Any),
+                ("demand", demand),
+                ("subject", parent as Any)
+            ]
+            return Mirror(self, children: children)
+        }
+
+        var playgroundDescription: Any { return description }
     }
 }
-
-extension CurrentValueSubject.Conduit: CustomStringConvertible {
-    fileprivate var description: String { return "CurrentValueSubject" }
-}

Sources/OpenCombine/Future.swift

@@ -6,113 +6,197 @@
 //
 
 /// A publisher that eventually produces one value and then finishes or fails.
-public final class Future<Output, Failure>: Publisher where Failure: Error {
+public final class Future<Output, Failure: Error>: Publisher {
 
     public typealias Promise = (Result<Output, Failure>) -> Void
 
-    private let _lock = UnfairRecursiveLock.allocate()
-    private var _subscriptions: [Conduit] = []
+    private let lock = UnfairLock.allocate()
+
+    private var downstreams = ConduitList<Output, Failure>.empty
 
     private var result: Result<Output, Failure>?
 
     public init(
         _ attemptToFulfill: @escaping (@escaping Promise) -> Void
     ) {
-        attemptToFulfill { result in
-            self._lock.do {
-                guard self.result == nil else { return }
-                self.result = result
-                self._publish(result)
-            }
-        }
+        attemptToFulfill(self.promise)
     }
 
     deinit {
-        _lock.deallocate()
+        lock.deallocate()
     }
 
-    /// This function is called to attach the specified `Subscriber` to this
-    /// `Publisher` by `subscribe(_:)`
-    ///
-    /// - SeeAlso: `subscribe(_:)`
-    /// - Parameters:
-    ///     - subscriber: The subscriber to attach to this `Publisher`.
-    ///                   once attached it can begin to receive values.
-    public func receive<Downstream: Subscriber>(
-        subscriber: Downstream
-    ) where Output == Downstream.Input, Failure == Downstream.Failure {
-        let subscription = Conduit(parent: self,
-                                   downstream: AnySubscriber(subscriber))
-
-        _subscriptions.append(subscription)
-       subscriber.receive(subscription: subscription)
-    }
-
-    private func _acknowledgeDownstreamDemand() {
-        _lock.do {
-            guard let result = result else { return }
-            _publish(result)
+    private func promise(_ result: Result<Output, Failure>) {
+        lock.lock()
+        guard self.result == nil else {
+            lock.unlock()
+            return
+        }
+        self.result = result
+        let downstreams = self.downstreams
+        self.downstreams.removeAll()
+        lock.unlock()
+        switch result {
+        case .success(let output):
+            downstreams.forEach { $0.offer(output) }
+        case .failure(let error):
+            downstreams.forEach { $0.finish(completion: .failure(error)) }
         }
     }
 
-    private func _publish(_ result: Result<Output, Failure>) {
-        for subscription in self._subscriptions where !subscription._isCompleted {
-            switch result {
-            case let .success(output) where subscription._demand > 0:
-                subscription._demand -= 1
-                subscription._demand += subscription._downstream?.receive(output) ?? .none
-                subscription._receive(completion: .finished)
-            case let .failure(error):
-                subscription._receive(completion: .failure(error))
-
-            // nothing to do if no demand
-            default: ()
-            }
+    public func receive<Downstream: Subscriber>(subscriber: Downstream)
+        where Output == Downstream.Input, Failure == Downstream.Failure
+    {
+        let conduit = Conduit(parent: self, downstream: subscriber)
+        lock.lock()
+        if let result = self.result {
+            downstreams.insert(conduit)
+            lock.unlock()
+            subscriber.receive(subscription: conduit)
+            conduit.fulfill(result)
+        } else {
+            downstreams.insert(conduit)
+            lock.unlock()
+            subscriber.receive(subscription: conduit)
         }
     }
+
+    private func disassociate(_ conduit: ConduitBase<Output, Failure>) {
+        lock.lock()
+        downstreams.remove(conduit)
+        lock.unlock()
+    }
 }
 
 extension Future {
 
-    fileprivate final class Conduit: Subscription {
+    private final class Conduit<Downstream: Subscriber>
+        : ConduitBase<Output, Failure>,
+          CustomStringConvertible,
+          CustomReflectable,
+          CustomPlaygroundDisplayConvertible
+        where Downstream.Input == Output, Downstream.Failure == Failure
+    {
 
-        fileprivate var _parent: Future<Output, Failure>?
+        fileprivate var parent: Future?
 
-        fileprivate var _downstream: AnySubscriber<Output, Failure>?
+        fileprivate var downstream: Downstream?
 
-        fileprivate var _demand: Subscribers.Demand = .none
+        fileprivate var hasAnyDemand = false
 
-        fileprivate var _isCompleted: Bool {
-            return _parent == nil
+        private var lock = UnfairLock.allocate()
+
+        private var downstreamLock = UnfairRecursiveLock.allocate()
+
+        fileprivate init(parent: Future, downstream: Downstream) {
+            self.parent = parent
+            self.downstream = downstream
         }
 
-        fileprivate init(parent: Future<Output, Failure>,
-                         downstream: AnySubscriber<Output, Failure>) {
-            _parent = parent
-            _downstream = downstream
+        deinit {
+            lock.deallocate()
+            downstreamLock.deallocate()
         }
 
-        fileprivate func _receive(completion: Subscribers.Completion<Failure>) {
-            if !_isCompleted {
-                _parent = nil
-                _downstream?.receive(completion: completion)
+        fileprivate func fulfill(_ result: Result<Output, Failure>) {
+            lock.lock()
+            guard let downstream = self.downstream else {
+                lock.unlock()
+                return
+            }
+            let parent = self.parent
+            if case .success = result, !hasAnyDemand {
+                lock.unlock()
+                return
+            }
+            self.downstream = nil
+            self.parent = nil
+            lock.unlock()
+            downstreamLock.lock()
+            switch result {
+            case .success(let output):
+                _ = downstream.receive(output)
+                downstream.receive(completion: .finished)
+            case .failure(let error):
+                downstream.receive(completion: .failure(error))
+            }
+            downstreamLock.unlock()
+            parent?.disassociate(self)
+        }
+
+        override func offer(_ output: Output) {
+            fulfill(.success(output))
+        }
+
+        override func finish(completion: Subscribers.Completion<Failure>) {
+            switch completion {
+            case .finished:
+                assertionFailure("unreachable")
+            case .failure(let error):
+                fulfill(.failure(error))
             }
         }
 
-        fileprivate func request(_ demand: Subscribers.Demand) {
+        override func request(_ demand: Subscribers.Demand) {
             demand.assertNonZero()
-            _parent?._lock.do {
-                _demand += demand
+            lock.lock()
+            guard let downstream = self.downstream, let parent = self.parent else {
+                lock.unlock()
+                return
             }
-            _parent?._acknowledgeDownstreamDemand()
+            hasAnyDemand = true
+
+            parent.lock.lock()
+            guard let result = parent.result else {
+                parent.lock.unlock()
+                lock.unlock()
+                return
+            }
+            parent.lock.unlock()
+            self.downstream = nil
+            self.parent = nil
+            lock.unlock()
+            downstreamLock.lock()
+            switch result {
+            case .success(let output):
+                _ = downstream.receive(output)
+                downstream.receive(completion: .finished)
+            case .failure(let error):
+                // This branch is not reachable under normal circumstances,
+                // but may be reachable in case of a race condition.
+                downstream.receive(completion: .failure(error))
+            }
+            downstreamLock.unlock()
+            parent.disassociate(self)
         }
 
-        fileprivate func cancel() {
-            _parent = nil
+        override func cancel() {
+            lock.lock()
+            if self.downstream == nil {
+                lock.unlock()
+                return
+            }
+            self.downstream = nil
+            let parent = self.parent
+            self.parent = nil
+            lock.unlock()
+            parent?.disassociate(self)
         }
+
+        var description: String { return "Future" }
+
+        var customMirror: Mirror {
+            lock.lock()
+            defer { lock.unlock() }
+            let children: [Mirror.Child] = [
+                ("parent", parent as Any),
+                ("downstream", downstream as Any),
+                ("hasAnyDemand", hasAnyDemand),
+                ("subject", parent as Any)
+            ]
+            return Mirror(self, children: children)
+        }
+
+        var playgroundDescription: Any { return description }
     }
 }
-
-extension Future.Conduit: CustomStringConvertible {
-    fileprivate var description: String { return "Future" }
-}

Sources/OpenCombine/Helpers/ConduitBase.swift

@@ -0,0 +1,40 @@
+//
+//  ConduitBase.swift
+//  
+//
+//  Created by Sergej Jaskiewicz on 25.06.2020.
+//
+
+internal class ConduitBase<Output, Failure: Error>: Subscription {
+
+    internal init() {}
+
+    internal func offer(_ output: Output) {
+        abstractMethod()
+    }
+
+    internal func finish(completion: Subscribers.Completion<Failure>) {
+        abstractMethod()
+    }
+
+    internal func request(_ demand: Subscribers.Demand) {
+        abstractMethod()
+    }
+
+    internal func cancel() {
+        abstractMethod()
+    }
+}
+
+extension ConduitBase: Equatable {
+    internal static func == (lhs: ConduitBase<Output, Failure>,
+                             rhs: ConduitBase<Output, Failure>) -> Bool {
+        return ObjectIdentifier(lhs) == ObjectIdentifier(rhs)
+    }
+}
+
+extension ConduitBase: Hashable {
+    internal func hash(into hasher: inout Hasher) {
+        hasher.combine(ObjectIdentifier(self))
+    }
+}

Sources/OpenCombine/Helpers/ConduitList.swift

@@ -0,0 +1,57 @@
+//
+//  ConduitList.swift
+//  
+//
+//  Created by Sergej Jaskiewicz on 25.06.2020.
+//
+
+internal enum ConduitList<Output, Failure: Error> {
+    case empty
+    case single(ConduitBase<Output, Failure>)
+    case many(Set<ConduitBase<Output, Failure>>)
+}
+
+extension ConduitList {
+    internal mutating func insert(_ conduit: ConduitBase<Output, Failure>) {
+        switch self {
+        case .empty:
+            self = .single(conduit)
+        case .single(conduit):
+            break // This element already exists.
+        case .single(let existingConduit):
+            self = .many([existingConduit, conduit])
+        case .many(var set):
+            set.insert(conduit)
+            self = .many(set)
+        }
+    }
+
+    internal func forEach(
+        _ body: (ConduitBase<Output, Failure>) throws -> Void
+    ) rethrows {
+        switch self {
+        case .empty:
+            break
+        case .single(let conduit):
+            try body(conduit)
+        case .many(let set):
+            try set.forEach(body)
+        }
+    }
+
+    internal mutating func remove(_ conduit: ConduitBase<Output, Failure>) {
+        switch self {
+        case .single(conduit):
+            self = .empty
+        case .empty, .single:
+            break
+        case .many(var set):
+            set.remove(conduit)
+            self = .many(set)
+        }
+    }
+
+    internal mutating func removeAll() {
+        self = .empty
+    }
+}

Sources/OpenCombine/Helpers/Locking.swift

@@ -11,13 +11,3 @@ import COpenCombineHelpers
 
 internal typealias UnfairLock = __UnfairLock
 internal typealias UnfairRecursiveLock = __UnfairRecursiveLock
-
-extension UnfairRecursiveLock {
-
-    @inlinable
-    internal func `do`<Result>(_ body: () throws -> Result) rethrows -> Result {
-        lock()
-        defer { unlock() }
-        return try body()
-    }
-}

Sources/OpenCombine/Helpers/SubjectSubscriber.swift

@@ -40,23 +40,21 @@ internal final class SubjectSubscriber<Downstream: Subject>
 
     internal func receive(_ input: Downstream.Output) -> Subscribers.Demand {
         lock.lock()
-        guard let downstreamSubject = downstreamSubject else {
+        guard let subject = downstreamSubject, upstreamSubscription != nil else {
             lock.unlock()
             return .none
         }
-        guard upstreamSubscription != nil else { APIViolationValueBeforeSubscription() }
         lock.unlock()
-        downstreamSubject.send(input)
+        subject.send(input)
         return .none
     }
 
     internal func receive(completion: Subscribers.Completion<Downstream.Failure>) {
         lock.lock()
-        guard let subject = downstreamSubject else {
+        guard let subject = downstreamSubject, upstreamSubscription != nil else {
             lock.unlock()
             return
         }
-        guard upstreamSubscription != nil else { APIViolationUnexpectedCompletion() }
         lock.unlock()
         subject.send(completion: completion)
         downstreamSubject = nil
@@ -87,11 +85,7 @@ internal final class SubjectSubscriber<Downstream: Subject>
 
     internal func cancel() {
         lock.lock()
-        if isCancelled {
-            lock.unlock()
-            return
-        }
-        guard let subscription = upstreamSubscription else {
+        guard !isCancelled, let subscription = upstreamSubscription else {
             lock.unlock()
             return
         }

Sources/OpenCombine/Helpers/SubscriptionStatus.swift

@@ -10,3 +10,14 @@ internal enum SubscriptionStatus {
     case subscribed(Subscription)
     case terminal
 }
+
+extension SubscriptionStatus {
+    internal var isAwaigingSubscription: Bool {
+        switch self {
+        case .awaitingSubscription:
+            return true
+        default:
+            return false
+        }
+    }
+}

Sources/OpenCombine/PassthroughSubject.swift

@@ -9,14 +9,15 @@
 ///
 /// Use a `PassthroughSubject` in unit tests when you want a publisher than can publish
 /// specific values on-demand during tests.
-public final class PassthroughSubject<Output, Failure: Error>: Subject  {
+public final class PassthroughSubject<Output, Failure: Error>: Subject {
 
-    private let _lock = UnfairRecursiveLock.allocate()
+    private let lock = UnfairLock.allocate()
 
-    private var _completion: Subscribers.Completion<Failure>?
+    private var active = true
 
-    // TODO: Combine uses bag data structure
-    private var _subscriptions: [Conduit] = []
+    private var completion: Subscribers.Completion<Failure>?
+
+    private var downstreams = ConduitList<Output, Failure>.empty
 
     internal var upstreamSubscriptions: [Subscription] = []
 
@@ -25,112 +26,197 @@ public final class PassthroughSubject<Output, Failure: Error>: Subject  {
     public init() {}
 
     deinit {
-        for subscription in _subscriptions {
-            subscription._downstream = nil
+        for subscription in upstreamSubscriptions {
+            subscription.cancel()
         }
-        _lock.deallocate()
+        lock.deallocate()
     }
 
     public func send(subscription: Subscription) {
-        _lock.do {
-            upstreamSubscriptions.append(subscription)
-            if hasAnyDownstreamDemand {
-                subscription.request(.unlimited)
-            }
+        lock.lock()
+        upstreamSubscriptions.append(subscription)
+        let hasAnyDownstreamDemand = self.hasAnyDownstreamDemand
+        lock.unlock()
+        if hasAnyDownstreamDemand {
+            subscription.request(.unlimited)
         }
     }
 
     public func receive<Downstream: Subscriber>(subscriber: Downstream)
         where Output == Downstream.Input, Failure == Downstream.Failure
     {
-        _lock.do {
-            if let completion = _completion {
-                subscriber.receive(subscription: Subscriptions.empty)
-                subscriber.receive(completion: completion)
-                return
-            } else {
-                let subscription = Conduit(parent: self,
-                                           downstream: AnySubscriber(subscriber))
-
-                _subscriptions.append(subscription)
-                subscriber.receive(subscription: subscription)
-            }
+        lock.lock()
+        if active {
+            let conduit = Conduit(parent: self, downstream: subscriber)
+            downstreams.insert(conduit)
+            lock.unlock()
+            subscriber.receive(subscription: conduit)
+        } else {
+            let completion = self.completion!
+            lock.unlock()
+            subscriber.receive(subscription: Subscriptions.empty)
+            subscriber.receive(completion: completion)
         }
     }
 
     public func send(_ input: Output) {
-        _lock.do {
-            for subscription in _subscriptions
-                where !subscription._isCompleted && subscription._demand > 0
-            {
-                let newDemand = subscription._downstream?.receive(input) ?? .none
-                subscription._demand += newDemand
-                subscription._demand -= 1
-            }
+        lock.lock()
+        guard active else {
+            lock.unlock()
+            return
+        }
+        let downstreams = self.downstreams
+        lock.unlock()
+        downstreams.forEach { conduit in
+            conduit.offer(input)
         }
     }
 
     public func send(completion: Subscribers.Completion<Failure>) {
-        _lock.do {
-            _completion = completion
-            for subscriber in _subscriptions {
-                subscriber._receive(completion: completion)
-            }
+        lock.lock()
+        guard active else {
+            lock.unlock()
+            return
+        }
+        active = false
+        self.completion = completion
+        let downstreams = self.downstreams
+        self.downstreams.removeAll()
+        lock.unlock()
+        downstreams.forEach { conduit in
+            conduit.finish(completion: completion)
         }
     }
 
-    private func _acknowledgeDownstreamDemand() {
-        _lock.do {
-            guard !hasAnyDownstreamDemand else { return }
-            hasAnyDownstreamDemand = true
-            for subscription in upstreamSubscriptions {
-                subscription.request(.unlimited)
-            }
+    private func acknowledgeDownstreamDemand() {
+        lock.lock()
+        if hasAnyDownstreamDemand {
+            lock.unlock()
+            return
         }
+        hasAnyDownstreamDemand = true
+        let upstreamSubscriptions = self.upstreamSubscriptions
+        lock.unlock()
+        for subscription in upstreamSubscriptions {
+            subscription.request(.unlimited)
+        }
+    }
+
+    private func disassociate(_ conduit: ConduitBase<Output, Failure>) {
+        lock.lock()
+        guard active else {
+            lock.unlock()
+            return
+        }
+        downstreams.remove(conduit)
+        lock.unlock()
     }
 }
 
 extension PassthroughSubject {
 
-    fileprivate final class Conduit: Subscription {
+    private final class Conduit<Downstream: Subscriber>
+        : ConduitBase<Output, Failure>,
+          CustomStringConvertible,
+          CustomReflectable,
+          CustomPlaygroundDisplayConvertible
+        where Downstream.Input == Output, Downstream.Failure == Failure
+    {
 
-        fileprivate var _parent: PassthroughSubject?
+        fileprivate var parent: PassthroughSubject?
 
-        fileprivate var _downstream: AnySubscriber<Output, Failure>?
+        fileprivate var downstream: Downstream?
 
-        fileprivate var _demand: Subscribers.Demand = .none
+        fileprivate var demand = Subscribers.Demand.none
 
-        fileprivate var _isCompleted: Bool {
-            return _parent == nil
-        }
+        private var lock = UnfairLock.allocate()
+
+        private var downstreamLock = UnfairRecursiveLock.allocate()
 
         fileprivate init(parent: PassthroughSubject,
-                         downstream: AnySubscriber<Output, Failure>) {
-            _parent = parent
-            _downstream = downstream
+                         downstream: Downstream) {
+            self.parent = parent
+            self.downstream = downstream
         }
 
-        fileprivate func _receive(completion: Subscribers.Completion<Failure>) {
-            if !_isCompleted {
-                _parent = nil
-                _downstream?.receive(completion: completion)
+        deinit {
+            lock.deallocate()
+            downstreamLock.deallocate()
+        }
+
+        override func offer(_ output: Output) {
+            lock.lock()
+            guard demand > 0, let downstream = self.downstream else {
+                lock.unlock()
+                return
             }
+            demand -= 1
+            lock.unlock()
+            downstreamLock.lock()
+            let newDemand = downstream.receive(output)
+            downstreamLock.unlock()
+            guard newDemand > 0 else { return }
+            lock.lock()
+            demand += newDemand
+            lock.unlock()
         }
 
-        fileprivate func request(_ demand: Subscribers.Demand) {
+        override func finish(completion: Subscribers.Completion<Failure>) {
+            lock.lock()
+            guard let downstream = self.downstream else {
+                lock.unlock()
+                return
+            }
+            self.downstream = nil
+            let parent = self.parent
+            self.parent = nil
+            lock.unlock()
+            parent?.disassociate(self)
+            downstreamLock.lock()
+            downstream.receive(completion: completion)
+            downstreamLock.unlock()
+        }
+
+        override func request(_ demand: Subscribers.Demand) {
             demand.assertNonZero()
-            _parent?._lock.do {
-                _demand += demand
+            lock.lock()
+            if self.downstream == nil {
+                lock.unlock()
+                return
             }
-            _parent?._acknowledgeDownstreamDemand()
+            self.demand += demand
+            let parent = self.parent
+            lock.unlock()
+            parent?.acknowledgeDownstreamDemand()
         }
 
-        fileprivate func cancel() {
-            _parent = nil
+        override func cancel() {
+            lock.lock()
+            if self.downstream == nil {
+                lock.unlock()
+                return
+            }
+            self.downstream = nil
+            let parent = self.parent
+            self.parent = nil
+            lock.unlock()
+            parent?.disassociate(self)
         }
+
+        var description: String { return "PassthroughSubject" }
+
+        var customMirror: Mirror {
+            lock.lock()
+            defer { lock.unlock() }
+            let children: [Mirror.Child] = [
+                ("parent", parent as Any),
+                ("downstream", downstream as Any),
+                ("demand", demand),
+                ("subject", parent as Any)
+            ]
+            return Mirror(self, children: children)
+        }
+
+        var playgroundDescription: Any { return description }
     }
 }
-
-extension PassthroughSubject.Conduit: CustomStringConvertible {
-    fileprivate var description: String { return "PassthroughSubject" }
-}

Sources/OpenCombine/Published.swift

@@ -18,8 +18,7 @@
 @propertyWrapper
 public struct Published<Value> {
 
-    /// Initialize the storage of the `Published` property as well as the corresponding
-    /// `Publisher`.
+    @inlinable // trivially forwarding
     public init(initialValue: Value) {
         self.init(wrappedValue: initialValue)
     }

Sources/OpenCombine/Publishers/GENERATED-Publishers.Catch.swift

@@ -0,0 +1,620 @@
+// ┏━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┓
+// ┃                                                                                     ┃
+// ┃                   Auto-generated from GYB template. DO NOT EDIT!                    ┃
+// ┃                                                                                     ┃
+// ┃                                                                                     ┃
+// ┗━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┛
+//
+//  Publishers.Catch.swift
+//  
+//
+//  Created by Sergej Jaskiewicz on 25.12.2019.
+//
+
+extension Publisher {
+
+    /// Handles errors from an upstream publisher by replacing it with another publisher.
+    ///
+    /// The following example replaces any error from the upstream publisher and replaces
+    /// the upstream with a `Just` publisher. This continues the stream by publishing
+    /// a single value and completing normally.
+    /// ```
+    /// enum SimpleError: Error { case error }
+    /// let errorPublisher = (0..<10).publisher.tryMap { v -> Int in
+    ///     if v < 5 {
+    ///         return v
+    ///     } else {
+    ///         throw SimpleError.error
+    ///     }
+    /// }
+    ///
+    /// let noErrorPublisher = errorPublisher.catch { _ in
+    ///     return Just(100)
+    /// }
+    /// ```
+    /// Backpressure note: This publisher passes through `request` and `cancel` to
+    /// the upstream. After receiving an error, the publisher sends sends any unfulfilled
+    /// demand to the new `Publisher`.
+    ///
+    /// - Parameter handler: A closure that accepts the upstream failure as input and
+    ///   returns a publisher to replace the upstream publisher.
+    /// - Returns: A publisher that handles errors from an upstream publisher by replacing
+    ///   the failed publisher with another publisher.
+    public func `catch`<NewPublisher: Publisher>(
+        _ handler: @escaping (Failure) -> NewPublisher
+    ) -> Publishers.Catch<Self, NewPublisher>
+        where NewPublisher.Output == Output
+    {
+        return .init(upstream: self, handler: handler)
+    }
+
+    /// Handles errors from an upstream publisher by either replacing it with another
+    /// publisher or `throw`ing  a new error.
+    ///
+    /// - Parameter handler: A `throw`ing closure that accepts the upstream failure as
+    ///   input and returns a publisher to replace the upstream publisher or if an error
+    ///   is thrown will send the error downstream.
+    /// - Returns: A publisher that handles errors from an upstream publisher by replacing
+    ///   the failed publisher with another publisher.
+    public func tryCatch<NewPublisher: Publisher>(
+        _ handler: @escaping (Failure) throws -> NewPublisher
+    ) -> Publishers.TryCatch<Self, NewPublisher>
+        where NewPublisher.Output == Output
+    {
+        return .init(upstream: self, handler: handler)
+    }
+}
+
+extension Publishers {
+
+    /// A publisher that handles errors from an upstream publisher by replacing the failed
+    /// publisher with another publisher.
+    public struct Catch<Upstream: Publisher, NewPublisher: Publisher>: Publisher
+        where Upstream.Output == NewPublisher.Output
+    {
+        public typealias Output = Upstream.Output
+
+        public typealias Failure = NewPublisher.Failure
+
+        /// The publisher that this publisher receives elements from.
+        public let upstream: Upstream
+
+        /// A closure that accepts the upstream failure as input and returns a publisher
+        /// to replace the upstream publisher.
+        public let handler: (Upstream.Failure) -> NewPublisher
+
+        /// Creates a publisher that handles errors from an upstream publisher by
+        /// replacing the failed publisher with another publisher.
+        ///
+        /// - Parameters:
+        ///   - upstream: The publisher that this publisher receives elements from.
+        ///   - handler: A closure that accepts the upstream failure as input and returns
+        ///     a publisher to replace the upstream publisher.
+        public init(upstream: Upstream,
+                    handler: @escaping (Upstream.Failure) -> NewPublisher) {
+            self.upstream = upstream
+            self.handler = handler
+        }
+
+        public func receive<Downstream: Subscriber>(subscriber: Downstream)
+            where Downstream.Input == Output, Downstream.Failure == Failure
+        {
+            let inner = Inner(downstream: subscriber, handler: handler)
+            let uncaughtS = Inner.UncaughtS(inner: inner)
+            upstream.subscribe(uncaughtS)
+        }
+    }
+
+    /// A publisher that handles errors from an upstream publisher by replacing the failed
+    /// publisher with another publisher or optionally producing a new error.
+    public struct TryCatch<Upstream: Publisher, NewPublisher: Publisher>: Publisher
+        where Upstream.Output == NewPublisher.Output
+    {
+        public typealias Output = Upstream.Output
+
+        public typealias Failure = Error
+
+        public let upstream: Upstream
+
+        public let handler: (Upstream.Failure) throws -> NewPublisher
+
+        public init(upstream: Upstream,
+                    handler: @escaping (Upstream.Failure) throws -> NewPublisher) {
+            self.upstream = upstream
+            self.handler = handler
+        }
+
+        public func receive<Downstream: Subscriber>(subscriber: Downstream)
+            where Downstream.Input == Output, Downstream.Failure == Failure
+        {
+            let inner = Inner(downstream: subscriber, handler: handler)
+            let uncaughtS = Inner.UncaughtS(inner: inner)
+            upstream.subscribe(uncaughtS)
+        }
+    }
+}
+
+extension Publishers.Catch {
+    private final class Inner<Downstream: Subscriber>
+        : Subscription,
+          CustomStringConvertible,
+          CustomReflectable,
+          CustomPlaygroundDisplayConvertible
+        where Downstream.Input == Upstream.Output,
+              Downstream.Failure == NewPublisher.Failure
+    {
+        struct UncaughtS: Subscriber,
+                          CustomStringConvertible,
+                          CustomReflectable,
+                          CustomPlaygroundDisplayConvertible
+        {
+            typealias Input = Upstream.Output
+
+            typealias Failure = Upstream.Failure
+
+            let inner: Inner
+
+            var combineIdentifier: CombineIdentifier { return inner.combineIdentifier }
+
+            func receive(subscription: Subscription) {
+                inner.receivePre(subscription: subscription)
+            }
+
+            func receive(_ input: Input) -> Subscribers.Demand {
+                return inner.receivePre(input)
+            }
+
+            func receive(completion: Subscribers.Completion<Failure>) {
+                return inner.receivePre(completion: completion)
+            }
+
+            var description: String { return inner.description }
+
+            var customMirror: Mirror { return inner.customMirror }
+
+            var playgroundDescription: Any { return description }
+        }
+
+        struct CaughtS: Subscriber,
+                        CustomStringConvertible,
+                        CustomReflectable,
+                        CustomPlaygroundDisplayConvertible
+        {
+            typealias Input = NewPublisher.Output
+
+            typealias Failure = NewPublisher.Failure
+
+            let inner: Inner
+
+            var combineIdentifier: CombineIdentifier { return inner.combineIdentifier }
+
+            func receive(subscription: Subscription) {
+                inner.receivePost(subscription: subscription)
+            }
+
+            func receive(_ input: Input) -> Subscribers.Demand {
+                return inner.receivePost(input)
+            }
+
+            func receive(completion: Subscribers.Completion<Failure>) {
+                inner.receivePost(completion: completion)
+            }
+
+            var description: String { return inner.description }
+
+            var customMirror: Mirror { return inner.customMirror }
+
+            var playgroundDescription: Any { return description }
+        }
+
+        private enum State {
+            case pendingPre
+            case pre(Subscription)
+            case pendingPost
+            case post(Subscription)
+            case cancelled
+        }
+
+        private let lock = UnfairLock.allocate()
+
+        private var demand = Subscribers.Demand.none
+
+        private var state = State.pendingPre
+
+        private let downstream: Downstream
+
+        private let handler: (Upstream.Failure) -> NewPublisher
+
+        init(downstream: Downstream,
+             handler: @escaping (Upstream.Failure) -> NewPublisher) {
+            self.downstream = downstream
+            self.handler = handler
+        }
+
+        deinit {
+            lock.deallocate()
+        }
+
+        func receivePre(subscription: Subscription) {
+            lock.lock()
+            guard case .pendingPre = state else {
+                lock.unlock()
+                subscription.cancel()
+                return
+            }
+            state = .pre(subscription)
+            lock.unlock()
+            downstream.receive(subscription: self)
+        }
+
+        func receivePre(_ input: Upstream.Output) -> Subscribers.Demand {
+            lock.lock()
+            demand -= 1
+            lock.unlock()
+            let newDemand = downstream.receive(input)
+            lock.lock()
+            demand += newDemand
+            lock.unlock()
+            return newDemand
+        }
+
+        func receivePre(completion: Subscribers.Completion<Upstream.Failure>) {
+            switch completion {
+            case .finished:
+                lock.lock()
+                switch state {
+                case .pre:
+                    state = .cancelled
+                    lock.unlock()
+                    downstream.receive(completion: .finished)
+                case .pendingPre, .pendingPost, .post, .cancelled:
+                    lock.unlock()
+                }
+            case .failure(let error):
+                lock.lock()
+                switch state {
+                case .pre:
+                    state = .pendingPost
+                    lock.unlock()
+                    handler(error).subscribe(CaughtS(inner: self))
+                case .cancelled:
+                    lock.unlock()
+                case .pendingPre, .post, .pendingPost:
+                    completionBeforeSubscription()
+                }
+            }
+        }
+
+        func receivePost(subscription: Subscription) {
+            lock.lock()
+            guard case .pendingPost = state else {
+                lock.unlock()
+                subscription.cancel()
+                return
+            }
+            state = .post(subscription)
+            let demand = self.demand
+            lock.unlock()
+            if demand > 0 {
+                subscription.request(demand)
+            }
+        }
+
+        func receivePost(_ input: NewPublisher.Output) -> Subscribers.Demand {
+            return downstream.receive(input)
+        }
+
+        func receivePost(completion: Subscribers.Completion<NewPublisher.Failure>) {
+            lock.lock()
+            guard case .post = state else {
+                lock.unlock()
+                return
+            }
+            state = .cancelled
+            lock.unlock()
+            downstream.receive(completion: completion)
+        }
+
+        func request(_ demand: Subscribers.Demand) {
+            demand.assertNonZero()
+            lock.lock()
+            switch state {
+            case .pendingPre:
+                // The client is only able to call the `request` method after we've sent
+                // `self` downstream. We only do it in the `receivePre(subscription:)`
+                // method, after setting `state` to `pre`.
+                // After that `state` never becomes `pendingPre`.
+                requestBeforeSubscription()
+            case let .pre(subscription):
+                self.demand += demand
+                lock.unlock()
+                subscription.request(demand)
+            case .pendingPost:
+                self.demand += demand
+                lock.unlock()
+            case let .post(subscription):
+                lock.unlock()
+                subscription.request(demand)
+            case .cancelled:
+                lock.unlock()
+            }
+        }
+
+        func cancel() {
+            lock.lock()
+            switch state {
+            case let .pre(subscription), let .post(subscription):
+                state = .cancelled
+                lock.unlock()
+                subscription.cancel()
+            case .pendingPre, .pendingPost, .cancelled:
+                lock.unlock()
+            }
+        }
+
+        var description: String { return "Catch" }
+
+        var customMirror: Mirror {
+            let children: [Mirror.Child] = [
+                ("downstream", downstream),
+                ("demand", demand)
+            ]
+            return Mirror(self, children: children)
+        }
+
+        var playgroundDescription: Any { return description }
+    }
+}
+
+extension Publishers.TryCatch {
+    private final class Inner<Downstream: Subscriber>
+        : Subscription,
+          CustomStringConvertible,
+          CustomReflectable,
+          CustomPlaygroundDisplayConvertible
+        where Downstream.Input == Upstream.Output,
+              Downstream.Failure == Error
+    {
+        struct UncaughtS: Subscriber,
+                          CustomStringConvertible,
+                          CustomReflectable,
+                          CustomPlaygroundDisplayConvertible
+        {
+            typealias Input = Upstream.Output
+
+            typealias Failure = Upstream.Failure
+
+            let inner: Inner
+
+            var combineIdentifier: CombineIdentifier { return inner.combineIdentifier }
+
+            func receive(subscription: Subscription) {
+                inner.receivePre(subscription: subscription)
+            }
+
+            func receive(_ input: Input) -> Subscribers.Demand {
+                return inner.receivePre(input)
+            }
+
+            func receive(completion: Subscribers.Completion<Failure>) {
+                return inner.receivePre(completion: completion)
+            }
+
+            var description: String { return inner.description }
+
+            var customMirror: Mirror { return inner.customMirror }
+
+            var playgroundDescription: Any { return description }
+        }
+
+        struct CaughtS: Subscriber,
+                        CustomStringConvertible,
+                        CustomReflectable,
+                        CustomPlaygroundDisplayConvertible
+        {
+            typealias Input = NewPublisher.Output
+
+            typealias Failure = NewPublisher.Failure
+
+            let inner: Inner
+
+            var combineIdentifier: CombineIdentifier { return inner.combineIdentifier }
+
+            func receive(subscription: Subscription) {
+                inner.receivePost(subscription: subscription)
+            }
+
+            func receive(_ input: Input) -> Subscribers.Demand {
+                return inner.receivePost(input)
+            }
+
+            func receive(completion: Subscribers.Completion<Failure>) {
+                inner.receivePost(completion: completion)
+            }
+
+            var description: String { return inner.description }
+
+            var customMirror: Mirror { return inner.customMirror }
+
+            var playgroundDescription: Any { return description }
+        }
+
+        private enum State {
+            case pendingPre
+            case pre(Subscription)
+            case pendingPost
+            case post(Subscription)
+            case cancelled
+        }
+
+        private let lock = UnfairLock.allocate()
+
+        private var demand = Subscribers.Demand.none
+
+        private var state = State.pendingPre
+
+        private let downstream: Downstream
+
+        private let handler: (Upstream.Failure) throws -> NewPublisher
+
+        init(downstream: Downstream,
+             handler: @escaping (Upstream.Failure) throws -> NewPublisher) {
+            self.downstream = downstream
+            self.handler = handler
+        }
+
+        deinit {
+            lock.deallocate()
+        }
+
+        func receivePre(subscription: Subscription) {
+            lock.lock()
+            guard case .pendingPre = state else {
+                lock.unlock()
+                subscription.cancel()
+                return
+            }
+            state = .pre(subscription)
+            lock.unlock()
+            downstream.receive(subscription: self)
+        }
+
+        func receivePre(_ input: Upstream.Output) -> Subscribers.Demand {
+            lock.lock()
+            demand -= 1
+            lock.unlock()
+            let newDemand = downstream.receive(input)
+            lock.lock()
+            demand += newDemand
+            lock.unlock()
+            return newDemand
+        }
+
+        func receivePre(completion: Subscribers.Completion<Upstream.Failure>) {
+            switch completion {
+            case .finished:
+                lock.lock()
+                switch state {
+                case .pre:
+                    state = .cancelled
+                    lock.unlock()
+                    downstream.receive(completion: .finished)
+                case .pendingPre, .pendingPost, .post, .cancelled:
+                    lock.unlock()
+                }
+            case .failure(let error):
+                lock.lock()
+                switch state {
+                case .pre:
+                    state = .pendingPost
+                    lock.unlock()
+                    do {
+                        try handler(error).subscribe(CaughtS(inner: self))
+                    } catch let anotherError {
+                        lock.lock()
+                        state = .cancelled
+                        lock.unlock()
+                        downstream.receive(completion: .failure(anotherError))
+                    }
+                case .cancelled:
+                    lock.unlock()
+                case .pendingPre, .post, .pendingPost:
+                    completionBeforeSubscription()
+                }
+            }
+        }
+
+        func receivePost(subscription: Subscription) {
+            lock.lock()
+            guard case .pendingPost = state else {
+                lock.unlock()
+                subscription.cancel()
+                return
+            }
+            state = .post(subscription)
+            let demand = self.demand
+            lock.unlock()
+            if demand > 0 {
+                subscription.request(demand)
+            }
+        }
+
+        func receivePost(_ input: NewPublisher.Output) -> Subscribers.Demand {
+            return downstream.receive(input)
+        }
+
+        func receivePost(completion: Subscribers.Completion<NewPublisher.Failure>) {
+            lock.lock()
+            guard case .post = state else {
+                lock.unlock()
+                return
+            }
+            state = .cancelled
+            lock.unlock()
+            downstream.receive(completion: completion.eraseError())
+        }
+
+        func request(_ demand: Subscribers.Demand) {
+            demand.assertNonZero()
+            lock.lock()
+            switch state {
+            case .pendingPre:
+                // The client is only able to call the `request` method after we've sent
+                // `self` downstream. We only do it in the `receivePre(subscription:)`
+                // method, after setting `state` to `pre`.
+                // After that `state` never becomes `pendingPre`.
+                requestBeforeSubscription()
+            case let .pre(subscription):
+                self.demand += demand
+                lock.unlock()
+                subscription.request(demand)
+            case .pendingPost:
+                self.demand += demand
+                lock.unlock()
+            case let .post(subscription):
+                lock.unlock()
+                subscription.request(demand)
+            case .cancelled:
+                lock.unlock()
+            }
+        }
+
+        func cancel() {
+            lock.lock()
+            switch state {
+            case let .pre(subscription), let .post(subscription):
+                state = .cancelled
+                lock.unlock()
+                subscription.cancel()
+            case .pendingPre, .pendingPost, .cancelled:
+                lock.unlock()
+            }
+        }
+
+        var description: String { return "TryCatch" }
+
+        var customMirror: Mirror {
+            let children: [Mirror.Child] = [
+                ("downstream", downstream),
+                ("demand", demand)
+            ]
+            return Mirror(self, children: children)
+        }
+
+        var playgroundDescription: Any { return description }
+    }
+}
+
+private func completionBeforeSubscription(file: StaticString = #file,
+                                          line: UInt = #line) -> Never {
+    fatalError("Unexpected state: received completion but do not have subscription",
+               file: file,
+               line: line)
+}
+
+private func requestBeforeSubscription(file: StaticString = #file,
+                                       line: UInt = #line) -> Never {
+    fatalError("Unexpected state: request before subscription sent",
+               file: file,
+               line: line)
+}

Sources/OpenCombine/Publishers/Publishers.Buffer.swift

@@ -0,0 +1,336 @@
+//
+//  Publishers.Buffer.swift
+//  
+//
+//  Created by Sergej Jaskiewicz on 08.01.2020.
+//
+
+extension Publisher {
+
+    /// Buffers elements received from an upstream publisher.
+    /// - Parameter size: The maximum number of elements to store.
+    /// - Parameter prefetch: The strategy for initially populating the buffer.
+    /// - Parameter whenFull: The action to take when the buffer becomes full.
+    public func buffer(
+        size: Int,
+        prefetch: Publishers.PrefetchStrategy,
+        whenFull: Publishers.BufferingStrategy<Failure>
+    ) -> Publishers.Buffer<Self> {
+        return .init(upstream: self,
+                     size: size,
+                     prefetch: prefetch,
+                     whenFull: whenFull)
+    }
+}
+
+extension Publishers {
+
+    /// A strategy for filling a buffer.
+    ///
+    /// * keepFull: A strategy to fill the buffer at subscription time, and keep it full
+    ///   thereafter.
+    /// * byRequest: A strategy that avoids prefetching and instead performs requests
+    ///   on demand.
+    public enum PrefetchStrategy {
+
+        /// A strategy to fill the buffer at subscription time, and keep it full
+        /// thereafter.
+        case keepFull
+
+        /// A strategy that avoids prefetching and instead performs requests
+        /// on demand.
+        case byRequest
+    }
+
+    /// A strategy for handling exhaustion of a buffer’s capacity.
+    ///
+    /// * dropNewest: When full, discard the newly-received element without buffering it.
+    /// * dropOldest: When full, remove the least recently-received element from the
+    ///   buffer.
+    /// * customError: When full, execute the closure to provide a custom error.
+    public enum BufferingStrategy<Failure: Error> {
+
+        /// When full, discard the newly-received element without buffering it.
+        case dropNewest
+
+        /// When full, remove the least recently-received element from the buffer.
+        case dropOldest
+
+        /// When full, execute the closure to provide a custom error.
+        case customError(() -> Failure)
+    }
+
+    /// A publisher that buffers elements received from an upstream publisher.
+    public struct Buffer<Upstream: Publisher>: Publisher {
+
+        public typealias Output = Upstream.Output
+
+        public typealias Failure = Upstream.Failure
+
+        /// The publisher from which this publisher receives elements.
+        public let upstream: Upstream
+
+        /// The maximum number of elements to store.
+        public let size: Int
+
+        /// The strategy for initially populating the buffer.
+        public let prefetch: Publishers.PrefetchStrategy
+
+        /// The action to take when the buffer becomes full.
+        public let whenFull: Publishers.BufferingStrategy<Failure>
+
+        /// Creates a publisher that buffers elements received from an upstream publisher.
+        /// - Parameter upstream: The publisher from which this publisher receives
+        ///   elements.
+        /// - Parameter size: The maximum number of elements to store.
+        /// - Parameter prefetch: The strategy for initially populating the buffer.
+        /// - Parameter whenFull: The action to take when the buffer becomes full.
+        public init(upstream: Upstream,
+                    size: Int,
+                    prefetch: Publishers.PrefetchStrategy,
+                    whenFull: Publishers.BufferingStrategy<Failure>) {
+            self.upstream = upstream
+            self.size = size
+            self.prefetch = prefetch
+            self.whenFull = whenFull
+        }
+
+        public func receive<Downstream: Subscriber>(subscriber: Downstream)
+            where Downstream.Input == Output, Downstream.Failure == Failure
+        {
+            upstream.subscribe(Inner(downstream: subscriber, buffer: self))
+        }
+    }
+}
+
+extension Publishers.PrefetchStrategy: Equatable {}
+
+extension Publishers.PrefetchStrategy: Hashable {}
+
+extension Publishers.Buffer {
+    private final class Inner<Downstream: Subscriber>
+        : Subscriber,
+          Subscription,
+          CustomStringConvertible,
+          CustomReflectable,
+          CustomPlaygroundDisplayConvertible
+        where Downstream.Input == Upstream.Output, Downstream.Failure == Upstream.Failure
+    {
+        typealias Input = Upstream.Output
+
+        typealias Failure = Upstream.Failure
+
+        private enum State {
+            case ready(Publishers.Buffer<Upstream>, Downstream)
+            case subscribed(Publishers.Buffer<Upstream>, Downstream, Subscription)
+            case terminal
+        }
+
+        private let lock = UnfairLock.allocate()
+
+        private var recursion = false
+
+        private var state: State
+
+        private var downstreamDemand = Subscribers.Demand.none
+
+        // TODO: Use a deque here?
+        // Need to measure performance with large buffers and `dropOldest` strategy.
+        private var values = [Input]()
+
+        private var upstreamFailed = false
+
+        private var terminal: Subscribers.Completion<Failure>?
+
+        init(downstream: Downstream, buffer: Publishers.Buffer<Upstream>) {
+            state = .ready(buffer, downstream)
+        }
+
+        deinit {
+            lock.deallocate()
+        }
+
+        func receive(subscription: Subscription) {
+            lock.lock()
+            guard case let .ready(buffer, downstream) = state else {
+                lock.unlock()
+                subscription.cancel()
+                return
+            }
+            state = .subscribed(buffer, downstream, subscription)
+            lock.unlock()
+
+            let upstreamDemand: Subscribers.Demand
+            switch buffer.prefetch {
+            case .keepFull:
+                upstreamDemand = .max(buffer.size)
+            case .byRequest:
+                upstreamDemand = .unlimited
+            }
+            subscription.request(upstreamDemand)
+            downstream.receive(subscription: self)
+        }
+
+        func receive(_ input: Input) -> Subscribers.Demand {
+            lock.lock()
+            guard case let .subscribed(buffer, _, subscription) = state else {
+                lock.unlock()
+                return .none
+            }
+            switch terminal {
+            case nil, .finished?:
+                if values.count >= buffer.size {
+                    switch buffer.whenFull {
+                    case .dropNewest:
+                        lock.unlock()
+                        return drain()
+                    case .dropOldest:
+                        values.removeFirst()
+                    case let .customError(makeError):
+                        terminal = .failure(makeError())
+                        lock.unlock()
+                        subscription.cancel()
+                        return .none
+                    }
+                }
+
+                values.append(input)
+                lock.unlock()
+                return drain()
+            case .failure?:
+                lock.unlock()
+                return .none
+            }
+        }
+
+        func receive(completion: Subscribers.Completion<Upstream.Failure>) {
+            lock.lock()
+            guard case .subscribed = state, terminal == nil else {
+                lock.unlock()
+                return
+            }
+            terminal = completion
+            lock.unlock()
+            _ = drain()
+        }
+
+        func request(_ demand: Subscribers.Demand) {
+            lock.lock()
+            guard case let .subscribed(_, _, subscription) = state else {
+                lock.unlock()
+                return
+            }
+            downstreamDemand += demand
+            let recursion = self.recursion
+            lock.unlock()
+            if recursion {
+                return
+            }
+
+            // Request the number of items just enough to fill the buffer.
+            subscription.request(drain() + demand)
+        }
+
+        func cancel() {
+            lock.lock()
+            guard case let .subscribed(_, _, subscription) = state else {
+                lock.unlock()
+                return
+            }
+            state = .terminal
+            values = []
+            lock.unlock()
+            subscription.cancel()
+        }
+
+        private func drain() -> Subscribers.Demand {
+            var upstreamDemand = Subscribers.Demand.none
+            lock.lock()
+            while true {
+                guard case let .subscribed(buffer, downstream, _) = state else {
+                    lock.unlock()
+                    return upstreamDemand
+                }
+
+                if downstreamDemand > 0 {
+                    if values.isEmpty {
+                        if let completion = terminal {
+                            state = .terminal
+                            lock.unlock()
+                            downstream.receive(completion: completion)
+                        } else {
+                            lock.unlock()
+                        }
+                        return upstreamDemand
+                    }
+                } else {
+                    if let completion = terminal, case .failure = completion {
+                        state = .terminal
+                        lock.unlock()
+                        downstream.receive(completion: completion)
+                    } else {
+                        lock.unlock()
+                    }
+                    return upstreamDemand
+                }
+
+                let poppedValues = lockedPop(downstreamDemand)
+                assert(poppedValues.count > 0,
+                       """
+                       We check that the buffer is not empty and downstreamDemand is \
+                       nonzero, how can this be triggered?
+                       """)
+
+                // This should not crash because `lockedPop(_:)` returns at most
+                // `downstreamDemand` items.
+                downstreamDemand -= poppedValues.count
+
+                recursion = true
+                lock.unlock()
+
+                var newDownstreamDemand = Subscribers.Demand.none
+                var additionalUpstreamDemand = 0
+
+                for value in poppedValues {
+                    newDownstreamDemand += downstream.receive(value)
+                    additionalUpstreamDemand += 1
+                }
+
+                if buffer.prefetch == .keepFull {
+                    upstreamDemand += additionalUpstreamDemand
+                }
+
+                lock.lock()
+                recursion = false
+                downstreamDemand += newDownstreamDemand
+            }
+        }
+
+        private func lockedPop(_ demand: Subscribers.Demand) -> [Input] {
+            assert(demand > 0)
+            guard let max = demand.max else {
+                let poppedValues = self.values
+                self.values = []
+                return poppedValues
+            }
+
+            let poppedValues = Array(values.prefix(max))
+            values.removeFirst(poppedValues.count)
+            return poppedValues
+        }
+
+        var description: String { return "Buffer" }
+
+        var customMirror: Mirror {
+            let children: [Mirror.Child] = [
+                ("values", values),
+                ("state", state),
+                ("downstreamDemand", downstreamDemand),
+                ("terminal", terminal as Any)
+            ]
+            return Mirror(self, children: children)
+        }
+
+        var playgroundDescription: Any { return description }
+    }
+}

Sources/OpenCombine/Publishers/Publishers.Catch.swift.gyb

@@ -0,0 +1,401 @@
+${template_header}
+//
+//  Publishers.Catch.swift
+//  
+//
+//  Created by Sergej Jaskiewicz on 25.12.2019.
+//
+
+%{
+instantiations = ['Catch', 'TryCatch']
+}%
+extension Publisher {
+
+    /// Handles errors from an upstream publisher by replacing it with another publisher.
+    ///
+    /// The following example replaces any error from the upstream publisher and replaces
+    /// the upstream with a `Just` publisher. This continues the stream by publishing
+    /// a single value and completing normally.
+    /// ```
+    /// enum SimpleError: Error { case error }
+    /// let errorPublisher = (0..<10).publisher.tryMap { v -> Int in
+    ///     if v < 5 {
+    ///         return v
+    ///     } else {
+    ///         throw SimpleError.error
+    ///     }
+    /// }
+    ///
+    /// let noErrorPublisher = errorPublisher.catch { _ in
+    ///     return Just(100)
+    /// }
+    /// ```
+    /// Backpressure note: This publisher passes through `request` and `cancel` to
+    /// the upstream. After receiving an error, the publisher sends sends any unfulfilled
+    /// demand to the new `Publisher`.
+    ///
+    /// - Parameter handler: A closure that accepts the upstream failure as input and
+    ///   returns a publisher to replace the upstream publisher.
+    /// - Returns: A publisher that handles errors from an upstream publisher by replacing
+    ///   the failed publisher with another publisher.
+    public func `catch`<NewPublisher: Publisher>(
+        _ handler: @escaping (Failure) -> NewPublisher
+    ) -> Publishers.Catch<Self, NewPublisher>
+        where NewPublisher.Output == Output
+    {
+        return .init(upstream: self, handler: handler)
+    }
+
+    /// Handles errors from an upstream publisher by either replacing it with another
+    /// publisher or `throw`ing  a new error.
+    ///
+    /// - Parameter handler: A `throw`ing closure that accepts the upstream failure as
+    ///   input and returns a publisher to replace the upstream publisher or if an error
+    ///   is thrown will send the error downstream.
+    /// - Returns: A publisher that handles errors from an upstream publisher by replacing
+    ///   the failed publisher with another publisher.
+    public func tryCatch<NewPublisher: Publisher>(
+        _ handler: @escaping (Failure) throws -> NewPublisher
+    ) -> Publishers.TryCatch<Self, NewPublisher>
+        where NewPublisher.Output == Output
+    {
+        return .init(upstream: self, handler: handler)
+    }
+}
+
+extension Publishers {
+
+    /// A publisher that handles errors from an upstream publisher by replacing the failed
+    /// publisher with another publisher.
+    public struct Catch<Upstream: Publisher, NewPublisher: Publisher>: Publisher
+        where Upstream.Output == NewPublisher.Output
+    {
+        public typealias Output = Upstream.Output
+
+        public typealias Failure = NewPublisher.Failure
+
+        /// The publisher that this publisher receives elements from.
+        public let upstream: Upstream
+
+        /// A closure that accepts the upstream failure as input and returns a publisher
+        /// to replace the upstream publisher.
+        public let handler: (Upstream.Failure) -> NewPublisher
+
+        /// Creates a publisher that handles errors from an upstream publisher by
+        /// replacing the failed publisher with another publisher.
+        ///
+        /// - Parameters:
+        ///   - upstream: The publisher that this publisher receives elements from.
+        ///   - handler: A closure that accepts the upstream failure as input and returns
+        ///     a publisher to replace the upstream publisher.
+        public init(upstream: Upstream,
+                    handler: @escaping (Upstream.Failure) -> NewPublisher) {
+            self.upstream = upstream
+            self.handler = handler
+        }
+
+        public func receive<Downstream: Subscriber>(subscriber: Downstream)
+            where Downstream.Input == Output, Downstream.Failure == Failure
+        {
+            let inner = Inner(downstream: subscriber, handler: handler)
+            let uncaughtS = Inner.UncaughtS(inner: inner)
+            upstream.subscribe(uncaughtS)
+        }
+    }
+
+    /// A publisher that handles errors from an upstream publisher by replacing the failed
+    /// publisher with another publisher or optionally producing a new error.
+    public struct TryCatch<Upstream: Publisher, NewPublisher: Publisher>: Publisher
+        where Upstream.Output == NewPublisher.Output
+    {
+        public typealias Output = Upstream.Output
+
+        public typealias Failure = Error
+
+        public let upstream: Upstream
+
+        public let handler: (Upstream.Failure) throws -> NewPublisher
+
+        public init(upstream: Upstream,
+                    handler: @escaping (Upstream.Failure) throws -> NewPublisher) {
+            self.upstream = upstream
+            self.handler = handler
+        }
+
+        public func receive<Downstream: Subscriber>(subscriber: Downstream)
+            where Downstream.Input == Output, Downstream.Failure == Failure
+        {
+            let inner = Inner(downstream: subscriber, handler: handler)
+            let uncaughtS = Inner.UncaughtS(inner: inner)
+            upstream.subscribe(uncaughtS)
+        }
+    }
+}
+% for instantiation in instantiations:
+%     throws_modifier = ' throws' if instantiation == 'TryCatch' else ''
+
+extension Publishers.${instantiation} {
+    private final class Inner<Downstream: Subscriber>
+        : Subscription,
+          CustomStringConvertible,
+          CustomReflectable,
+          CustomPlaygroundDisplayConvertible
+        where Downstream.Input == Upstream.Output,
+%     if instantiation == 'Catch':
+              Downstream.Failure == NewPublisher.Failure
+%     else:
+              Downstream.Failure == Error
+%     end
+    {
+        struct UncaughtS: Subscriber,
+                          CustomStringConvertible,
+                          CustomReflectable,
+                          CustomPlaygroundDisplayConvertible
+        {
+            typealias Input = Upstream.Output
+
+            typealias Failure = Upstream.Failure
+
+            let inner: Inner
+
+            var combineIdentifier: CombineIdentifier { return inner.combineIdentifier }
+
+            func receive(subscription: Subscription) {
+                inner.receivePre(subscription: subscription)
+            }
+
+            func receive(_ input: Input) -> Subscribers.Demand {
+                return inner.receivePre(input)
+            }
+
+            func receive(completion: Subscribers.Completion<Failure>) {
+                return inner.receivePre(completion: completion)
+            }
+
+            var description: String { return inner.description }
+
+            var customMirror: Mirror { return inner.customMirror }
+
+            var playgroundDescription: Any { return description }
+        }
+
+        struct CaughtS: Subscriber,
+                        CustomStringConvertible,
+                        CustomReflectable,
+                        CustomPlaygroundDisplayConvertible
+        {
+            typealias Input = NewPublisher.Output
+
+            typealias Failure = NewPublisher.Failure
+
+            let inner: Inner
+
+            var combineIdentifier: CombineIdentifier { return inner.combineIdentifier }
+
+            func receive(subscription: Subscription) {
+                inner.receivePost(subscription: subscription)
+            }
+
+            func receive(_ input: Input) -> Subscribers.Demand {
+                return inner.receivePost(input)
+            }
+
+            func receive(completion: Subscribers.Completion<Failure>) {
+                inner.receivePost(completion: completion)
+            }
+
+            var description: String { return inner.description }
+
+            var customMirror: Mirror { return inner.customMirror }
+
+            var playgroundDescription: Any { return description }
+        }
+
+        private enum State {
+            case pendingPre
+            case pre(Subscription)
+            case pendingPost
+            case post(Subscription)
+            case cancelled
+        }
+
+        private let lock = UnfairLock.allocate()
+
+        private var demand = Subscribers.Demand.none
+
+        private var state = State.pendingPre
+
+        private let downstream: Downstream
+
+        private let handler: (Upstream.Failure)${throws_modifier} -> NewPublisher
+
+        init(downstream: Downstream,
+             handler: @escaping (Upstream.Failure)${throws_modifier} -> NewPublisher) {
+            self.downstream = downstream
+            self.handler = handler
+        }
+
+        deinit {
+            lock.deallocate()
+        }
+
+        func receivePre(subscription: Subscription) {
+            lock.lock()
+            guard case .pendingPre = state else {
+                lock.unlock()
+                subscription.cancel()
+                return
+            }
+            state = .pre(subscription)
+            lock.unlock()
+            downstream.receive(subscription: self)
+        }
+
+        func receivePre(_ input: Upstream.Output) -> Subscribers.Demand {
+            lock.lock()
+            demand -= 1
+            lock.unlock()
+            let newDemand = downstream.receive(input)
+            lock.lock()
+            demand += newDemand
+            lock.unlock()
+            return newDemand
+        }
+
+        func receivePre(completion: Subscribers.Completion<Upstream.Failure>) {
+            switch completion {
+            case .finished:
+                lock.lock()
+                switch state {
+                case .pre:
+                    state = .cancelled
+                    lock.unlock()
+                    downstream.receive(completion: .finished)
+                case .pendingPre, .pendingPost, .post, .cancelled:
+                    lock.unlock()
+                }
+            case .failure(let error):
+                lock.lock()
+                switch state {
+                case .pre:
+                    state = .pendingPost
+                    lock.unlock()
+%     if instantiation == 'Catch':
+                    handler(error).subscribe(CaughtS(inner: self))
+%     else:
+                    do {
+                        try handler(error).subscribe(CaughtS(inner: self))
+                    } catch let anotherError {
+                        lock.lock()
+                        state = .cancelled
+                        lock.unlock()
+                        downstream.receive(completion: .failure(anotherError))
+                    }
+%     end
+                case .cancelled:
+                    lock.unlock()
+                case .pendingPre, .post, .pendingPost:
+                    completionBeforeSubscription()
+                }
+            }
+        }
+
+        func receivePost(subscription: Subscription) {
+            lock.lock()
+            guard case .pendingPost = state else {
+                lock.unlock()
+                subscription.cancel()
+                return
+            }
+            state = .post(subscription)
+            let demand = self.demand
+            lock.unlock()
+            if demand > 0 {
+                subscription.request(demand)
+            }
+        }
+
+        func receivePost(_ input: NewPublisher.Output) -> Subscribers.Demand {
+            return downstream.receive(input)
+        }
+
+        func receivePost(completion: Subscribers.Completion<NewPublisher.Failure>) {
+            lock.lock()
+            guard case .post = state else {
+                lock.unlock()
+                return
+            }
+            state = .cancelled
+            lock.unlock()
+%     if instantiation == 'Catch':
+            downstream.receive(completion: completion)
+%     else:
+            downstream.receive(completion: completion.eraseError())
+%     end
+        }
+
+        func request(_ demand: Subscribers.Demand) {
+            demand.assertNonZero()
+            lock.lock()
+            switch state {
+            case .pendingPre:
+                // The client is only able to call the `request` method after we've sent
+                // `self` downstream. We only do it in the `receivePre(subscription:)`
+                // method, after setting `state` to `pre`.
+                // After that `state` never becomes `pendingPre`.
+                requestBeforeSubscription()
+            case let .pre(subscription):
+                self.demand += demand
+                lock.unlock()
+                subscription.request(demand)
+            case .pendingPost:
+                self.demand += demand
+                lock.unlock()
+            case let .post(subscription):
+                lock.unlock()
+                subscription.request(demand)
+            case .cancelled:
+                lock.unlock()
+            }
+        }
+
+        func cancel() {
+            lock.lock()
+            switch state {
+            case let .pre(subscription), let .post(subscription):
+                state = .cancelled
+                lock.unlock()
+                subscription.cancel()
+            case .pendingPre, .pendingPost, .cancelled:
+                lock.unlock()
+            }
+        }
+
+        var description: String { return "${instantiation}" }
+
+        var customMirror: Mirror {
+            let children: [Mirror.Child] = [
+                ("downstream", downstream),
+                ("demand", demand)
+            ]
+            return Mirror(self, children: children)
+        }
+
+        var playgroundDescription: Any { return description }
+    }
+}
+% end
+
+private func completionBeforeSubscription(file: StaticString = #file,
+                                          line: UInt = #line) -> Never {
+    fatalError("Unexpected state: received completion but do not have subscription",
+               file: file,
+               line: line)
+}
+
+private func requestBeforeSubscription(file: StaticString = #file,
+                                       line: UInt = #line) -> Never {
+    fatalError("Unexpected state: request before subscription sent",
+               file: file,
+               line: line)
+}

Sources/OpenCombine/Publishers/Publishers.Concatenate.swift

@@ -110,8 +110,8 @@ extension Publishers {
             where Suffix.Failure == Downstream.Failure, Suffix.Output == Downstream.Input
         {
             let inner = Inner(downstream: subscriber, suffix: suffix)
-            prefix.subscribe(inner)
             subscriber.receive(subscription: inner)
+            prefix.subscribe(inner)
         }
     }
 }

Sources/OpenCombine/Publishers/Publishers.Debounce.swift

@@ -0,0 +1,264 @@
+//
+//  Publishers.Debounce.swift
+//  
+//
+//  Created by Sergej Jaskiewicz on 17.12.2019.
+//
+
+extension Publisher {
+
+    /// Publishes elements only after a specified time interval elapses between events.
+    ///
+    /// Use this operator when you want to wait for a pause in the delivery of events from
+    /// the upstream publisher. For example, call `debounce` on the publisher from a text
+    /// field to only receive elements when the user pauses or stops typing. When they
+    /// start typing again, the `debounce` holds event delivery until the next pause.
+    ///
+    /// - Parameters:
+    ///   - dueTime: The time the publisher should wait before publishing an element.
+    ///   - scheduler: The scheduler on which this publisher delivers elements
+    ///   - options: Scheduler options that customize this publisher’s delivery
+    ///     of elements.
+    /// - Returns: A publisher that publishes events only after a specified time elapses.
+    public func debounce<Context: Scheduler>(
+        for dueTime: Context.SchedulerTimeType.Stride,
+        scheduler: Context,
+        options: Context.SchedulerOptions? = nil
+    ) -> Publishers.Debounce<Self, Context> {
+        return .init(upstream: self,
+                     dueTime: dueTime,
+                     scheduler: scheduler,
+                     options: options)
+    }
+}
+
+extension Publishers {
+
+    /// A publisher that publishes elements only after a specified time interval elapses
+    /// between events.
+    public struct Debounce<Upstream: Publisher, Context: Scheduler>: Publisher {
+
+        public typealias Output = Upstream.Output
+
+        public typealias Failure = Upstream.Failure
+
+        /// The publisher from which this publisher receives elements.
+        public let upstream: Upstream
+
+        /// The amount of time the publisher should wait before publishing an element.
+        public let dueTime: Context.SchedulerTimeType.Stride
+
+        /// The scheduler on which this publisher delivers elements.
+        public let scheduler: Context
+
+        /// Scheduler options that customize this publisher’s delivery of elements.
+        public let options: Context.SchedulerOptions?
+
+        public init(upstream: Upstream,
+                    dueTime: Context.SchedulerTimeType.Stride,
+                    scheduler: Context,
+                    options: Context.SchedulerOptions?) {
+            self.upstream = upstream
+            self.dueTime = dueTime
+            self.scheduler = scheduler
+            self.options = options
+        }
+
+        public func receive<Downstream: Subscriber>(subscriber: Downstream)
+            where Downstream.Failure == Failure, Downstream.Input == Output
+        {
+            let inner = Inner(downstream: subscriber,
+                              dueTime: dueTime,
+                              scheduler: scheduler,
+                              options: options)
+            upstream.subscribe(inner)
+        }
+    }
+}
+
+extension Publishers.Debounce {
+    private final class Inner<Downstream: Subscriber>
+        : Subscriber,
+          Subscription,
+          CustomStringConvertible,
+          CustomReflectable,
+          CustomPlaygroundDisplayConvertible
+        where Upstream.Output == Downstream.Input,
+              Upstream.Failure == Downstream.Failure
+    {
+        typealias Input = Upstream.Output
+
+        typealias Failure = Upstream.Failure
+
+        private typealias Generation = UInt64
+
+        private let lock = UnfairLock.allocate()
+
+        private let downstreamLock = UnfairRecursiveLock.allocate()
+
+        private let downstream: Downstream
+
+        private let dueTime: Context.SchedulerTimeType.Stride
+
+        private let scheduler: Context
+
+        private let options: Context.SchedulerOptions?
+
+        private var state = SubscriptionStatus.awaitingSubscription
+
+        private var currentCanceller: Cancellable?
+
+        private var currentValue: Output?
+
+        private var currentGeneration: Generation = 0
+
+        private var downstreamDemand = Subscribers.Demand.none
+
+        init(downstream: Downstream,
+             dueTime: Context.SchedulerTimeType.Stride,
+             scheduler: Context,
+             options: Context.SchedulerOptions?) {
+            self.downstream = downstream
+            self.dueTime = dueTime
+            self.scheduler = scheduler
+            self.options = options
+        }
+
+        deinit {
+            lock.deallocate()
+            downstreamLock.deallocate()
+        }
+
+        func receive(subscription: Subscription) {
+            lock.lock()
+            guard case .awaitingSubscription = state else {
+                lock.unlock()
+                subscription.cancel()
+                return
+            }
+            state = .subscribed(subscription)
+            lock.unlock()
+            downstreamLock.lock()
+            downstream.receive(subscription: self)
+            downstreamLock.unlock()
+            subscription.request(.unlimited)
+        }
+
+        func receive(_ input: Input) -> Subscribers.Demand {
+            lock.lock()
+            precondition(!state.isAwaigingSubscription)
+            guard case .subscribed = state else {
+                lock.unlock()
+                return .none
+            }
+            currentGeneration += 1
+            let generation = currentGeneration
+            currentValue = input
+            let due = scheduler.now.advanced(by: dueTime)
+            lock.unlock()
+            let newCanceller = scheduler.schedule(after: due,
+                                                  interval: dueTime,
+                                                  tolerance: scheduler.minimumTolerance,
+                                                  options: options) { [weak self] in
+                self?.due(generation: generation)
+            }
+            lock.lock()
+            let canceller = currentCanceller
+            currentCanceller = newCanceller
+            lock.unlock()
+            canceller?.cancel()
+            return .none
+        }
+
+        func receive(completion: Subscribers.Completion<Upstream.Failure>) {
+            lock.lock()
+            precondition(!state.isAwaigingSubscription)
+            guard case .subscribed = state else {
+                lock.unlock()
+                return
+            }
+            state = .terminal
+            let canceller = currentCanceller
+            lock.unlock()
+            canceller?.cancel()
+            scheduler.schedule {
+                self.downstreamLock.lock()
+                self.downstream.receive(completion: completion)
+                self.downstreamLock.unlock()
+            }
+        }
+
+        func request(_ demand: Subscribers.Demand) {
+            lock.lock()
+            precondition(!state.isAwaigingSubscription)
+            guard case .subscribed = state else {
+                lock.unlock()
+                return
+            }
+            downstreamDemand += demand
+            lock.unlock()
+        }
+
+        func cancel() {
+            lock.lock()
+            guard case .subscribed(let subscription) = state else {
+                lock.unlock()
+                return
+            }
+            state = .terminal
+            lock.unlock()
+            subscription.cancel()
+        }
+
+        var description: String { return "Debounce" }
+
+        var customMirror: Mirror {
+            let children: [Mirror.Child] = [
+                ("downstream", downstream),
+                ("downstreamDemand", downstreamDemand),
+                ("currentValue", currentValue as Any)
+            ]
+            return Mirror(self, children: children)
+        }
+
+        var playgroundDescription: Any { return description }
+
+        private func due(generation: Generation) {
+            lock.lock()
+            guard case .subscribed = state else {
+                lock.unlock()
+                return
+            }
+
+            // If this condition holds, it means that no values were received
+            // in this time frame => we should propagate the current value downstream.
+            guard generation == currentGeneration, let value = currentValue else {
+                let canceller = currentCanceller
+                lock.unlock()
+                canceller?.cancel()
+                return
+            }
+
+            let hasAnyDemand = downstreamDemand > 0
+            if hasAnyDemand {
+                downstreamDemand -= 1
+            }
+
+            let canceller = currentCanceller!
+            lock.unlock()
+            canceller.cancel()
+
+            guard hasAnyDemand else { return }
+
+            downstreamLock.lock()
+            let newDemand = downstream.receive(value)
+            downstreamLock.unlock()
+
+            if newDemand == .none { return }
+
+            lock.lock()
+            downstreamDemand += newDemand
+            lock.unlock()
+        }
+    }
+}

Sources/OpenCombine/Publishers/Publishers.DropUntilOutput.swift

@@ -65,9 +65,9 @@ extension Publishers {
                   Other.Failure == Downstream.Failure
         {
             let inner = Inner(downstream: subscriber)
+            subscriber.receive(subscription: inner)
             other.subscribe(Inner.OtherSubscriber(inner: inner))
             upstream.subscribe(inner)
-            subscriber.receive(subscription: inner)
         }
     }
 }

Sources/OpenCombine/Publishers/Publishers.FlatMap.swift

@@ -78,9 +78,8 @@ extension Publishers.FlatMap {
         /// acquired.
         private var outerSubscription: Subscription?
 
-        // Must be recursive lock. Probably a bug in Combine.
         /// The lock for requesting from `outerSubscription`.
-        private let outerLock = UnfairLock.allocate()
+        private let outerLock = UnfairRecursiveLock.allocate()
 
         /// The lock for modifying the state. All mutable state here should be
         /// read and modified with this lock acquired.

Sources/OpenCombine/Publishers/Publishers.ReceiveOn.swift

@@ -129,8 +129,8 @@ extension Publishers.ReceiveOn {
                 return .none
             }
             lock.unlock()
-            receiveOn.scheduler.schedule(options: receiveOn.options) { [weak self] in
-                self?.scheduledReceive(input, downstream: downstream)
+            receiveOn.scheduler.schedule(options: receiveOn.options) {
+                self.scheduledReceive(input, downstream: downstream)
             }
             return .none
         }
@@ -159,8 +159,8 @@ extension Publishers.ReceiveOn {
             }
             state = .terminal
             lock.unlock()
-            receiveOn.scheduler.schedule(options: receiveOn.options) { [weak self] in
-                self?.scheduledReceive(completion: completion, downstream: downstream)
+            receiveOn.scheduler.schedule(options: receiveOn.options) {
+                self.scheduledReceive(completion: completion, downstream: downstream)
             }
         }
 

Sources/OpenCombine/Publishers/Publishers.SwitchToLatest.swift

@@ -0,0 +1,336 @@
+//
+//  Publishers.SwitchToLatest.swift
+//  
+//
+//  Created by Sergej Jaskiewicz on 07.01.2020.
+//
+
+extension Publisher where Output: Publisher, Output.Failure == Failure {
+
+    /// Flattens the stream of events from multiple upstream publishers to appear as if
+    /// they were coming from a single stream of events.
+    ///
+    /// This operator switches the inner publisher as new ones arrive but keeps the outer
+    /// one constant for downstream subscribers.
+    /// For example, given the type `Publisher<Publisher<Data, NSError>, Never>`,
+    /// calling `switchToLatest()` will result in the type `Publisher<Data, NSError>`.
+    /// The downstream subscriber sees a continuous stream of values even though they may
+    /// be coming from different upstream publishers.
+    public func switchToLatest() -> Publishers.SwitchToLatest<Output, Self> {
+        return .init(upstream: self)
+    }
+}
+
+extension Publishers {
+
+    /// A publisher that “flattens” nested publishers.
+    ///
+    /// Given a publisher that publishes Publishers, the `SwitchToLatest` publisher
+    /// produces a sequence of events from only the most recent one.
+    ///
+    /// For example, given the type `Publisher<Publisher<Data, NSError>, Never>`,
+    /// calling `switchToLatest()` will result in the type `Publisher<Data, NSError>`.
+    /// The downstream subscriber sees a continuous stream of values even though they may
+    /// be coming from different upstream publishers.
+    public struct SwitchToLatest<NestedPublisher: Publisher, Upstream: Publisher>
+        : Publisher
+        where Upstream.Output == NestedPublisher,
+              Upstream.Failure == NestedPublisher.Failure
+    {
+        public typealias Output = NestedPublisher.Output
+
+        public typealias Failure = NestedPublisher.Failure
+
+        /// The publisher from which this publisher receives elements.
+        public let upstream: Upstream
+
+        /// Creates a publisher that “flattens” nested publishers.
+        ///
+        /// - Parameter upstream: The publisher from which this publisher receives
+        ///   elements.
+        public init(upstream: Upstream) {
+            self.upstream = upstream
+        }
+
+        public func receive<Downstream: Subscriber>(subscriber: Downstream)
+            where Downstream.Input == Output, Downstream.Failure == Failure
+        {
+            let outer = Outer(downstream: subscriber)
+            subscriber.receive(subscription: outer)
+            upstream.subscribe(outer)
+        }
+    }
+}
+
+extension Publishers.SwitchToLatest {
+    fileprivate final class Outer<Downstream: Subscriber>
+        : Subscriber,
+          Subscription,
+          CustomStringConvertible,
+          CustomReflectable,
+          CustomPlaygroundDisplayConvertible
+        where Downstream.Input == NestedPublisher.Output,
+              Downstream.Failure == Upstream.Failure
+    {
+        typealias Input = Upstream.Output
+
+        typealias Failure = Upstream.Failure
+
+        private let downstream: Downstream
+        private var outerSubscription: Subscription?
+        private var currentInnerSubscription: Subscription?
+        private var currentInnerIndex: UInt64 = 0
+        private var nextInnerIndex: UInt64 = 1
+        private let lock = UnfairLock.allocate()
+        private let downstreamLock = UnfairRecursiveLock.allocate()
+        private var cancelled = false
+        private var finished = false
+        private var sentCompletion = false
+        private var awaitingInnerSubscription = false
+        private var downstreamDemand = Subscribers.Demand.none
+
+        init(downstream: Downstream) {
+            self.downstream = downstream
+        }
+
+        deinit {
+            lock.deallocate()
+            downstreamLock.deallocate()
+        }
+
+        func receive(subscription: Subscription) {
+            lock.lock()
+            guard outerSubscription == nil && !cancelled else {
+                lock.unlock()
+                subscription.cancel()
+                return
+            }
+            outerSubscription = subscription
+            lock.unlock()
+            subscription.request(.unlimited)
+        }
+
+        func receive(_ input: Input) -> Subscribers.Demand {
+            lock.lock()
+            if cancelled || finished {
+                lock.unlock()
+                return .none
+            }
+
+            if let currentInnerSubscription = self.currentInnerSubscription  {
+                self.currentInnerSubscription = nil
+                lock.unlock()
+                currentInnerSubscription.cancel()
+                lock.lock()
+            }
+
+            let index = nextInnerIndex
+            currentInnerIndex = index
+            nextInnerIndex += 1
+            awaitingInnerSubscription = true
+            lock.unlock()
+            input.subscribe(Side(inner: self, index: index))
+            return .none
+        }
+
+        func receive(completion: Subscribers.Completion<Failure>) {
+            lock.lock()
+            outerSubscription = nil
+            finished = true
+
+            if cancelled {
+                lock.unlock()
+                return
+            }
+
+            switch completion {
+            case .finished:
+                if awaitingInnerSubscription {
+                    lock.unlock()
+                    return
+                }
+                if currentInnerSubscription == nil {
+                    sentCompletion = true
+                    lock.unlock()
+                    downstreamLock.lock()
+                    downstream.receive(completion: completion)
+                    downstreamLock.unlock()
+                } else {
+                    lock.unlock()
+                }
+            case .failure:
+                let currentInnerSubscription = self.currentInnerSubscription
+                self.currentInnerSubscription = nil
+                sentCompletion = true
+                lock.unlock()
+                currentInnerSubscription?.cancel()
+                downstreamLock.lock()
+                downstream.receive(completion: completion)
+                downstreamLock.unlock()
+            }
+        }
+
+        func request(_ demand: Subscribers.Demand) {
+            demand.assertNonZero()
+            lock.lock()
+            downstreamDemand += demand
+            if let currentInnerSubscription = self.currentInnerSubscription {
+                lock.unlock()
+                currentInnerSubscription.request(demand)
+            } else {
+                lock.unlock()
+            }
+        }
+
+        func cancel() {
+            lock.lock()
+            cancelled = true
+            let currentInnerSubscription = self.currentInnerSubscription
+            self.currentInnerSubscription = nil
+            let outerSubscription = self.outerSubscription
+            self.outerSubscription = nil
+            lock.unlock()
+
+            currentInnerSubscription?.cancel()
+            outerSubscription?.cancel()
+        }
+
+        var description: String { return "SwitchToLatest" }
+
+        var customMirror: Mirror {
+            return Mirror(self, children: EmptyCollection())
+        }
+
+        var playgroundDescription: Any { return description }
+
+        private func receiveInner(subscription: Subscription, _ index: UInt64) {
+            lock.lock()
+            guard currentInnerIndex == index &&
+                  !cancelled &&
+                  currentInnerSubscription == nil else {
+                lock.unlock()
+                subscription.cancel()
+                return
+            }
+
+            currentInnerSubscription = subscription
+            awaitingInnerSubscription = false
+            let downstreamDemand = self.downstreamDemand
+            lock.unlock()
+            if downstreamDemand > 0 {
+                subscription.request(downstreamDemand)
+            }
+        }
+
+        private func receiveInner(_ input: NestedPublisher.Output,
+                                  _ index: UInt64) -> Subscribers.Demand {
+            lock.lock()
+            guard currentInnerIndex == index && !cancelled else {
+                lock.unlock()
+                return .none
+            }
+
+            // This will crash if we don't have any demand yet.
+            // Combine crashes here too.
+            downstreamDemand -= 1
+
+            lock.unlock()
+            downstreamLock.lock()
+            let newDemand = downstream.receive(input)
+            downstreamLock.unlock()
+            if newDemand > 0 {
+                lock.lock()
+                downstreamDemand += newDemand
+                lock.unlock()
+            }
+
+            return newDemand
+        }
+
+        private func receiveInner(completion: Subscribers.Completion<Failure>,
+                                  _ index: UInt64) {
+            lock.lock()
+            guard currentInnerIndex == index && !cancelled else {
+                lock.unlock()
+                return
+            }
+            precondition(!awaitingInnerSubscription, "Unexpected completion")
+            currentInnerSubscription = nil
+            switch completion {
+            case .finished:
+                if sentCompletion || !finished {
+                    lock.unlock()
+                    return
+                }
+                sentCompletion = true
+                lock.unlock()
+                downstreamLock.lock()
+                downstream.receive(completion: completion)
+                downstreamLock.unlock()
+            case .failure:
+                if sentCompletion {
+                    lock.unlock()
+                    return
+                }
+                cancelled = true
+                let outerSubscription = self.outerSubscription
+                self.outerSubscription = nil
+                sentCompletion = true
+                lock.unlock()
+                outerSubscription?.cancel()
+                downstreamLock.lock()
+                downstream.receive(completion: completion)
+                downstreamLock.unlock()
+            }
+        }
+    }
+}
+
+extension Publishers.SwitchToLatest.Outer {
+    private struct Side
+        : Subscriber,
+          CustomStringConvertible,
+          CustomReflectable,
+          CustomPlaygroundDisplayConvertible
+    {
+        typealias Input = NestedPublisher.Output
+
+        typealias Failure = NestedPublisher.Failure
+
+        typealias Outer =
+            Publishers.SwitchToLatest<NestedPublisher, Upstream>.Outer<Downstream>
+
+        private let index: UInt64
+        private let outer: Outer
+
+        let combineIdentifier = CombineIdentifier()
+
+        init(inner: Outer, index: UInt64) {
+            self.index = index
+            self.outer = inner
+        }
+
+        func receive(subscription: Subscription) {
+            outer.receiveInner(subscription: subscription, index)
+        }
+
+        func receive(_ input: Input) -> Subscribers.Demand {
+            return outer.receiveInner(input, index)
+        }
+
+        func receive(completion: Subscribers.Completion<Failure>) {
+            outer.receiveInner(completion: completion, index)
+        }
+
+        var description: String { return "SwitchToLatest" }
+
+        var customMirror: Mirror {
+            let children = CollectionOfOne<Mirror.Child>(
+                ("parentSubscription", outer.combineIdentifier)
+            )
+            return Mirror(self, children: children)
+        }
+
+        var playgroundDescription: Any { return description }
+    }
+}

Sources/OpenCombine/Publishers/Publishers.Timeout.swift

@@ -0,0 +1,229 @@
+//
+//  Publishers.Timeout.swift
+//  
+//
+//  Created by Sergej Jaskiewicz on 14.06.2020.
+//
+
+extension Publisher {
+
+    /// Terminates publishing if the upstream publisher exceeds the specified time
+    /// interval without producing an element.
+    ///
+    /// - Parameters:
+    ///   - interval: The maximum time interval the publisher can go without emitting
+    ///     an element, expressed in the time system of the scheduler.
+    ///   - scheduler: The scheduler to deliver events on.
+    ///   - options: Scheduler options that customize the delivery of elements.
+    ///   - customError: A closure that executes if the publisher times out.
+    ///     The publisher sends the failure returned by this closure to the subscriber as
+    ///     the reason for termination.
+    /// - Returns: A publisher that terminates if the specified interval elapses with no
+    ///   events received from the upstream publisher.
+    public func timeout<Context: Scheduler>(
+        _ interval: Context.SchedulerTimeType.Stride,
+        scheduler: Context,
+        options: Context.SchedulerOptions? = nil,
+        customError: (() -> Self.Failure)? = nil
+    ) -> Publishers.Timeout<Self, Context> {
+        return .init(upstream: self,
+                     interval: interval,
+                     scheduler: scheduler,
+                     options: options,
+                     customError: customError)
+    }
+}
+
+extension Publishers {
+
+    public struct Timeout<Upstream: Publisher, Context: Scheduler>: Publisher {
+
+        public typealias Output = Upstream.Output
+
+        public typealias Failure = Upstream.Failure
+
+        public let upstream: Upstream
+
+        public let interval: Context.SchedulerTimeType.Stride
+
+        public let scheduler: Context
+
+        public let options: Context.SchedulerOptions?
+
+        public let customError: (() -> Upstream.Failure)?
+
+        public init(upstream: Upstream,
+                    interval: Context.SchedulerTimeType.Stride,
+                    scheduler: Context,
+                    options: Context.SchedulerOptions?,
+                    customError: (() -> Publishers.Timeout<Upstream, Context>.Failure)?) {
+            self.upstream = upstream
+            self.interval = interval
+            self.scheduler = scheduler
+            self.options = options
+            self.customError = customError
+        }
+
+        public func receive<Downsteam: Subscriber>(subscriber: Downsteam)
+            where Downsteam.Failure == Failure, Downsteam.Input == Output
+        {
+            let inner = Inner(downstream: subscriber,
+                              interval: interval,
+                              scheduler: scheduler,
+                              options: options,
+                              customError: customError)
+            upstream.subscribe(inner)
+        }
+    }
+}
+
+extension Publishers.Timeout {
+    private final class Inner<Downstream: Subscriber>
+        : Subscriber,
+          Subscription,
+          CustomStringConvertible,
+          CustomReflectable,
+          CustomPlaygroundDisplayConvertible
+        where Downstream.Input == Upstream.Output, Downstream.Failure == Upstream.Failure
+    {
+        typealias Input = Upstream.Output
+
+        typealias Failure = Upstream.Failure
+
+        private let lock = UnfairLock.allocate()
+
+        private let downstreamLock = UnfairRecursiveLock.allocate()
+
+        private let downstream: Downstream
+
+        private let interval: Context.SchedulerTimeType.Stride
+
+        private let scheduler: Context
+
+        private let options: Context.SchedulerOptions?
+
+        private let customError: (() -> Upstream.Failure)?
+
+        private var state = SubscriptionStatus.awaitingSubscription
+
+        private var didTimeout = false
+
+        private var timer: AnyCancellable?
+
+        init(downstream: Downstream,
+             interval: Context.SchedulerTimeType.Stride,
+             scheduler: Context,
+             options: Context.SchedulerOptions?,
+             customError: (() -> Upstream.Failure)?) {
+            self.downstream = downstream
+            self.interval = interval
+            self.scheduler = scheduler
+            self.options = options
+            self.customError = customError
+        }
+
+        deinit {
+            lock.deallocate()
+            downstreamLock.deallocate()
+        }
+
+        func receive(subscription: Subscription) {
+            lock.lock()
+            guard case .awaitingSubscription = state else {
+                lock.unlock()
+                subscription.cancel()
+                return
+            }
+            state = .subscribed(subscription)
+            timer = timeoutClock()
+            lock.unlock()
+            downstreamLock.lock()
+            downstream.receive(subscription: self)
+            downstreamLock.unlock()
+            subscription.request(.unlimited)
+        }
+
+        func receive(_ input: Upstream.Output) -> Subscribers.Demand {
+            lock.lock()
+            guard !didTimeout, case .subscribed = state else {
+                lock.unlock()
+                return .none
+            }
+            timer?.cancel()
+            didTimeout = false
+            timer = timeoutClock()
+            lock.unlock()
+            scheduler.schedule(options: options) {
+                self.downstreamLock.lock()
+                _ = self.downstream.receive(input)
+                self.downstreamLock.unlock()
+            }
+            return .unlimited
+        }
+
+        func receive(completion: Subscribers.Completion<Upstream.Failure>) {
+            lock.lock()
+            timer?.cancel()
+            state = .terminal
+            lock.unlock()
+            scheduler.schedule(options: options) {
+                self.downstreamLock.lock()
+                self.downstream.receive(completion: completion)
+                self.downstreamLock.unlock()
+            }
+        }
+
+        func request(_ demand: Subscribers.Demand) {
+            lock.lock()
+            guard case let .subscribed(subscription) = state else {
+                lock.unlock()
+                return
+            }
+            lock.unlock()
+            subscription.request(demand)
+        }
+
+        func cancel() {
+            lock.lock()
+            guard case let .subscribed(subscription) = state else {
+                lock.unlock()
+                return
+            }
+            state = .terminal
+            lock.unlock()
+            subscription.cancel()
+        }
+
+        var description: String { return "Timeout" }
+
+        var customMirror: Mirror { return Mirror(self, children: EmptyCollection()) }
+
+        var playgroundDescription: Any { return description }
+
+        private func timedOut() {
+            lock.lock()
+            guard !didTimeout, case let .subscribed(subscription) = state else {
+                lock.unlock()
+                return
+            }
+            didTimeout = true
+            state = .terminal
+            lock.unlock()
+            subscription.cancel()
+            downstreamLock.lock()
+            downstream
+                .receive(completion: customError.map { .failure($0()) } ?? .finished)
+            downstreamLock.unlock()
+        }
+
+        private func timeoutClock() -> AnyCancellable {
+            let cancellable = scheduler
+                .schedule(after: scheduler.now.advanced(by: interval),
+                          interval: interval,
+                          tolerance: scheduler.minimumTolerance,
+                          options: options,
+                          { [weak self] in self?.timedOut() })
+            return AnyCancellable { cancellable.cancel() }
+        }
+    }
+}

Sources/OpenCombine/Subscribers/Subscribers.Demand.swift

@@ -61,6 +61,7 @@ extension Subscribers {
             }
         }
 
+        /// Returns the result of adding two demands.
         /// When adding any value to `.unlimited`, the result is `.unlimited`.
         @inline(__always)
         @inlinable
@@ -77,6 +78,7 @@ extension Subscribers {
             }
         }
 
+        /// Adds two demands, and assigns the result to the first demand.
         /// When adding any value to `.unlimited`, the result is `.unlimited`.
         @inline(__always)
         @inlinable
@@ -85,6 +87,7 @@ extension Subscribers {
             lhs = lhs + rhs
         }
 
+        /// Returns the result of adding an integer to a demand.
         /// When adding any value to` .unlimited`, the result is `.unlimited`.
         @inline(__always)
         @inlinable
@@ -96,6 +99,7 @@ extension Subscribers {
             return isOverflow ? .unlimited : .max(sum)
         }
 
+        /// Adds an integer to a demand, and assigns the result to the demand.
         /// When adding any value to `.unlimited`, the result is `.unlimited`.
         @inline(__always)
         @inlinable
@@ -103,6 +107,9 @@ extension Subscribers {
             lhs = lhs + rhs
         }
 
+        /// Returns the result of multiplying a demand by an integer.
+        /// When multiplying any value by `.unlimited`, the result is `.unlimited`. If
+        /// the multiplication operation overflows, the result is `.unlimited`.
         public static func * (lhs: Demand, rhs: Int) -> Demand {
             if lhs == .unlimited {
                 return .unlimited
@@ -112,12 +119,16 @@ extension Subscribers {
             return isOverflow ? .unlimited : .max(product)
         }
 
+        /// Multiplies a demand by an integer, and assigns the result to the demand.
+        /// When multiplying any value by `.unlimited`, the result is `.unlimited`. If
+        /// the multiplication operation overflows, the result is `.unlimited`.
         @inline(__always)
         @inlinable
         public static func *= (lhs: inout Demand, rhs: Int) {
             lhs = lhs * rhs
         }
 
+        /// Returns the result of subtracting one demand from another.
         /// When subtracting any value (including `.unlimited`) from `.unlimited`,
         /// the result is still `.unlimited`. Subtracting `.unlimited` from any value
         /// (except `.unlimited`) results in `.max(0)`. A negative demand is not possible;
@@ -137,6 +148,7 @@ extension Subscribers {
             }
         }
 
+        /// Subtracts one demand from another, and assigns the result to the first demand.
         /// When subtracting any value (including `.unlimited`) from `.unlimited`,
         /// the result is still `.unlimited`. Subtracting unlimited from any value
         /// (except `.unlimited`) results in `.max(0)`. A negative demand is not possible;
@@ -148,6 +160,7 @@ extension Subscribers {
             lhs = lhs - rhs
         }
 
+        /// Returns the result of subtracting an integer from a demand.
         /// When subtracting any value from `.unlimited`, the result is still
         /// `.unlimited`.
         /// A negative demand is not possible; any operation that would result in
@@ -164,6 +177,7 @@ extension Subscribers {
             return isOverflow ? .none : .max(difference)
         }
 
+        /// Subtracts an integer from a demand, and assigns the result to the demand.
         /// When subtracting any value from `.unlimited,` the result is still
         /// `.unlimited`.
         /// A negative demand is not possible; any operation that would result in
@@ -175,6 +189,10 @@ extension Subscribers {
             lhs = lhs - rhs
         }
 
+        /// Returns a Boolean that indicates whether the demand requests more than
+        /// the given number of elements.
+        /// If `lhs` is `.unlimited`, then the result is always `true`.
+        /// Otherwise, the operator compares the demand’s `max` value to `rhs`.
         @inline(__always)
         @inlinable
         public static func > (lhs: Demand, rhs: Int) -> Bool {
@@ -185,6 +203,10 @@ extension Subscribers {
             }
         }
 
+        /// Returns a Boolean that indicates whether the first demand requests more or
+        /// the same number of elements as the second.
+        /// If `lhs` is `.unlimited`, then the result is always `true`.
+        /// Otherwise, the operator compares the demand’s `max` value to `rhs`.
         @inline(__always)
         @inlinable
         public static func >= (lhs: Demand, rhs: Int) -> Bool {
@@ -195,6 +217,10 @@ extension Subscribers {
             }
         }
 
+        /// Returns a Boolean that indicates a given number of elements is greater than
+        /// the maximum specified by the demand.
+        /// If `rhs` is `.unlimited`, then the result is always `false`.
+        /// Otherwise, the operator compares the demand’s `max` value to `lhs`.
         @inline(__always)
         @inlinable
         public static func > (lhs: Int, rhs: Demand) -> Bool {
@@ -205,6 +231,10 @@ extension Subscribers {
             }
         }
 
+        /// Returns a Boolean that indicates a given number of elements is greater than
+        /// or equal to the maximum specified by the demand.
+        /// If `rhs` is `.unlimited`, then the result is always `false`.
+        /// Otherwise, the operator compares the demand’s `max` value to `lhs`.
         @inline(__always)
         @inlinable
         public static func >= (lhs: Int, rhs: Demand) -> Bool {
@@ -215,6 +245,10 @@ extension Subscribers {
             }
         }
 
+        /// Returns a Boolean that indicates whether the demand requests fewer than
+        /// the given number of elements.
+        /// If `lhs` is `.unlimited`, then the result is always `false`.
+        /// Otherwise, the operator compares the demand’s `max` value to `rhs`.
         @inline(__always)
         @inlinable
         public static func < (lhs: Demand, rhs: Int) -> Bool {
@@ -225,6 +259,10 @@ extension Subscribers {
             }
         }
 
+        /// Returns a Boolean that indicates a given number of elements is less than
+        /// the maximum specified by the demand.
+        /// If `rhs` is `.unlimited`, then the result is always `true`.
+        /// Otherwise, the operator compares the demand’s `max` value to `lhs`.
         @inline(__always)
         @inlinable
         public static func < (lhs: Int, rhs: Demand) -> Bool {
@@ -235,6 +273,10 @@ extension Subscribers {
             }
         }
 
+        /// Returns a Boolean that indicates whether the demand requests fewer or
+        /// the same number of elements as the given integer.
+        /// If `lhs` is `.unlimited`, then the result is always `false`.
+        /// Otherwise, the operator compares the demand’s `max` value to `rhs`.
         @inline(__always)
         @inlinable
         public static func <= (lhs: Demand, rhs: Int) -> Bool {
@@ -245,6 +287,10 @@ extension Subscribers {
             }
         }
 
+        /// Returns a Boolean value that indicates a given number of elements is less
+        /// than or equal the maximum specified by the demand.
+        /// If `rhs` is `.unlimited`, then the result is always `true`.
+        /// Otherwise, the operator compares the demand’s `max` value to `lhs`.
         @inline(__always)
         @inlinable
         public static func <= (lhs: Int, rhs: Demand) -> Bool {
@@ -255,9 +301,12 @@ extension Subscribers {
             }
         }
 
+        /// Returns a Boolean value that indicates whether the first demand requests fewer
+        /// elements than the second.
+        /// If both sides are `.unlimited`, the result is always `false`.
         /// If `lhs` is `.unlimited`, then the result is always `false`.
-        /// If `rhs` is `.unlimited` then the result is `false` iff `lhs` is `.unlimited`
-        /// Otherwise, the two `.max` values are compared.
+        /// If `rhs` is `.unlimited`, then the result is always `true`.
+        /// Otherwise, this operator compares the demands’ `max` values.
         @inline(__always)
         @inlinable
         public static func < (lhs: Demand, rhs: Demand) -> Bool {
@@ -271,6 +320,12 @@ extension Subscribers {
             }
         }
 
+        /// Returns a Boolean value that indicates whether the first demand requests fewer
+        /// or the same number of elements as the second.
+        /// If both sides are `.unlimited`, the result is always `true`.
+        /// If `lhs` is `.unlimited`, then the result is always `false`.
+        /// If `rhs` is `.unlimited` then the result is always `true`.
+        /// Otherwise, this operator compares the demands’ `max` values.
         @inline(__always)
         @inlinable
         public static func <= (lhs: Demand, rhs: Demand) -> Bool {
@@ -286,6 +341,12 @@ extension Subscribers {
             }
         }
 
+        /// Returns a Boolean that indicates whether the first demand requests more or
+        /// the same number of elements as the second.
+        /// If both sides are `.unlimited`, the result is always `false`.
+        /// If `lhs` is `.unlimited`, then the result is always `true`.
+        /// If rhs is `.unlimited` then the result is always `false`.
+        /// Otherwise, this operator compares the demands’ `max` values.
         @inline(__always)
         @inlinable
         public static func >= (lhs: Demand, rhs: Demand) -> Bool {
@@ -301,6 +362,12 @@ extension Subscribers {
             }
         }
 
+        /// Returns a Boolean that indicates whether the first demand requests more
+        /// elements than the second.
+        /// If both sides are `.unlimited`, the result is always `false`.
+        /// If `lhs` is `.unlimited`, then the result is always `true`.
+        /// If `rhs` is `.unlimited` then the result is always `false`.
+        /// Otherwise, this operator compares the demands’ `max` values.
         @inline(__always)
         @inlinable
         public static func > (lhs: Demand, rhs: Demand) -> Bool {
@@ -316,8 +383,9 @@ extension Subscribers {
             }
         }
 
-        /// Returns `true` if `lhs` and `rhs` are equal. `.unlimited` is not equal to any
-        /// integer.
+        /// Returns a Boolean value indicating whether a demand requests the given number
+        /// of elements.
+        /// An `.unlimited` demand doesn’t match any integer.
         @inline(__always)
         @inlinable
         public static func == (lhs: Demand, rhs: Int) -> Bool {
@@ -328,8 +396,9 @@ extension Subscribers {
             }
         }
 
-        /// Returns `true` if `lhs` and `rhs` are not equal. `.unlimited` is not equal to
-        /// any integer.
+        /// Returns a Boolean value indicating whether a demand is not equal to
+        /// an integer.
+        /// The `.unlimited` value isn’t equal to any integer.
         @inlinable
         public static func != (lhs: Demand, rhs: Int) -> Bool {
             if lhs == .unlimited {
@@ -339,8 +408,9 @@ extension Subscribers {
             }
         }
 
-        /// Returns `true` if `lhs` and `rhs` are equal. `.unlimited` is not equal to any
-        /// integer.
+        /// Returns a Boolean value indicating whether a given number of elements matches
+        /// the request of a given demand.
+        /// An `.unlimited` demand doesn’t match any integer.
         @inlinable
         public static func == (lhs: Int, rhs: Demand) -> Bool {
             if rhs == .unlimited {
@@ -350,8 +420,9 @@ extension Subscribers {
             }
         }
 
-        /// Returns `true` if `lhs` and `rhs` are not equal. `.unlimited` is not equal to
-        /// any integer.
+        /// Returns a Boolean value indicating whether an integer is not equal to
+        /// a demand.
+        /// The `.unlimited` value isn’t equal to any integer.
         @inlinable
         public static func != (lhs: Int, rhs: Demand) -> Bool {
             if rhs == .unlimited {
@@ -366,7 +437,7 @@ extension Subscribers {
             return lhs.rawValue == rhs.rawValue
         }
 
-        /// Returns the number of requested values, or `nil` if `.unlimited`.
+        /// The number of requested values, or nil if `.unlimited`.
         @inlinable public var max: Int? {
             if self == .unlimited {
                 return nil

Sources/OpenCombineDispatch/DispatchQueue+Scheduler.swift

@@ -344,8 +344,10 @@ extension DispatchQueue {
 #if !canImport(Combine)
 extension DispatchQueue: OpenCombine.Scheduler {
 
+    /// Options that affect the operation of the dispatch queue scheduler.
     public typealias SchedulerOptions = OCombine.SchedulerOptions
 
+    /// The scheduler time type used by the dispatch queue.
     public typealias SchedulerTimeType = OCombine.SchedulerTimeType
 
     public var minimumTolerance: OCombine.SchedulerTimeType.Stride {

Sources/OpenCombineFoundation/OperationQueue+Scheduler.swift

@@ -0,0 +1,311 @@
+//
+//  OperationQueue+Scheduler.swift
+//  
+//
+//  Created by Sergej Jaskiewicz on 14.06.2020.
+//
+
+import Foundation
+import OpenCombine
+
+extension OperationQueue {
+
+    /// A namespace for disambiguation when both OpenCombine and Combine are imported.
+    ///
+    /// Foundation overlay for Combine extends `OperationQueue` with new methods and
+    /// nested types.
+    /// If you import both OpenCombine and Foundation, you will not be able
+    /// to write `OperationQueue.SchedulerTimeType`,
+    /// because Swift is unable to understand which `SchedulerTimeType`
+    /// you're referring to.
+    ///
+    /// So you have to write `OperationQueue.OCombine.SchedulerTimeType`.
+    ///
+    /// This bug is tracked [here](https://bugs.swift.org/browse/SR-11183).
+    ///
+    /// You can omit this whenever Combine is not available (e. g. on Linux).
+    public struct OCombine: Scheduler {
+
+        public let queue: OperationQueue
+
+        public init(_ queue: OperationQueue) {
+            self.queue = queue
+        }
+
+        /// The scheduler time type used by the operation queue.
+        public struct SchedulerTimeType: Strideable, Codable, Hashable {
+
+            /// The date represented by this type.
+            public var date: Date
+
+            /// Initializes a operation queue scheduler time with the given date.
+            ///
+            /// - Parameter date: The date to represent.
+            public init(_ date: Date) {
+                self.date = date
+            }
+
+            /// Returns the distance to another operation queue scheduler time.
+            ///
+            /// - Parameter other: Another operation queue time.
+            /// - Returns: The time interval between this time and the provided time.
+            public func distance(to other: SchedulerTimeType) -> Stride {
+                let absoluteSelf = date.timeIntervalSinceReferenceDate
+                let absoluteOther = other.date.timeIntervalSinceReferenceDate
+                return Stride(absoluteSelf.distance(to: absoluteOther))
+            }
+
+            /// Returns an operation queue scheduler time calculated by advancing this
+            /// instance’s time by the given interval.
+            ///
+            /// - Parameter n: A time interval to advance.
+            /// - Returns: An operation queue time advanced by the given interval from
+            ///   this instance’s time.
+            public func advanced(by value: Stride) -> SchedulerTimeType {
+                return SchedulerTimeType(date + value.magnitude)
+            }
+
+            /// The interval by which operation queue times advance.
+            public struct Stride: SchedulerTimeIntervalConvertible,
+                                  Comparable,
+                                  SignedNumeric,
+                                  ExpressibleByFloatLiteral,
+                                  Codable {
+
+                public typealias FloatLiteralType = TimeInterval
+
+                public typealias IntegerLiteralType = TimeInterval
+
+                public typealias Magnitude = TimeInterval
+
+                /// The value of this time interval in seconds.
+                public var magnitude: TimeInterval
+
+                /// The value of this time interval in seconds.
+                public var timeInterval: TimeInterval {
+                    return magnitude
+                }
+
+                public init(integerLiteral value: TimeInterval) {
+                    magnitude = value
+                }
+
+                public init(floatLiteral value: TimeInterval) {
+                    magnitude = value
+                }
+
+                public init(_ timeInterval: TimeInterval) {
+                    magnitude = timeInterval
+                }
+
+                public init?<Source: BinaryInteger>(exactly source: Source) {
+                    guard let value = TimeInterval(exactly: source) else { return nil }
+                    magnitude = value
+                }
+
+                public static func < (lhs: Stride, rhs: Stride) -> Bool {
+                    return lhs.magnitude < rhs.magnitude
+                }
+
+                public static func * (lhs: Stride, rhs: Stride) -> Stride {
+                    return Stride(lhs.magnitude * rhs.magnitude)
+                }
+
+                public static func + (lhs: Stride, rhs: Stride) -> Stride {
+                    return Stride(lhs.magnitude + rhs.magnitude)
+                }
+
+                public static func - (lhs: Stride, rhs: Stride) -> Stride {
+                    return Stride(lhs.magnitude - rhs.magnitude)
+                }
+
+                public static func *= (lhs: inout Stride, rhs: Stride) {
+                    lhs.magnitude *= rhs.magnitude
+                }
+
+                public static func += (lhs: inout Stride, rhs: Stride) {
+                    lhs.magnitude += rhs.magnitude
+                }
+
+                public static func -= (lhs: inout Stride, rhs: Stride) {
+                    lhs.magnitude -= rhs.magnitude
+                }
+
+                public static func seconds(_ value: Int) -> Stride {
+                    return Stride(TimeInterval(value))
+                }
+
+                public static func seconds(_ value: Double) -> Stride {
+                    return Stride(TimeInterval(value))
+                }
+
+                public static func milliseconds(_ value: Int) -> Stride {
+                    return Stride(TimeInterval(value) / 1_000)
+                }
+
+                public static func microseconds(_ value: Int) -> Stride {
+                    return Stride(TimeInterval(value) / 1_000_000)
+                }
+
+                public static func nanoseconds(_ value: Int) -> Stride {
+                    return Stride(TimeInterval(value) / 1_000_000_000)
+                }
+            }
+        }
+
+        /// Options that affect the operation of the operation queue scheduler.
+        public struct SchedulerOptions {
+        }
+
+        private final class DelayReadyOperation: Operation, Cancellable {
+
+            private static let readySchedulingQueue =
+                DispatchQueue(label: "DelayReadyOperation")
+
+            private var action: (() -> Void)?
+
+            private var readyFromAfter = false
+            private let lock = UnfairLock.allocate()
+
+            init(_ action: @escaping() -> Void, after: SchedulerTimeType) {
+                self.action = action
+                super.init()
+                let deadline = DispatchTime.now() + after.date.timeIntervalSinceNow
+                DelayReadyOperation.readySchedulingQueue
+                    .asyncAfter(deadline: deadline) { [weak self] in
+                        self?.becomeReady()
+                    }
+            }
+
+            deinit {
+                lock.deallocate()
+            }
+
+            override func main() {
+                action!()
+                action = nil
+            }
+
+            private func becomeReady() {
+#if canImport(Darwin)
+                // The smart key paths don't work with NSOperation on OS versions prior to
+                // iOS 11. The string key paths work fine everywhere.
+                // https://forums.swift.org/t/keypath-translation-for-kvo-notification-seems-to-not-work-properly-on-ios-10/15898
+                willChangeValue(forKey: "isReady")
+#else
+                willChangeValue(for: \.isReady)
+#endif
+                lock.lock()
+                readyFromAfter = true
+                lock.unlock()
+#if canImport(Darwin)
+                // The smart key paths don't work with NSOperation on OS versions prior to
+                // iOS 11. The string key paths work fine everywhere.
+                // https://forums.swift.org/t/keypath-translation-for-kvo-notification-seems-to-not-work-properly-on-ios-10/15898
+                didChangeValue(forKey: "isReady")
+#else
+                didChangeValue(for: \.isReady)
+#endif
+            }
+
+            override var isReady: Bool {
+                guard super.isReady else { return false }
+                lock.lock()
+                defer { lock.unlock() }
+                return readyFromAfter
+            }
+        }
+
+        public func schedule(options: SchedulerOptions?,
+                             _ action: @escaping () -> Void) {
+            let op = BlockOperation(block: action)
+            queue.addOperation(op)
+        }
+
+        public func schedule(after date: SchedulerTimeType,
+                             tolerance: SchedulerTimeType.Stride,
+                             options: SchedulerOptions?,
+                             _ action: @escaping () -> Void) {
+            let op = DelayReadyOperation(action, after: date)
+            queue.addOperation(op)
+        }
+
+        public func schedule(after date: SchedulerTimeType,
+                             interval: SchedulerTimeType.Stride,
+                             tolerance: SchedulerTimeType.Stride,
+                             options: SchedulerOptions?,
+                             _ action: @escaping () -> Void) -> Cancellable {
+            let op = DelayReadyOperation(action, after: date.advanced(by: interval))
+            queue.addOperation(op)
+            return AnyCancellable(op)
+        }
+
+        public var now: SchedulerTimeType {
+            return .init(Date())
+        }
+
+        public var minimumTolerance: SchedulerTimeType.Stride {
+            return .init(0.0)
+        }
+    }
+
+    /// A namespace for disambiguation when both OpenCombine and Foundation are imported.
+    ///
+    /// Foundation overlay for Combine extends `OperationQueue` with new methods and
+    /// nested types.
+    /// If you import both OpenCombine and Foundation, you will not be able
+    /// to write `OperationQueue.main.schedule { doThings() }`,
+    /// because Swift is unable to understand which `schedule` method
+    /// you're referring to.
+    ///
+    /// So you have to write `OperationQueue.main.ocombine.schedule { doThings() }`.
+    ///
+    /// This bug is tracked [here](https://bugs.swift.org/browse/SR-11183).
+    ///
+    /// You can omit this whenever Combine is not available (e. g. on Linux).
+    public var ocombine: OCombine {
+        return OCombine(self)
+    }
+}
+
+#if !canImport(Combine)
+extension OperationQueue: OpenCombine.Scheduler {
+
+    /// Options that affect the operation of the run loop scheduler.
+    public typealias SchedulerOptions = OCombine.SchedulerOptions
+
+    /// The scheduler time type used by the run loop.
+    public typealias SchedulerTimeType = OCombine.SchedulerTimeType
+
+    public func schedule(options: SchedulerOptions?, _ action: @escaping () -> Void) {
+        ocombine.schedule(options: options, action)
+    }
+
+    public func schedule(after date: SchedulerTimeType,
+                         tolerance: SchedulerTimeType.Stride,
+                         options: SchedulerOptions?,
+                         _ action: @escaping () -> Void) {
+        ocombine.schedule(after: date, tolerance: tolerance, options: options, action)
+    }
+
+    public func schedule(after date: SchedulerTimeType,
+                         interval: SchedulerTimeType.Stride,
+                         tolerance: SchedulerTimeType.Stride,
+                         options: SchedulerOptions?,
+                         _ action: @escaping () -> Void) -> Cancellable {
+        return ocombine.schedule(after: date,
+                                 interval: interval,
+                                 tolerance: tolerance,
+                                 options: options,
+                                 action)
+    }
+
+    public var now: SchedulerTimeType {
+        return ocombine.now
+    }
+
+    public var minimumTolerance: SchedulerTimeType.Stride {
+        return ocombine.minimumTolerance
+    }
+}
+#endif

Sources/OpenCombineFoundation/RunLoop+Scheduler.swift

@@ -0,0 +1,290 @@
+//
+//  RunLoop+Scheduler.swift
+//  
+//
+//  Created by Sergej Jaskiewicz on 13.12.2019.
+//
+
+import CoreFoundation
+import Foundation
+import OpenCombine
+
+extension RunLoop {
+
+    /// A namespace for disambiguation when both OpenCombine and Combine are imported.
+    ///
+    /// Foundation overlay for Combine extends `RunLoop` with new methods and nested
+    /// types.
+    /// If you import both OpenCombine and Foundation, you will not be able
+    /// to write `RunLoop.SchedulerTimeType`,
+    /// because Swift is unable to understand which `SchedulerTimeType`
+    /// you're referring to.
+    ///
+    /// So you have to write `RunLoop.OCombine.SchedulerTimeType`.
+    ///
+    /// This bug is tracked [here](https://bugs.swift.org/browse/SR-11183).
+    ///
+    /// You can omit this whenever Combine is not available (e. g. on Linux).
+    public struct OCombine: Scheduler {
+
+        public let runLoop: RunLoop
+
+        public init(_ runLoop: RunLoop) {
+            self.runLoop = runLoop
+        }
+
+        /// The scheduler time type used by the run loop.
+        public struct SchedulerTimeType: Strideable, Codable, Hashable {
+
+            /// The date represented by this type.
+            public var date: Date
+
+            /// Initializes a run loop scheduler time with the given date.
+            ///
+            /// - Parameter date: The date to represent.
+            public init(_ date: Date) {
+                self.date = date
+            }
+
+            /// Returns the distance to another run loop scheduler time.
+            ///
+            /// - Parameter other: Another run loop time.
+            /// - Returns: The time interval between this time and the provided time.
+            public func distance(to other: SchedulerTimeType) -> Stride {
+                let absoluteSelf = date.timeIntervalSinceReferenceDate
+                let absoluteOther = other.date.timeIntervalSinceReferenceDate
+                return Stride(absoluteSelf.distance(to: absoluteOther))
+            }
+
+            /// Returns a run loop scheduler time calculated by advancing this instance’s
+            /// time by the given interval.
+            ///
+            /// - Parameter value: A time interval to advance.
+            /// - Returns: A run loop time advanced by the given interval from this
+            ///   instance’s time.
+            public func advanced(by value: Stride) -> SchedulerTimeType {
+                return SchedulerTimeType(date + value.magnitude)
+            }
+
+            /// The interval by which run loop times advance.
+            public struct Stride: SchedulerTimeIntervalConvertible,
+                                  Comparable,
+                                  SignedNumeric,
+                                  ExpressibleByFloatLiteral,
+                                  Codable {
+
+                public typealias FloatLiteralType = TimeInterval
+
+                public typealias IntegerLiteralType = TimeInterval
+
+                /// A type that can represent the absolute value of any possible value
+                /// of the conforming type.
+                public typealias Magnitude = TimeInterval
+
+                /// The value of this time interval in seconds.
+                public var magnitude: TimeInterval
+
+                /// The value of this time interval in seconds.
+                public var timeInterval: TimeInterval { return magnitude }
+
+                public init(integerLiteral value: TimeInterval) {
+                    self.magnitude = value
+                }
+
+                public init(floatLiteral value: TimeInterval) {
+                    self.magnitude = value
+                }
+
+                public init(_ timeInterval: TimeInterval) {
+                    self.magnitude = timeInterval
+                }
+
+                public init?<Source: BinaryInteger>(exactly source: Source) {
+                    guard let value = TimeInterval(exactly: source) else { return nil }
+                    magnitude = value
+                }
+
+                public static func < (lhs: Stride, rhs: Stride) -> Bool {
+                    return lhs.magnitude < rhs.magnitude
+                }
+
+                public static func * (lhs: Stride, rhs: Stride) -> Stride {
+                    return Stride(lhs.magnitude * rhs.magnitude)
+                }
+
+                public static func + (lhs: Stride, rhs: Stride) -> Stride {
+                    return Stride(lhs.magnitude + rhs.magnitude)
+                }
+
+                public static func - (lhs: Stride, rhs: Stride) -> Stride {
+                    return Stride(lhs.magnitude - rhs.magnitude)
+                }
+
+                public static func *= (lhs: inout Stride, rhs: Stride) {
+                    lhs.magnitude *= rhs.magnitude
+                }
+
+                public static func += (lhs: inout Stride, rhs: Stride) {
+                    lhs.magnitude += rhs.magnitude
+                }
+
+                public static func -= (lhs: inout Stride, rhs: Stride) {
+                    lhs.magnitude -= rhs.magnitude
+                }
+
+                public static func seconds(_ value: Int) -> Stride {
+                    return Stride(TimeInterval(value))
+                }
+
+                public static func seconds(_ value: Double) -> Stride {
+                    return Stride(TimeInterval(value))
+                }
+
+                public static func milliseconds(_ value: Int) -> Stride {
+                    return Stride(TimeInterval(value) / 1_000)
+                }
+
+                public static func microseconds(_ value: Int) -> Stride {
+                    return Stride(TimeInterval(value) / 1_000_000)
+                }
+
+                public static func nanoseconds(_ value: Int) -> Stride {
+                    return Stride(TimeInterval(value) / 1_000_000_000)
+                }
+            }
+        }
+
+        /// Options that affect the operation of the run loop scheduler.
+        public struct SchedulerOptions {
+        }
+
+        public func schedule(options: SchedulerOptions?, _ action: @escaping () -> Void) {
+            let cfRunLoop = runLoop.getCFRunLoop()
+            CFRunLoopPerformBlock(cfRunLoop, defaultRunLoopModeString, action)
+            CFRunLoopWakeUp(cfRunLoop)
+        }
+
+        public func schedule(after date: SchedulerTimeType,
+                             tolerance: SchedulerTimeType.Stride,
+                             options: SchedulerOptions?,
+                             _ action: @escaping () -> Void) {
+            let timer = CFRunLoopTimerCreateWithHandler(
+                nil,
+                date.date.timeIntervalSinceReferenceDate,
+                0,
+                0,
+                0,
+                { _ in action() }
+            )
+            // A bug in Combine. The schedule(after:tolerance:options:_:) methods
+            // always executes the action on the current runloop.
+            // (FB7493579 if Apple folks are watching)
+            let theWrongRunLoop = CFRunLoopGetCurrent()
+            CFRunLoopAddTimer(theWrongRunLoop, timer, defaultRunLoopMode)
+            CFRunLoopWakeUp(theWrongRunLoop)
+        }
+
+        public func schedule(after date: SchedulerTimeType,
+                             interval: SchedulerTimeType.Stride,
+                             tolerance: SchedulerTimeType.Stride,
+                             options: SchedulerOptions?,
+                             _ action: @escaping () -> Void) -> Cancellable {
+            let timer = CFRunLoopTimerCreateWithHandler(
+                nil,
+                date.date.timeIntervalSinceReferenceDate,
+                interval.magnitude,
+                0,
+                0,
+                { _ in action() }
+            )
+            let cfRunLoop = runLoop.getCFRunLoop()
+            CFRunLoopAddTimer(cfRunLoop, timer, defaultRunLoopMode)
+            CFRunLoopWakeUp(cfRunLoop)
+            return AnyCancellable { CFRunLoopTimerInvalidate(timer) }
+        }
+
+        public var now: SchedulerTimeType {
+            return .init(Date())
+        }
+
+        public var minimumTolerance: SchedulerTimeType.Stride {
+            return .init(0)
+        }
+    }
+
+    /// A namespace for disambiguation when both OpenCombine and Foundation are imported.
+    ///
+    /// Foundation overlay for Combine extends `RunLoop` with new methods and nested
+    /// types.
+    /// If you import both OpenCombine and Foundation, you will not be able
+    /// to write `RunLoop.main.schedule { doThings() }`,
+    /// because Swift is unable to understand which `schedule` method
+    /// you're referring to.
+    ///
+    /// So you have to write `RunLoop.main.ocombine.schedule { doThings() }`.
+    ///
+    /// This bug is tracked [here](https://bugs.swift.org/browse/SR-11183).
+    ///
+    /// You can omit this whenever Combine is not available (e. g. on Linux).
+    public var ocombine: OCombine {
+        return OCombine(self)
+    }
+}
+
+#if !canImport(Combine)
+extension RunLoop: OpenCombine.Scheduler {
+
+    /// Options that affect the operation of the run loop scheduler.
+    public typealias SchedulerOptions = OCombine.SchedulerOptions
+
+    /// The scheduler time type used by the run loop.
+    public typealias SchedulerTimeType = OCombine.SchedulerTimeType
+
+    public func schedule(options: SchedulerOptions?, _ action: @escaping () -> Void) {
+        ocombine.schedule(options: options, action)
+    }
+
+    public func schedule(after date: SchedulerTimeType,
+                         tolerance: SchedulerTimeType.Stride,
+                         options: SchedulerOptions?,
+                         _ action: @escaping () -> Void) {
+        ocombine.schedule(after: date, tolerance: tolerance, options: options, action)
+    }
+
+    public func schedule(after date: SchedulerTimeType,
+                         interval: SchedulerTimeType.Stride,
+                         tolerance: SchedulerTimeType.Stride,
+                         options: SchedulerOptions?,
+                         _ action: @escaping () -> Void) -> Cancellable {
+        return ocombine.schedule(after: date,
+                                 interval: interval,
+                                 tolerance: tolerance,
+                                 options: options,
+                                 action)
+    }
+
+    public var now: SchedulerTimeType {
+        return ocombine.now
+    }
+
+    public var minimumTolerance: SchedulerTimeType.Stride {
+        return ocombine.minimumTolerance
+    }
+}
+#endif
+
+private var defaultRunLoopMode: CFRunLoopMode {
+#if canImport(Darwin)
+    return CFRunLoopMode.defaultMode
+#else
+    return kCFRunLoopDefaultMode
+#endif
+}
+
+private var defaultRunLoopModeString: CFString {
+#if canImport(Darwin)
+    return CFRunLoopMode.defaultMode.rawValue
+#else
+    return kCFRunLoopDefaultMode
+#endif
+}

Sources/OpenCombineFoundation/Timer+Publisher.swift

@@ -0,0 +1,348 @@
+//
+//  Timer+Publisher.swift
+//
+//
+//  Created by Sergej Jaskiewicz on 23.06.2020.
+//
+
+import CoreFoundation
+import Foundation
+import OpenCombine
+
+extension Timer {
+
+    /// Returns a publisher that repeatedly emits the current date on the given interval.
+    ///
+    /// - Parameters:
+    ///   - interval: The time interval on which to publish events. For example,
+    ///     a value of `0.5` publishes an event approximately every half-second.
+    ///   - tolerance: The allowed timing variance when emitting events.
+    ///     Defaults to `nil`, which allows any variance.
+    ///   - runLoop: The run loop on which the timer runs.
+    ///   - mode: The run loop mode in which to run the timer.
+    ///   - options: Scheduler options passed to the timer. Defaults to `nil`.
+    /// - Returns: A publisher that repeatedly emits the current date on the given
+    ///   interval.
+    public static func publish(
+        every interval: TimeInterval,
+        tolerance _: TimeInterval? = nil,
+        on runLoop: RunLoop,
+        in mode: RunLoop.Mode,
+        options: RunLoop.OCombine.SchedulerOptions? = nil
+    ) -> OCombine.TimerPublisher {
+        // A bug in Combine: tolerance is ignored.
+        return .init(interval: interval, runLoop: runLoop, mode: mode, options: options)
+    }
+
+    /// A namespace for disambiguation when both OpenCombine and Combine are imported.
+    ///
+    /// Foundation overlay for Combine extends `Timer` with new methods and nested
+    /// types.
+    /// If you import both OpenCombine and Foundation, you will not be able
+    /// to write `Timer.TimerPublisher`,
+    /// because Swift is unable to understand which `TimerPublisher`
+    /// you're referring to.
+    ///
+    /// So you have to write `Timer.OCombine.TimerPublisher`.
+    ///
+    /// This bug is tracked [here](https://bugs.swift.org/browse/SR-11183).
+    ///
+    /// You can omit this whenever Combine is not available (e. g. on Linux).
+    public enum OCombine {
+
+        /// A publisher that repeatedly emits the current date on a given interval.
+        public final class TimerPublisher: ConnectablePublisher {
+            public typealias Output = Date
+            public typealias Failure = Never
+
+            public let interval: TimeInterval
+            public let tolerance: TimeInterval?
+            public let runLoop: RunLoop
+            public let mode: RunLoop.Mode
+            public let options: RunLoop.OCombine.SchedulerOptions?
+
+            private lazy var routingSubscription: RoutingSubscription = {
+                RoutingSubscription(parent: self)
+            }()
+
+            /// Creates a publisher that repeatedly emits the current date
+            /// on the given interval.
+            ///
+            /// - Parameters:
+            ///   - interval: The interval on which to publish events.
+            ///   - tolerance: The allowed timing variance when emitting events.
+            ///     Defaults to `nil`, which allows any variance.
+            ///   - runLoop: The run loop on which the timer runs.
+            ///   - mode: The run loop mode in which to run the timer.
+            ///   - options: Scheduler options passed to the timer. Defaults to `nil`.
+            public init(
+                interval: TimeInterval,
+                tolerance: TimeInterval? = nil,
+                runLoop: RunLoop,
+                mode: RunLoop.Mode,
+                options: RunLoop.OCombine.SchedulerOptions? = nil
+            ) {
+                self.interval = interval
+                self.tolerance = tolerance
+                self.runLoop = runLoop
+                self.mode = mode
+                self.options = options
+            }
+
+            /// Adapter subscription to allow `Timer` to multiplex to multiple subscribers
+            /// the values produced by a single `TimerPublisher.Inner`
+            private final class RoutingSubscription
+                : Subscription,
+                  CustomStringConvertible,
+                  CustomReflectable,
+                  CustomPlaygroundDisplayConvertible
+            {
+                typealias Input = Date
+                typealias Failure = Never
+
+                private typealias ErasedSubscriber = AnySubscriber<Output, Failure>
+
+                private let lock = UnfairLock.allocate()
+
+                // Inner is IUP due to init requirements
+                // swiftlint:disable:next implicitly_unwrapped_optional
+                private var inner: Inner!
+
+                private var subscribers: [ErasedSubscriber] = []
+
+                private var isConnected = false
+
+                init(parent: TimerPublisher) {
+                    inner = Inner(parent: parent, downstream: self)
+                }
+
+                deinit {
+                    lock.deallocate()
+                }
+
+                func addSubscriber<Downstream: Subscriber>(_ downstream: Downstream)
+                    where Downstream.Failure == Failure, Downstream.Input == Output
+                {
+                    lock.lock()
+                    subscribers.append(AnySubscriber(downstream))
+                    lock.unlock()
+
+                    downstream.receive(subscription: self)
+                }
+
+                func receive(_ value: Input) -> Subscribers.Demand {
+                    var resultingDemand = Subscribers.Demand.none
+                    lock.lock()
+                    let subscribers = self.subscribers
+                    let isConnected = self.isConnected
+                    lock.unlock()
+
+                    guard isConnected else {
+                        // This branch is only reachable in case of a race condition.
+                        return .none
+                    }
+
+                    for subscriber in subscribers {
+                        resultingDemand += subscriber.receive(value)
+                    }
+                    return resultingDemand
+                }
+
+                func request(_ demand: Subscribers.Demand) {
+                    lock.lock()
+                    let inner = self.inner!
+                    lock.unlock()
+
+                    inner.request(demand)
+                }
+
+                func cancel() {
+                    lock.lock()
+                    let inner = self.inner!
+                    isConnected = false
+                    subscribers = []
+                    lock.unlock()
+
+                    inner.cancel()
+                }
+
+                var description: String { return "Timer" }
+
+                var customMirror: Mirror { return inner.customMirror }
+
+                var playgroundDescription: Any { return description }
+
+                var combineIdentifier: CombineIdentifier {
+                    return inner.combineIdentifier
+                }
+
+                func startPublishing() {
+                    lock.lock()
+                    let isConnected = self.isConnected
+                    self.isConnected = true
+                    let inner = self.inner!
+                    lock.unlock()
+                    if isConnected { return }
+                    inner.startPublishing()
+                }
+            }
+
+            public func receive<Downstream: Subscriber>(subscriber: Downstream)
+                where Failure == Downstream.Failure, Output == Downstream.Input
+            {
+                routingSubscription.addSubscriber(subscriber)
+            }
+
+            public func connect() -> Cancellable {
+                routingSubscription.startPublishing()
+                return routingSubscription
+            }
+
+            private typealias Parent = TimerPublisher
+
+            private final class Inner
+                : NSObject,
+                  Subscription,
+                  CustomReflectable,
+                  CustomPlaygroundDisplayConvertible
+            {
+                private lazy var timer: CFRunLoopTimer? = {
+                    let timer = CFRunLoopTimerCreateWithHandler(
+                        nil,
+                        Date().timeIntervalSinceReferenceDate,
+                        parent?.interval ?? 0,
+                        0,
+                        0,
+                        { [weak self] _ in self?.timerFired() }
+                    )!
+                    CFRunLoopTimerSetTolerance(timer, parent?.tolerance ?? 0)
+                    return timer
+                }()
+
+                private let lock = UnfairLock.allocate()
+
+                private var downstream: RoutingSubscription?
+
+                private var parent: Parent?
+
+                private var started = false
+
+                private var demand = Subscribers.Demand.none
+
+                init(parent: Parent, downstream: RoutingSubscription) {
+                    self.parent = parent
+                    self.downstream = downstream
+                }
+
+                deinit {
+                    lock.deallocate()
+                }
+
+                func startPublishing() {
+                    lock.lock()
+                    guard let timer = self.timer,
+                          let parent = self.parent,
+                          !started else {
+                        lock.unlock()
+                        return
+                    }
+
+                    started = true
+                    lock.unlock()
+
+                    CFRunLoopAddTimer(parent.runLoop.getCFRunLoop(),
+                                      timer,
+                                      parent.mode.asCFRunLoopMode())
+                }
+
+                func cancel() {
+                    lock.lock()
+                    guard let timer = self.timer else {
+                        lock.unlock()
+                        return
+                    }
+
+                    downstream = nil
+                    parent = nil
+                    started = false
+                    demand = .none
+                    self.timer = nil
+                    lock.unlock()
+
+                    CFRunLoopTimerInvalidate(timer)
+                }
+
+                func request(_ demand: Subscribers.Demand) {
+                    lock.lock()
+                    defer { lock.unlock() }
+                    guard parent != nil else {
+                        return
+                    }
+                    self.demand += demand
+                }
+
+                override var description: String { return "Timer" }
+
+                var customMirror: Mirror {
+                    lock.lock()
+                    defer { lock.unlock() }
+                    let children: [Mirror.Child] = [
+                        ("downstream", downstream as Any),
+                        ("interval", parent?.interval as Any),
+                        ("tolerance", parent?.tolerance as Any),
+                    ]
+                    return Mirror(self, children: children)
+                }
+
+                var playgroundDescription: Any { return description }
+
+                private func timerFired() {
+                    lock.lock()
+                    guard let downstream = self.downstream,
+                          parent != nil,
+                          demand > 0
+                    else {
+                        lock.unlock()
+                        return
+                    }
+
+                    demand -= 1
+                    lock.unlock()
+
+                    let newDemand = downstream.receive(Date())
+                    guard newDemand > 0 else {
+                        return
+                    }
+
+                    lock.lock()
+                    demand += newDemand
+                    lock.unlock()
+                }
+            }
+        }
+    }
+}
+
+#if !canImport(Combine)
+extension Timer {
+
+    /// A publisher that repeatedly emits the current date on a given interval.
+    public typealias TimerPublisher = OCombine.TimerPublisher
+}
+#endif
+
+extension RunLoop.Mode {
+    fileprivate func asCFRunLoopMode() -> CFRunLoopMode {
+#if canImport(Darwin)
+        return CFRunLoopMode(rawValue as CFString)
+#else
+        return rawValue.withCString {
+            CFStringCreateWithCString(
+                nil,
+                $0,
+                CFStringEncoding(kCFStringEncodingUTF8)
+            )
+        }
+#endif
+    }
+}

Tests/OpenCombineTests/CurrentValueSubjectTests.swift

@@ -102,6 +102,36 @@ final class CurrentValueSubjectTests: XCTestCase {
         XCTAssertEqual(numberOfInputsHistory, expectedNumberOfInputsHistory)
     }
 
+    func testRequestSeveralTimes() throws {
+        let cvs = Sut(-1)
+        var downstreamSubscription: Subscription?
+        let tracking = TrackingSubscriber(
+            receiveSubscription: { downstreamSubscription = $0 }
+        )
+        cvs.subscribe(tracking)
+
+        XCTAssertEqual(tracking.history, [.subscription("CurrentValueSubject")])
+
+        try XCTUnwrap(downstreamSubscription).request(.max(2))
+        try XCTUnwrap(downstreamSubscription).request(.max(3))
+        try XCTUnwrap(downstreamSubscription).request(.max(1))
+
+        XCTAssertEqual(tracking.history, [.subscription("CurrentValueSubject"),
+                                          .value(-1)])
+
+        for i in 0 ..< 10 {
+            cvs.send(i)
+        }
+
+        XCTAssertEqual(tracking.history, [.subscription("CurrentValueSubject"),
+                                          .value(-1),
+                                          .value(0),
+                                          .value(1),
+                                          .value(2),
+                                          .value(3),
+                                          .value(4)])
+    }
+
     func testCrashOnZeroInitialDemand() {
         assertCrashes {
             let subscriber = TrackingSubscriber(
@@ -139,6 +169,20 @@ final class CurrentValueSubjectTests: XCTestCase {
                                             .completion(.failure(.oops))])
     }
 
+    func testChangeValueAfterCompletion() {
+        let cvs = Sut(0)
+        cvs.send(completion: .finished)
+        cvs.value = 42
+        XCTAssertEqual(cvs.value, 42)
+    }
+
+    func testSendValueAfterCompletion() {
+        let cvs = Sut(0)
+        cvs.send(completion: .finished)
+        cvs.send(42)
+        XCTAssertEqual(cvs.value, 0)
+    }
+
     func testMultipleSubscriptions() {
 
         let cvs = Sut(112)
@@ -224,6 +268,42 @@ final class CurrentValueSubjectTests: XCTestCase {
                         .subscription("CurrentValueSubject"),
                         .value(112),
                         .subscription("CurrentValueSubject")])
+
+        for (i, subscription) in subscriber.tracking.subscriptions.enumerated()
+            where i.isMultiple(of: 2)
+        {
+            subscription.cancel()
+        }
+        cvs.value = 200
+
+        XCTAssertEqual(subscriber.tracking.history,
+                       [.value(112),
+                        .subscription("CurrentValueSubject"),
+                        .value(112),
+                        .subscription("CurrentValueSubject"),
+                        .value(112),
+                        .subscription("CurrentValueSubject"),
+                        .value(112),
+                        .subscription("CurrentValueSubject"),
+                        .value(112),
+                        .subscription("CurrentValueSubject"),
+                        .value(112),
+                        .subscription("CurrentValueSubject"),
+                        .value(112),
+                        .subscription("CurrentValueSubject"),
+                        .value(112),
+                        .subscription("CurrentValueSubject"),
+                        .value(112),
+                        .subscription("CurrentValueSubject"),
+                        .value(112),
+                        .subscription("CurrentValueSubject"),
+                        .value(112),
+                        .subscription("CurrentValueSubject"),
+                        .value(200),
+                        .value(200),
+                        .value(200),
+                        .value(200),
+                        .value(200)])
     }
 
     // Reactive Streams Spec: Rule #6
@@ -375,6 +455,100 @@ final class CurrentValueSubjectTests: XCTestCase {
         XCTAssertEqual(subscription2.history, [.requested(.unlimited)])
     }
 
+    func testCompletion() throws {
+        let passthrough = Sut(42)
+        var downstreamSubscription: Subscription?
+        let tracking = TrackingSubscriber(
+            receiveSubscription: { downstreamSubscription = $0 }
+        )
+
+        passthrough.subscribe(tracking)
+
+        try XCTUnwrap(downstreamSubscription).request(.max(12))
+
+        passthrough.send(1)
+
+        expectedChildren(
+            ("parent", .contains("CurrentValueSubject")),
+            ("downstream", .contains("TrackingSubscriberBase")),
+            ("demand", "max(10)"),
+            ("subject", .contains("CurrentValueSubject"))
+        )(Mirror(reflecting: try XCTUnwrap(downstreamSubscription)))
+
+        passthrough.send(completion: .finished)
+
+        if !hasCustomMirrorUseAfterFreeBug {
+            expectedChildren(
+                ("parent", "nil"),
+                ("downstream", "nil"),
+                ("demand", "max(10)"),
+                ("subject", "nil")
+            )(Mirror(reflecting: try XCTUnwrap(downstreamSubscription)))
+        }
+
+        XCTAssertEqual(tracking.history, [.subscription("CurrentValueSubject"),
+                                          .value(42),
+                                          .value(1),
+                                          .completion(.finished)])
+
+        passthrough.send(completion: .failure(.oops))
+        try XCTUnwrap(downstreamSubscription).cancel()
+        try XCTUnwrap(downstreamSubscription).request(.max(3))
+
+        if !hasCustomMirrorUseAfterFreeBug {
+            expectedChildren(
+                ("parent", "nil"),
+                ("downstream", "nil"),
+                ("demand", "max(10)"),
+                ("subject", "nil")
+            )(Mirror(reflecting: try XCTUnwrap(downstreamSubscription)))
+        }
+
+        XCTAssertEqual(tracking.history, [.subscription("CurrentValueSubject"),
+                                          .value(42),
+                                          .value(1),
+                                          .completion(.finished)])
+    }
+
+    func testCancellation() throws {
+        let cvs = Sut(42)
+        var downstreamSubscription: Subscription?
+        let tracking = TrackingSubscriber(
+            receiveSubscription: { downstreamSubscription = $0 }
+        )
+
+        cvs.subscribe(tracking)
+
+        try XCTUnwrap(downstreamSubscription).request(.max(12))
+
+        cvs.send(1)
+
+        expectedChildren(
+            ("parent", .contains("CurrentValueSubject")),
+            ("downstream", .contains("TrackingSubscriberBase")),
+            ("demand", "max(10)"),
+            ("subject", .contains("CurrentValueSubject"))
+        )(Mirror(reflecting: try XCTUnwrap(downstreamSubscription)))
+
+        try XCTUnwrap(downstreamSubscription).cancel()
+        try XCTUnwrap(downstreamSubscription).cancel()
+        try XCTUnwrap(downstreamSubscription).request(.max(3))
+        try XCTUnwrap(downstreamSubscription).request(.max(4))
+
+        if !hasCustomMirrorUseAfterFreeBug {
+            expectedChildren(
+                ("parent", "nil"),
+                ("downstream", "nil"),
+                ("demand", "max(10)"),
+                ("subject", "nil")
+            )(Mirror(reflecting: try XCTUnwrap(downstreamSubscription)))
+        }
+
+        XCTAssertEqual(tracking.history, [.subscription("CurrentValueSubject"),
+                                          .value(42),
+                                          .value(1)])
+    }
+
     func testLifecycle() throws {
 
         var deinitCounter = 0
@@ -425,68 +599,87 @@ final class CurrentValueSubjectTests: XCTestCase {
         XCTAssertEqual(deinitCounter, 2)
     }
 
-    func testSynchronization() {
-
-        let subscriptions = Atomic<[Subscription]>([])
-        let inputs = Atomic<[Int]>([])
-        let completions = Atomic<[Subscribers.Completion<TestingError>]>([])
-
-        let cvs = Sut(112)
-        let subscriber = AnySubscriber<Int, TestingError>(
-            receiveSubscription: { subscription in
-                subscriptions.do { $0.append(subscription) }
-                subscription.request(.unlimited)
-            },
-            receiveValue: { value in
-                inputs.do { $0.append(value) }
-                return .none
-            },
-            receiveCompletion: { completion in
-                completions.do { $0.append(completion) }
+    func testCancelsUpstreamSubscriptionsOnDeinit() {
+        let subscription = CustomSubscription()
+        do {
+            let cvs = Sut(42)
+            for _ in 0 ..< 5 {
+                cvs.send(subscription: subscription)
             }
-        )
+            XCTAssertEqual(subscription.history, [.requested(.unlimited),
+                                                  .requested(.unlimited),
+                                                  .requested(.unlimited),
+                                                  .requested(.unlimited),
+                                                  .requested(.unlimited)])
+        }
 
-        race(
-            {
-                cvs.subscribe(subscriber)
-            },
-            {
-                cvs.subscribe(subscriber)
+        XCTAssertEqual(subscription.history, [.requested(.unlimited),
+                                              .requested(.unlimited),
+                                              .requested(.unlimited),
+                                              .requested(.unlimited),
+                                              .requested(.unlimited),
+                                              .cancelled,
+                                              .cancelled,
+                                              .cancelled,
+                                              .cancelled,
+                                              .cancelled])
+    }
+
+    func testReleasesEverythingOnTermination() {
+
+        enum TerminationReason: CaseIterable {
+            case cancelled
+            case finished
+            case failed
+        }
+
+        for reason in TerminationReason.allCases {
+            weak var weakSubscriber: TrackingSubscriber?
+            weak var weakSubject: Sut?
+            weak var weakSubscription: AnyObject?
+
+            do {
+                let subject = Sut(42)
+                do {
+                    let subscriber = TrackingSubscriber(
+                        receiveSubscription: {
+                            weakSubscription = $0 as AnyObject
+                        }
+                    )
+                    weakSubscriber = subscriber
+                    weakSubject = subject
+
+                    subject.subscribe(subscriber)
+                }
+
+                switch reason {
+                case .cancelled:
+                    (weakSubscription as? Subscription)?.cancel()
+                case .finished:
+                    subject.send(completion: .finished)
+                case .failed:
+                    subject.send(completion: .failure(.oops))
+                }
+
+                XCTAssertNil(weakSubscriber, "Subscriber leaked - \(reason)")
+                XCTAssertNil(weakSubscription, "Subscription leaked - \(reason)")
             }
+
+            XCTAssertNil(weakSubject, "Subject leaked - \(reason)")
+        }
+    }
+
+    func testConduitReflection() throws {
+        try testSubscriptionReflection(
+            description: "CurrentValueSubject",
+            customMirror: expectedChildren(
+                ("parent", .contains("CurrentValueSubject")),
+                ("downstream", .contains("TrackingSubscriberBase")),
+                ("demand", "max(0)"),
+                ("subject", .contains("CurrentValueSubject"))
+            ),
+            playgroundDescription:  "CurrentValueSubject",
+            sut: CurrentValueSubject<Int, Error>(42)
         )
-
-        XCTAssertEqual(subscriptions.value.count, 200)
-
-        race(
-            {
-                cvs.value = 42
-            },
-            {
-                cvs.value = 42
-            }
-        )
-
-        XCTAssertEqual(inputs.value.count, 40200)
-        XCTAssertEqual(cvs.value, 42)
-
-        race(
-            {
-                subscriptions.value[0].request(.max(4))
-            },
-            {
-                subscriptions.value[0].request(.max(10))
-            }
-        )
-
-        race(
-            {
-                cvs.send(completion: .finished)
-            },
-            {
-                cvs.send(completion: .failure(""))
-            }
-        )
-
-        XCTAssertEqual(completions.value.count, 200)
     }
 }

Tests/OpenCombineTests/FoundationTests/OperationQueueSchedulerTests.swift

@@ -0,0 +1,453 @@
+//
+//  OperationQueueSchedulerTests.swift
+//  
+//
+//  Created by Sergej Jaskiewicz on 14.06.2020.
+//
+
+import Foundation
+import XCTest
+
+#if OPENCOMBINE_COMPATIBILITY_TEST
+import Combine
+#else
+import OpenCombine
+import OpenCombineFoundation
+#endif
+
+@available(macOS 10.15, iOS 13.0, *)
+final class OperationQueueSchedulerTests: XCTestCase {
+
+    // MARK: - Scheduler.SchedulerTimeType
+
+    func testSchedulerTimeTypeDistance() {
+        RunLoopSchedulerTests.testSchedulerTimeTypeDistance(OperationQueueScheduler.self)
+    }
+
+    func testSchedulerTimeTypeAdvanced() {
+        RunLoopSchedulerTests.testSchedulerTimeTypeAdvanced(OperationQueueScheduler.self)
+    }
+
+    func testSchedulerTimeTypeEquatable() {
+        RunLoopSchedulerTests.testSchedulerTimeTypeEquatable(OperationQueueScheduler.self)
+    }
+
+    func testSchedulerTimeTypeCodable() throws {
+        try RunLoopSchedulerTests
+            .testSchedulerTimeTypeCodable(OperationQueueScheduler.self)
+    }
+
+    // MARK: - Scheduler.SchedulerTimeType.Stride
+
+    func testStrideToTimeInterval() {
+        RunLoopSchedulerTests.testStrideToTimeInterval(OperationQueueScheduler.self)
+    }
+
+    func testStrideFromTimeInterval() {
+        RunLoopSchedulerTests.testStrideFromTimeInterval(OperationQueueScheduler.self)
+    }
+
+    func testStrideFromNumericValue() {
+        RunLoopSchedulerTests.testStrideFromNumericValue(OperationQueueScheduler.self)
+    }
+
+    func testStrideComparable() {
+        RunLoopSchedulerTests.testStrideComparable(OperationQueueScheduler.self)
+    }
+
+    func testStrideMultiplication() {
+        RunLoopSchedulerTests.testStrideMultiplication(OperationQueueScheduler.self)
+    }
+
+    func testStrideAddition() {
+        RunLoopSchedulerTests.testStrideAddition(OperationQueueScheduler.self)
+    }
+
+    func testStrideSubtraction() {
+        RunLoopSchedulerTests.testStrideSubtraction(OperationQueueScheduler.self)
+    }
+
+    func testStrideCodable() throws {
+        try RunLoopSchedulerTests.testStrideCodable(OperationQueueScheduler.self)
+    }
+
+    // MARK: - Scheduler
+
+#if canImport(Darwin)
+    // FIXME: These tests crash with swift-corelibs-foundation.
+    // The issue has been resolved in
+    // https://github.com/apple/swift-corelibs-foundation/pull/2779
+    // but it hasn't made it into an official release yet.
+
+    func testScheduleActionOnceNowWithTestQueue() {
+        let queue = TestOperationQueue()
+        let scheduler = makeScheduler(queue)
+
+        let counter = Atomic(0)
+        scheduler.schedule {
+            counter.do { $0 += 1 }
+        }
+
+        XCTAssertEqual(queue.history.count, 1)
+
+        guard case let .addOperation(op as BlockOperation)? = queue.history.first else {
+            XCTFail("Unexpected history")
+            return
+        }
+
+        queue.waitUntilAllOperationsAreFinished()
+
+        XCTAssertEqual(counter.value, 1)
+        op.main()
+        XCTAssertEqual(counter.value, 2)
+        op.main()
+        XCTAssertEqual(counter.value, 3)
+    }
+
+    func testScheduleActionOnceNowWithRealQueue() {
+        let mainQueue = OperationQueue.main
+        let now = Date()
+        var actualDate = Date.distantPast
+        executeOnBackgroundThread {
+            makeScheduler(mainQueue).schedule {
+                XCTAssertTrue(Thread.isMainThread)
+                actualDate = Date()
+                XCTAssertNotNil(OperationQueue.current)
+                RunLoop.current.run(until: Date() + 0.01)
+            }
+        }
+
+        XCTAssertEqual(actualDate, .distantPast)
+        RunLoop.main.run(until: Date() + 0.05)
+        XCTAssertEqual(actualDate.timeIntervalSinceReferenceDate,
+                       now.timeIntervalSinceReferenceDate,
+                       accuracy: 0.1)
+    }
+
+    func testScheduleActionOnceLaterWithTestQueue() {
+        let queue = TestOperationQueue()
+        let scheduler = makeScheduler(queue)
+        let desiredDelay: TimeInterval = 0.6
+
+        let counter = Atomic(0)
+        scheduler.schedule(after: scheduler.now.advanced(by: .init(desiredDelay))) {
+            counter.do { $0 += 1 }
+        }
+
+        XCTAssertEqual(queue.history.count, 1)
+
+        guard case let .addOperation(op)? = queue.history.first else {
+            XCTFail("Unexpected history")
+            return
+        }
+        XCTAssertFalse(op is BlockOperation)
+        XCTAssertFalse(op.isReady)
+        XCTAssertFalse(op.isFinished)
+        XCTAssertFalse(op.isCancelled)
+        XCTAssertFalse(op.isAsynchronous)
+        XCTAssertFalse(op.isConcurrent)
+        XCTAssert(op is Cancellable)
+
+        XCTAssertEqual(counter.value, 0)
+        let now = Date()
+        queue.waitUntilAllOperationsAreFinished()
+        XCTAssertEqual(counter.value, 1)
+        XCTAssertEqual(Date().timeIntervalSinceReferenceDate,
+                       (now + desiredDelay).timeIntervalSinceReferenceDate,
+                       accuracy: desiredDelay / 3)
+
+        assertCrashes {
+            op.main()
+        }
+    }
+
+    func testScheduleActionOnceLaterWithRealQueue() {
+        let mainQueue = OperationQueue.main
+        let startDate = Date()
+        var actualDate = Date.distantPast
+        let desiredDelay: TimeInterval = 2
+        executeOnBackgroundThread {
+            let scheduler = makeScheduler(mainQueue)
+            scheduler
+                .schedule(after: scheduler.now.advanced(by: .init(desiredDelay))) {
+                    XCTAssertTrue(Thread.isMainThread)
+                    actualDate = Date()
+                    XCTAssertNotNil(OperationQueue.current)
+                }
+        }
+
+        XCTAssertEqual(actualDate, .distantPast)
+        RunLoop.main.run(until: Date() + desiredDelay * 2)
+        XCTAssertEqual(
+            actualDate.timeIntervalSinceReferenceDate -
+                startDate.timeIntervalSinceReferenceDate,
+            desiredDelay,
+            accuracy: desiredDelay / 3
+        )
+    }
+
+    func testScheduleRepeatingWithTestQueue() {
+        let queue = TestOperationQueue()
+        let scheduler = makeScheduler(queue)
+        let desiredDelay: TimeInterval = 0.7
+        let desiredInterval: TimeInterval = 0.3
+
+        let counter = Atomic(0)
+        let cancellable = scheduler
+            .schedule(after: scheduler.now.advanced(by: .init(desiredDelay)),
+                      interval: .init(desiredInterval)) {
+                counter.do { $0 += 1 }
+            }
+
+        XCTAssertEqual(queue.history.count, 1)
+
+        guard case let .addOperation(op)? = queue.history.first else {
+            XCTFail("Unexpected history")
+            return
+        }
+        XCTAssertFalse(op is BlockOperation)
+        XCTAssertFalse(op.isReady)
+        XCTAssertFalse(op.isFinished)
+        XCTAssertFalse(op.isCancelled)
+        XCTAssertFalse(op.isAsynchronous)
+        XCTAssertFalse(op.isConcurrent)
+        XCTAssert(op is Cancellable)
+        XCTAssert(cancellable is AnyCancellable)
+
+        XCTAssertEqual(counter.value, 0)
+        let now = Date()
+        queue.waitUntilAllOperationsAreFinished()
+        XCTAssertEqual(counter.value, 1)
+        let expectedDelay = desiredDelay + desiredInterval
+        XCTAssertEqual(Date().timeIntervalSinceReferenceDate,
+                       (now + expectedDelay).timeIntervalSinceReferenceDate,
+                       accuracy: expectedDelay / 3)
+
+        assertCrashes {
+            op.main()
+        }
+    }
+
+    func testScheduleRepeatingWithRealQueue() {
+        let mainQueue = OperationQueue.main
+        let startDate = Date()
+
+        let desiredDelay: TimeInterval = 0.7
+        let desiredInterval: TimeInterval = 0.3
+
+        let ticks = Atomic([TimeInterval]())
+
+        let cancellable = executeOnBackgroundThread { () -> Cancellable in
+            let scheduler = makeScheduler(mainQueue)
+            return scheduler
+                .schedule(after: scheduler.now.advanced(by: .init(desiredDelay)),
+                          interval: .init(desiredInterval)) {
+                    XCTAssertTrue(Thread.isMainThread)
+                    ticks.do { $0.append(Date().timeIntervalSinceReferenceDate) }
+                    XCTAssertNotNil(OperationQueue.current)
+                }
+        }
+
+        XCTAssert(cancellable is AnyCancellable)
+
+        XCTAssertEqual(ticks.value.count, 0)
+        RunLoop.main.run(until: Date() + 0.001)
+        XCTAssertEqual(ticks.value.count, 0)
+
+        // The OperationQueue scheduler doesn't repeat actions.
+        // Wait some extra time to make sure this is the case.
+        RunLoop.main.run(until: Date() + desiredDelay + desiredInterval * 5)
+
+        if ticks.value.isEmpty {
+            XCTFail("The scheduler doesn't work")
+            return
+        }
+
+        XCTAssertEqual(ticks.value.count, 1)
+
+        let expectedDelay = desiredDelay + desiredInterval
+        XCTAssertEqual(
+            ticks.value[0],
+            (startDate + expectedDelay).timeIntervalSinceReferenceDate,
+            accuracy: expectedDelay / 3
+        )
+    }
+#endif // canImport(Darwin)
+
+    func testMinimumTolerance() {
+        let scheduler = makeScheduler(.main)
+        XCTAssertEqual(scheduler.minimumTolerance, .init(0))
+    }
+
+    func testNow() {
+        let scheduler = makeScheduler(.main)
+        XCTAssertEqual(scheduler.now.date.timeIntervalSinceReferenceDate,
+                       Date().timeIntervalSinceReferenceDate,
+                       accuracy: 0.001)
+    }
+}
+
+#if OPENCOMBINE_COMPATIBILITY_TEST || !canImport(Combine)
+
+private typealias OperationQueueScheduler = OperationQueue
+
+private func makeScheduler(_ queue: OperationQueue) -> OperationQueueScheduler {
+    return queue
+}
+
+#else
+
+private typealias OperationQueueScheduler = OperationQueue.OCombine
+
+private func makeScheduler(_ queue: OperationQueue) -> OperationQueueScheduler {
+    return queue.ocombine
+}
+
+#endif
+
+@available(macOS 10.15, iOS 13.0, *)
+extension OperationQueueScheduler.SchedulerTimeType.Stride
+    : TimeIntervalBackedSchedulerStride
+{}
+
+@available(macOS 10.15, iOS 13.0, *)
+extension OperationQueueScheduler.SchedulerTimeType: DateBackedSchedulerTimeType {}
+
+extension OperationQueueScheduler: RunLoopLikeScheduler {}
+
+private final class TestOperationQueue: OperationQueue {
+
+    enum Event {
+        case progress
+        case addOperation(Operation)
+        case addOperations([Operation], waitUntilFinished: Bool)
+        case addBlockOperation(() -> Void)
+        case addBarrierBloack(() -> Void)
+        case getMaxConcurrentOperationCount
+        case setMaxConcurrentOperationCount(Int)
+        case getIsSuspended
+        case setIsSuspended(Bool)
+        case getName
+        case setName(String?)
+        case getQualityOfService
+        case setQualityOfService(QualityOfService)
+        case getUnderlyingQueue
+        case setUnderlyingQueue(DispatchQueue?)
+        case cancelAllOperations
+        case waitUntilAllOperationsAreFinished
+        case operations
+        case operationCount
+    }
+
+    private(set) var history = [Event]()
+
+#if swift(>=5.1)
+    @available(macOS 10.15, iOS 13.0, *)
+    override var progress: Progress {
+        history.append(.progress)
+        return super.progress
+    }
+#endif // swift(>=5.1)
+
+    override func addOperation(_ op: Operation) {
+        history.append(.addOperation(op))
+        super.addOperation(op)
+    }
+
+    override func addOperations(_ ops: [Operation], waitUntilFinished wait: Bool) {
+        history.append(.addOperations(ops, waitUntilFinished: wait))
+        super.addOperations(ops, waitUntilFinished: wait)
+    }
+
+    override func addOperation(_ block: @escaping () -> Void) {
+        history.append(.addBlockOperation(block))
+        super.addOperation(block)
+    }
+
+#if swift(>=5.1)
+    @available(macOS 10.15, iOS 13.0, *)
+    override func addBarrierBlock(_ barrier: @escaping () -> Void) {
+        history.append(.addBarrierBloack(barrier))
+        super.addBarrierBlock(barrier)
+    }
+#endif // swift(>=5.1)
+
+    override var maxConcurrentOperationCount: Int {
+        get {
+            history.append(.getMaxConcurrentOperationCount)
+            return super.maxConcurrentOperationCount
+        }
+        set {
+            history.append(.setMaxConcurrentOperationCount(newValue))
+            super.maxConcurrentOperationCount = newValue
+        }
+    }
+
+    override var isSuspended: Bool {
+        get {
+            history.append(.getIsSuspended)
+            return super.isSuspended
+        }
+        set {
+            history.append(.setIsSuspended(newValue))
+            super.isSuspended = newValue
+        }
+    }
+
+    override var name: String? {
+        get {
+            history.append(.getName)
+            return super.name
+        }
+        set {
+            history.append(.setName(newValue))
+            super.name = newValue
+        }
+    }
+
+    override var qualityOfService: QualityOfService {
+        get {
+            history.append(.getQualityOfService)
+            return super.qualityOfService
+        }
+        set {
+            history.append(.setQualityOfService(newValue))
+            super.qualityOfService = newValue
+        }
+    }
+
+    override var underlyingQueue: DispatchQueue? {
+        get {
+            history.append(.getUnderlyingQueue)
+            return super.underlyingQueue
+        }
+        set {
+            history.append(.setUnderlyingQueue(newValue))
+            super.underlyingQueue = newValue
+        }
+    }
+
+    override func cancelAllOperations() {
+        history.append(.cancelAllOperations)
+        super.cancelAllOperations()
+    }
+
+    override func waitUntilAllOperationsAreFinished() {
+        history.append(.waitUntilAllOperationsAreFinished)
+        super.waitUntilAllOperationsAreFinished()
+    }
+
+    // These properties are declared in an extension in swift-corelibs-foundation,
+    // so they can't be overridden.
+#if canImport(Darwin)
+    override var operations: [Operation] {
+        history.append(.operations)
+        return super.operations
+    }
+
+    override var operationCount: Int {
+        history.append(.operationCount)
+        return super.operationCount
+    }
+#endif // canImport(Darwin)
+}

Tests/OpenCombineTests/FoundationTests/RunLoopSchedulerTests.swift

@@ -0,0 +1,633 @@
+//
+//  RunLoopSchedulerTests.swift
+//  
+//
+//  Created by Sergej Jaskiewicz on 14.12.2019.
+//
+
+import Foundation
+import XCTest
+
+#if OPENCOMBINE_COMPATIBILITY_TEST
+import Combine
+#else
+import OpenCombine
+import OpenCombineFoundation
+#endif
+
+@available(macOS 10.15, iOS 13.0, *)
+final class RunLoopSchedulerTests: XCTestCase {
+
+    // MARK: - Scheduler.SchedulerTimeType
+
+    func testSchedulerTimeTypeDistance() {
+        RunLoopSchedulerTests.testSchedulerTimeTypeDistance(RunLoopScheduler.self)
+    }
+
+    static func testSchedulerTimeTypeDistance<Context: RunLoopLikeScheduler>(
+        _ schedulerType: Context.Type
+    ) {
+        let time1 = Context.SchedulerTimeType(Date(timeIntervalSince1970: 10_000))
+        let time2 = Context.SchedulerTimeType(Date(timeIntervalSince1970: 10_431))
+        let distantFuture = Context.SchedulerTimeType(.distantFuture)
+        let notSoDistantFuture = Context.SchedulerTimeType(
+            Date.distantFuture - 1024
+        )
+
+        XCTAssertEqual(time1.distance(to: time2).timeInterval, 431)
+        XCTAssertEqual(time2.distance(to: time1).timeInterval, -431)
+
+        XCTAssertEqual(time1.distance(to: distantFuture).timeInterval, 64_092_201_200)
+        XCTAssertEqual(distantFuture.distance(to: time1).timeInterval, -64_092_201_200)
+        XCTAssertEqual(time2.distance(to: distantFuture).timeInterval, 64_092_200_769)
+        XCTAssertEqual(distantFuture.distance(to: time2).timeInterval, -64_092_200_769)
+
+        XCTAssertEqual(time1.distance(to: notSoDistantFuture).timeInterval,
+                       64_092_200_176)
+        XCTAssertEqual(notSoDistantFuture.distance(to: time1).timeInterval,
+                       -64_092_200_176)
+        XCTAssertEqual(time2.distance(to: notSoDistantFuture).timeInterval,
+                       64_092_199_745)
+        XCTAssertEqual(notSoDistantFuture.distance(to: time2).timeInterval,
+                       -64_092_199_745)
+
+        XCTAssertEqual(distantFuture.distance(to: distantFuture).timeInterval,
+                       0)
+        XCTAssertEqual(notSoDistantFuture.distance(to: notSoDistantFuture).timeInterval,
+                       0)
+    }
+
+    func testSchedulerTimeTypeAdvanced() {
+        RunLoopSchedulerTests.testSchedulerTimeTypeAdvanced(RunLoopScheduler.self)
+    }
+
+    static func testSchedulerTimeTypeAdvanced<Context: RunLoopLikeScheduler>(
+        _ schedulerType: Context.Type
+    ) {
+        let time =
+            Context.SchedulerTimeType(Date(timeIntervalSinceReferenceDate: 10_000))
+        let beginningOfTime =
+            Context.SchedulerTimeType(Date(timeIntervalSinceReferenceDate: 1))
+        let stride1 = Context.SchedulerTimeType.Stride.seconds(431)
+        let stride2 = Context.SchedulerTimeType.Stride.seconds(-220)
+
+        XCTAssertEqual(time.advanced(by: stride1),
+                       .init(Date(timeIntervalSinceReferenceDate: 10431)))
+
+        XCTAssertEqual(time.advanced(by: stride2),
+                       .init(Date(timeIntervalSinceReferenceDate: 9780)))
+
+#if arch(x86_64) || arch(arm64) || arch(s390x) || arch(powerpc64) || arch(powerpc64le)
+        // 64-bit platforms
+        XCTAssertEqual(time.advanced(by: .nanoseconds(.max)).date,
+                       Date(timeIntervalSinceReferenceDate: 9223382036.854776))
+
+        XCTAssertEqual(time.advanced(by: .seconds(.max)).date,
+                       Date(timeIntervalSinceReferenceDate: 9.223372036854786E+18))
+#elseif arch(i386) || arch(arm)
+        // 32-bit platforms
+        XCTAssertEqual(time.advanced(by: .nanoseconds(.max)).date,
+                       Date(timeIntervalSinceReferenceDate: 10002.147483647))
+
+        XCTAssertEqual(time.advanced(by: .seconds(.max)).date,
+                       Date(timeIntervalSinceReferenceDate: 2147493647))
+#else
+#error("This architecture isn't known. Add it to the 32-bit or 64-bit line.")
+#endif
+
+        XCTAssertEqual(beginningOfTime.advanced(by: .nanoseconds(-1000)).date,
+                       Date(timeIntervalSinceReferenceDate: 0.999999))
+
+        XCTAssertEqual(beginningOfTime.advanced(by: .seconds(-1000)).date,
+                       Date(timeIntervalSinceReferenceDate: -999.0))
+    }
+
+    func testSchedulerTimeTypeEquatable() {
+        RunLoopSchedulerTests.testSchedulerTimeTypeEquatable(RunLoopScheduler.self)
+    }
+
+    static func testSchedulerTimeTypeEquatable<Context: RunLoopLikeScheduler>(
+        _ schedulerType: Context.Type
+    ) {
+        let time1 = Context.SchedulerTimeType(Date(timeIntervalSinceReferenceDate: 10000))
+        let time2 = Context.SchedulerTimeType(Date(timeIntervalSinceReferenceDate: 10000))
+        let time3 = Context.SchedulerTimeType(Date(timeIntervalSinceReferenceDate: 10001))
+
+        XCTAssertEqual(time1, time1)
+        XCTAssertEqual(time2, time2)
+        XCTAssertEqual(time3, time3)
+
+        XCTAssertEqual(time1, time2)
+        XCTAssertEqual(time2, time1)
+        XCTAssertNotEqual(time1, time3)
+        XCTAssertNotEqual(time3, time1)
+    }
+
+    func testSchedulerTimeTypeCodable() throws {
+        try RunLoopSchedulerTests.testSchedulerTimeTypeCodable(RunLoopScheduler.self)
+    }
+
+    static func testSchedulerTimeTypeCodable<Context: RunLoopLikeScheduler>(
+        _ schedulerType: Context.Type
+    ) throws {
+        let encoder = JSONEncoder()
+        let decoder = JSONDecoder()
+
+        let time =
+            Context.SchedulerTimeType(Date(timeIntervalSinceReferenceDate: 1024.75))
+        let encodedData = try encoder
+            .encode(time)
+        let encodedString = String(decoding: encodedData, as: UTF8.self)
+
+        XCTAssertEqual(encodedString,
+                       #"{"date":1024.75}"#)
+
+        let decodedTime = try decoder
+            .decode(Context.SchedulerTimeType.self, from: encodedData)
+
+        XCTAssertEqual(decodedTime, time)
+    }
+
+    // MARK: - Scheduler.SchedulerTimeType.Stride
+
+    func testStrideToTimeInterval() {
+        RunLoopSchedulerTests.testStrideToTimeInterval(RunLoopScheduler.self)
+    }
+
+    static func testStrideToTimeInterval<Context: RunLoopLikeScheduler>(
+        _ schedulerType: Context.Type
+    ) {
+        typealias Stride = Context.SchedulerTimeType.Stride
+        XCTAssertEqual(Stride.seconds(2).timeInterval, 2)
+        XCTAssertEqual(Stride.seconds(2.2).timeInterval, 2.2)
+        XCTAssertEqual(Stride.seconds(Double.infinity).timeInterval, .infinity)
+        XCTAssertEqual(Stride.milliseconds(2).timeInterval, 0.002)
+        XCTAssertEqual(Stride.microseconds(2).timeInterval, 2E-06)
+        XCTAssertEqual(Stride.nanoseconds(2).timeInterval, 2E-09)
+#if arch(x86_64) || arch(arm64) || arch(s390x) || arch(powerpc64) || arch(powerpc64le)
+        // 64-bit platforms
+        XCTAssertEqual(Stride.seconds(Int.max).timeInterval, 9.223372036854776E+18)
+        XCTAssertEqual(Stride.milliseconds(.max).timeInterval, 9.223372036854776E+15)
+        XCTAssertEqual(Stride.microseconds(.max).timeInterval, 9223372036854.775)
+        XCTAssertEqual(Stride.nanoseconds(.max).timeInterval, 9223372036.854776)
+#elseif arch(i386) || arch(arm)
+        // 32-bit platforms
+        XCTAssertEqual(Stride.seconds(Int.max).timeInterval, 2147483647)
+        XCTAssertEqual(Stride.milliseconds(.max).timeInterval, 2147483.647)
+        XCTAssertEqual(Stride.microseconds(.max).timeInterval, 2147.483647)
+        XCTAssertEqual(Stride.nanoseconds(.max).timeInterval, 2.147483647)
+#else
+#error("This architecture isn't known. Add it to the 32-bit or 64-bit line.")
+#endif
+    }
+
+    func testStrideFromTimeInterval() {
+        RunLoopSchedulerTests.testStrideFromTimeInterval(RunLoopScheduler.self)
+    }
+
+    static func testStrideFromTimeInterval<Context: RunLoopLikeScheduler>(
+        _ schedulerType: Context.Type
+    ) {
+        typealias Stride = Context.SchedulerTimeType.Stride
+        XCTAssertEqual(Stride(2).magnitude, 2)
+        XCTAssertEqual(Stride(2.2).magnitude, 2.2)
+        XCTAssertEqual(Stride(.infinity).magnitude, .infinity)
+        XCTAssertEqual(Stride(0.002).magnitude, 0.002)
+        XCTAssertEqual(Stride(2E-06).magnitude, 2E-06)
+        XCTAssertEqual(Stride(2E-09).magnitude, 2E-09)
+        XCTAssertEqual(Stride(9.223372036854776E+18).magnitude, 9.223372036854776E+18)
+    }
+
+    func testStrideFromNumericValue() {
+        RunLoopSchedulerTests.testStrideFromNumericValue(RunLoopScheduler.self)
+    }
+
+    static func testStrideFromNumericValue<Context: RunLoopLikeScheduler>(
+        _ schedulerType: Context.Type
+    ) {
+        typealias Stride = Context.SchedulerTimeType.Stride
+
+        XCTAssertEqual((1.2 as Stride).magnitude, 1.2)
+        XCTAssertEqual((2 as Stride).magnitude, 2)
+
+        XCTAssertNil(Stride(exactly: UInt64.max))
+        XCTAssertEqual(Stride(exactly: 871 as UInt64)?.magnitude, 871)
+    }
+
+    func testStrideComparable() {
+        RunLoopSchedulerTests.testStrideComparable(RunLoopScheduler.self)
+    }
+
+    static func testStrideComparable<Context: RunLoopLikeScheduler>(
+        _ schedulerType: Context.Type
+    ) {
+        typealias Stride = Context.SchedulerTimeType.Stride
+        XCTAssertLessThan(Stride.nanoseconds(1), .nanoseconds(2))
+        XCTAssertGreaterThan(Stride.nanoseconds(-2), .microseconds(-10))
+        XCTAssertLessThan(Stride.milliseconds(2), .seconds(2))
+    }
+
+    func testStrideMultiplication() {
+        RunLoopSchedulerTests.testStrideMultiplication(RunLoopScheduler.self)
+    }
+
+    static func testStrideMultiplication<Context: RunLoopLikeScheduler>(
+        _ schedulerType: Context.Type
+    ) {
+        typealias Stride = Context.SchedulerTimeType.Stride
+
+        XCTAssertEqual((Stride.nanoseconds(0) * .nanoseconds(61346)).magnitude, 0)
+        XCTAssertEqual((Stride.nanoseconds(61346) * .nanoseconds(0)).magnitude, 0)
+        XCTAssertEqual((Stride.nanoseconds(18) * .nanoseconds(1)).magnitude, 1.8E-17)
+        XCTAssertEqual((Stride.nanoseconds(18) * .microseconds(1)).magnitude, 1.8E-14)
+        XCTAssertEqual((Stride.nanoseconds(1) * .nanoseconds(18)).magnitude, 1.8E-17)
+        XCTAssertEqual((Stride.microseconds(1) * .nanoseconds(18)).magnitude, 1.8E-14)
+        XCTAssertEqual((Stride.nanoseconds(15) * .nanoseconds(2)).magnitude, 3E-17)
+        XCTAssertEqual((Stride.microseconds(-3) * .nanoseconds(10)).magnitude, -3E-14)
+
+        do {
+            var stride = Stride.nanoseconds(0)
+            stride *= .nanoseconds(61346)
+            XCTAssertEqual(stride.magnitude, 0)
+        }
+
+        do {
+            var stride = Stride.nanoseconds(61346)
+            stride *= .nanoseconds(0)
+            XCTAssertEqual(stride.magnitude, 0)
+        }
+
+        do {
+            var stride = Stride.nanoseconds(18)
+            stride *= .nanoseconds(1)
+            XCTAssertEqual(stride.magnitude, 1.8E-17)
+        }
+
+        do {
+            var stride = Stride.nanoseconds(18)
+            stride *= .microseconds(1)
+            XCTAssertEqual(stride.magnitude, 1.8E-14)
+        }
+
+        do {
+            var stride = Stride.nanoseconds(1)
+            stride *= .nanoseconds(18)
+            XCTAssertEqual(stride.magnitude, 1.8E-17)
+        }
+
+        do {
+            var stride = Stride.microseconds(1)
+            stride *= .nanoseconds(18)
+            XCTAssertEqual(stride.magnitude, 1.8E-14)
+        }
+
+        do {
+            var stride = Stride.nanoseconds(15)
+            stride *= .nanoseconds(2)
+            XCTAssertEqual(stride.magnitude, 3E-17)
+        }
+
+        do {
+            var stride = Stride.microseconds(-3)
+            stride *= .nanoseconds(10)
+            XCTAssertEqual(stride.magnitude, -3E-14)
+        }
+    }
+
+    func testStrideAddition() {
+        RunLoopSchedulerTests.testStrideAddition(RunLoopScheduler.self)
+    }
+
+    static func testStrideAddition<Context: RunLoopLikeScheduler>(
+        _ schedulerType: Context.Type
+    ) {
+        typealias Stride = Context.SchedulerTimeType.Stride
+
+        XCTAssertEqual((Stride.nanoseconds(0) + .microseconds(2)).magnitude, 2E-06)
+        XCTAssertEqual((Stride.nanoseconds(2) + .microseconds(0)).magnitude, 2E-09)
+        XCTAssertEqual((Stride.nanoseconds(7) + .nanoseconds(12)).magnitude,
+                       1.8999999999999998E-08)
+        XCTAssertEqual((Stride.nanoseconds(12) + .nanoseconds(7)).magnitude,
+                       1.8999999999999998E-08)
+        XCTAssertEqual((Stride.nanoseconds(7) + .nanoseconds(-12)).magnitude, -5E-09)
+        XCTAssertEqual((Stride.nanoseconds(-12) + .nanoseconds(7)).magnitude, -5E-09)
+        XCTAssertEqual((Stride.milliseconds(-12) + .seconds(7)).magnitude, 6.988)
+        XCTAssertEqual((Stride.seconds(-12) + .milliseconds(7)).magnitude, -11.993)
+
+        do {
+            var stride = Stride.nanoseconds(0)
+            stride += .microseconds(2)
+            XCTAssertEqual(stride.magnitude, 2E-06)
+        }
+
+        do {
+            var stride = Stride.nanoseconds(2)
+            stride += .microseconds(0)
+            XCTAssertEqual(stride.magnitude, 2E-09)
+        }
+
+        do {
+            var stride = Stride.nanoseconds(7)
+            stride += .nanoseconds(12)
+            XCTAssertEqual(stride.magnitude, 1.8999999999999998E-08)
+        }
+
+        do {
+            var stride = Stride.nanoseconds(12)
+            stride += .nanoseconds(7)
+            XCTAssertEqual(stride.magnitude, 1.8999999999999998E-08)
+        }
+
+        do {
+            var stride = Stride.nanoseconds(7)
+            stride += .nanoseconds(-12)
+            XCTAssertEqual(stride.magnitude, -5E-09)
+        }
+
+        do {
+            var stride = Stride.nanoseconds(-12)
+            stride += .nanoseconds(7)
+            XCTAssertEqual(stride.magnitude, -5E-09)
+        }
+
+        do {
+            var stride = Stride.seconds(-12)
+            stride += .milliseconds(7)
+            XCTAssertEqual(stride.magnitude, -11.993)
+        }
+
+        do {
+            var stride = Stride.milliseconds(-12)
+            stride += .seconds(7)
+            XCTAssertEqual(stride.magnitude, 6.988)
+        }
+    }
+
+    func testStrideSubtraction() {
+        RunLoopSchedulerTests.testStrideSubtraction(RunLoopScheduler.self)
+    }
+
+    static func testStrideSubtraction<Context: RunLoopLikeScheduler>(
+        _ schedulerType: Context.Type
+    ) {
+        typealias Stride = Context.SchedulerTimeType.Stride
+
+        XCTAssertEqual((Stride.nanoseconds(0) - .microseconds(2)).magnitude, -2E-06)
+        XCTAssertEqual((Stride.nanoseconds(2) - .microseconds(0)).magnitude, 2E-09)
+        XCTAssertEqual((Stride.nanoseconds(7) - .nanoseconds(12)).magnitude, -5E-09)
+        XCTAssertEqual((Stride.nanoseconds(12) - .nanoseconds(7)).magnitude, 5E-09)
+        XCTAssertEqual((Stride.nanoseconds(7) - .nanoseconds(-12)).magnitude,
+                       1.8999999999999998E-08)
+        XCTAssertEqual((Stride.nanoseconds(-12) - .nanoseconds(7)).magnitude,
+                       -1.8999999999999998E-08)
+        XCTAssertEqual((Stride.seconds(-12) - .milliseconds(7)).magnitude, -12.007)
+        XCTAssertEqual((Stride.milliseconds(-12) - .seconds(7)).magnitude, -7.012)
+
+        do {
+            var stride = Stride.nanoseconds(0)
+            stride -= .microseconds(2)
+            XCTAssertEqual(stride.magnitude, -2E-06)
+        }
+
+        do {
+            var stride = Stride.nanoseconds(2)
+            stride -= .microseconds(0)
+            XCTAssertEqual(stride.magnitude, 2E-09)
+        }
+
+        do {
+            var stride = Stride.nanoseconds(7)
+            stride -= .nanoseconds(12)
+            XCTAssertEqual(stride.magnitude, -5E-09)
+        }
+
+        do {
+            var stride = Stride.nanoseconds(12)
+            stride -= .nanoseconds(7)
+            XCTAssertEqual(stride.magnitude, 5E-09)
+        }
+
+        do {
+            var stride = Stride.nanoseconds(7)
+            stride -= .nanoseconds(-12)
+            XCTAssertEqual(stride.magnitude, 1.8999999999999998E-08)
+        }
+
+        do {
+            var stride = Stride.nanoseconds(-12)
+            stride -= .nanoseconds(7)
+            XCTAssertEqual(stride.magnitude, -1.8999999999999998E-08)
+        }
+
+        do {
+            var stride = Stride.seconds(-12)
+            stride -= .milliseconds(7)
+            XCTAssertEqual(stride.magnitude, -12.007)
+        }
+
+        do {
+            var stride = Stride.milliseconds(-12)
+            stride -= .seconds(7)
+            XCTAssertEqual(stride.magnitude, -7.012)
+        }
+    }
+
+    func testStrideCodable() throws {
+        try RunLoopSchedulerTests.testStrideCodable(RunLoopScheduler.self)
+    }
+
+    static func testStrideCodable<Context: RunLoopLikeScheduler>(
+        _ schedulerType: Context.Type
+    ) throws {
+        typealias Stride = Context.SchedulerTimeType.Stride
+
+        let encoder = JSONEncoder()
+        let decoder = JSONDecoder()
+
+        let stride = Stride.seconds(1024.5)
+        let encodedData = try encoder.encode(stride)
+        let encodedString = String(decoding: encodedData, as: UTF8.self)
+
+        XCTAssertEqual(encodedString, #"{"magnitude":1024.5}"#)
+
+        let decodedStride = try decoder
+            .decode(Stride.self, from: encodedData)
+
+        XCTAssertEqual(decodedStride, stride)
+    }
+
+    // MARK: - Scheduler
+
+    func testScheduleActionOnceNow() {
+        let mainRunLoop = RunLoop.main
+        let now = Date()
+        var actualDate = Date.distantPast
+        executeOnBackgroundThread {
+            makeScheduler(mainRunLoop).schedule {
+                XCTAssertTrue(Thread.isMainThread)
+                actualDate = Date()
+                RunLoop.current.run(until: Date() + 0.01)
+            }
+        }
+        XCTAssertEqual(actualDate, .distantPast)
+        mainRunLoop.run(until: Date() + 0.05)
+        XCTAssertEqual(actualDate.timeIntervalSinceReferenceDate,
+                       now.timeIntervalSinceReferenceDate,
+                       accuracy: 0.1)
+    }
+
+    func testScheduleActionOnceLater() {
+        let mainRunLoop = RunLoop.main
+        let now = Date()
+        var actualDate = Date.distantPast
+        let desiredDelay: TimeInterval = 0.6
+        executeOnBackgroundThread {
+            let scheduler = makeScheduler(mainRunLoop)
+            scheduler
+                .schedule(after: scheduler.now.advanced(by: .init(desiredDelay))) {
+                    // This is a bug in Combine! (FB7493579)
+                    // This should be XCTAssertTrue. When they fix it, this test will fail
+                    // and we'll know to fix our implementation.
+                    XCTAssertFalse(Thread.isMainThread)
+                    actualDate = Date()
+                }
+            RunLoop.current.run(until: Date() + 1)
+        }
+
+        XCTAssertEqual(
+            actualDate.timeIntervalSinceReferenceDate -
+                now.timeIntervalSinceReferenceDate,
+            desiredDelay,
+            accuracy: desiredDelay / 3
+        )
+    }
+
+    func testScheduleRepeating() {
+        let mainRunLoop = RunLoop.main
+
+        let expectation5ticks = expectation(description: "5 ticks")
+        expectation5ticks.expectedFulfillmentCount = 10
+
+        let startDate = Date().timeIntervalSinceReferenceDate
+
+        let ticks = Atomic([TimeInterval]())
+
+        let desiredDelay: TimeInterval = 0.7
+        let desiredInterval: TimeInterval = 0.3
+
+        let cancellable = executeOnBackgroundThread { () -> Cancellable in
+            let scheduler = makeScheduler(mainRunLoop)
+            return scheduler
+                .schedule(after: scheduler.now.advanced(by: .init(desiredDelay)),
+                          interval: .init(desiredInterval)) {
+                    XCTAssertTrue(Thread.isMainThread)
+                    ticks.do {
+                        $0.append(Date().timeIntervalSinceReferenceDate)
+                    }
+                    expectation5ticks.fulfill()
+                    RunLoop.current.run(until: Date() + 0.001)
+                }
+        }
+
+        XCTAssertEqual(ticks.value.count, 0)
+        mainRunLoop.run(until: Date() + 0.001)
+        XCTAssertEqual(ticks.value.count, 0)
+
+        wait(for: [expectation5ticks], timeout: 5)
+
+        if ticks.value.isEmpty {
+            XCTFail("The scheduler doesn't work")
+            return
+        }
+
+        let actualDelay = ticks.value[0] - startDate
+        let actualIntervals = zip(ticks.value.dropFirst(), ticks.value.dropLast()).map(-)
+        let averageInterval = actualIntervals.reduce(0, +) / Double(actualIntervals.count)
+
+        XCTAssertEqual(actualDelay,
+                       desiredDelay,
+                       accuracy: desiredDelay / 3,
+                       """
+                       Actual delay (\(actualDelay)) deviates from desired delay \
+                       (\(desiredDelay)) too much
+                       """)
+
+        XCTAssertEqual(averageInterval,
+                       desiredInterval,
+                       accuracy: desiredInterval / 3,
+                       """
+                       Actual average interval (\(averageInterval)) deviates from \
+                       desired interval (\(desiredInterval)) too much.
+
+                       Actual intervals: \(actualIntervals)
+                       """)
+
+        cancellable.cancel()
+        let numberOfTicksRightAfterCancellation = ticks.value.count
+        mainRunLoop.run(until: Date() + 1)
+        let numberOfTicksOneSecondAfterCancellation = ticks.value.count
+        XCTAssertEqual(numberOfTicksRightAfterCancellation,
+                       numberOfTicksOneSecondAfterCancellation)
+    }
+
+    func testMinimumTolerance() {
+        let scheduler = makeScheduler(.main)
+        XCTAssertEqual(scheduler.minimumTolerance, .init(0))
+    }
+
+    func testNow() {
+        let scheduler = makeScheduler(.main)
+        XCTAssertEqual(scheduler.now.date.timeIntervalSinceReferenceDate,
+                       Date().timeIntervalSinceReferenceDate,
+                       accuracy: 0.001)
+    }
+}
+
+#if OPENCOMBINE_COMPATIBILITY_TEST || !canImport(Combine)
+
+private typealias RunLoopScheduler = RunLoop
+
+private func makeScheduler(_ runLoop: RunLoop) -> RunLoopScheduler {
+    return runLoop
+}
+
+#else
+
+private typealias RunLoopScheduler = RunLoop.OCombine
+
+private func makeScheduler(_ runLoop: RunLoop) -> RunLoopScheduler {
+    return runLoop.ocombine
+}
+
+#endif
+
+protocol DateBackedSchedulerTimeType: Strideable, Codable, Hashable {
+    init(_ date: Date)
+
+    var date: Date { get }
+}
+
+protocol TimeIntervalBackedSchedulerStride: SchedulerTimeIntervalConvertible,
+                                            Comparable,
+                                            SignedNumeric,
+                                            ExpressibleByFloatLiteral,
+                                            Codable
+    where Magnitude == TimeInterval
+{
+    init(_ timeInterval: TimeInterval)
+
+    var timeInterval: TimeInterval { get }
+}
+
+protocol RunLoopLikeScheduler: Scheduler
+    where SchedulerTimeType: DateBackedSchedulerTimeType,
+          SchedulerTimeType.Stride: TimeIntervalBackedSchedulerStride {
+}
+
+@available(macOS 10.15, iOS 13.0, *)
+extension RunLoopScheduler.SchedulerTimeType.Stride: TimeIntervalBackedSchedulerStride {}
+
+@available(macOS 10.15, iOS 13.0, *)
+extension RunLoopScheduler.SchedulerTimeType: DateBackedSchedulerTimeType {}
+
+extension RunLoopScheduler: RunLoopLikeScheduler {}

Tests/OpenCombineTests/FoundationTests/TimerPublisherTests.swift

@@ -0,0 +1,224 @@
+//
+//  TimerPublisherTests.swift
+//  
+//
+//  Created by Sergej Jaskiewicz on 23.06.2020.
+//
+
+import Foundation
+import XCTest
+
+#if OPENCOMBINE_COMPATIBILITY_TEST
+import Combine
+#else
+import OpenCombine
+import OpenCombineFoundation
+#endif
+
+@available(macOS 10.15, iOS 13.0, *)
+final class TimerPublisherTests: XCTestCase {
+
+    func testPublishMethod() {
+        let publisher: TimerPublisher = Timer
+            .publish(every: 0.25,
+                     tolerance: 0.02,
+                     on: .main,
+                     in: RunLoop.Mode(rawValue: "testMode"),
+                     options: nil)
+
+        XCTAssertEqual(publisher.interval, 0.25)
+        XCTAssertEqual(publisher.tolerance, nil)
+        XCTAssertEqual(publisher.runLoop, .main)
+        XCTAssertEqual(publisher.mode, RunLoop.Mode(rawValue: "testMode"))
+        XCTAssertNil(publisher.options)
+    }
+
+    func testConnectAndPublish() {
+        let desiredInterval: TimeInterval = 0.5
+        var ticks = [TimeInterval]()
+
+        let tracking1 = TrackingSubscriberBase<Date, Never>(
+            receiveSubscription: {
+                $0.request(.max(3))
+            },
+            receiveValue: {
+                ticks.append($0.timeIntervalSinceReferenceDate)
+                return ticks.count < 3 ? .max(1) : .none
+            }
+        )
+
+        let tracking2 = TrackingSubscriberBase<Date, Never>(
+            receiveSubscription: {
+                $0.request(.max(2))
+            }
+        )
+        let tracking3 = TrackingSubscriberBase<Date, Never>(
+            receiveSubscription: {
+                $0.request(.max(1))
+            },
+            receiveValue: { _ in
+                ticks.count < 3 ? .max(1) : .none
+            }
+        )
+
+        let publisher: TimerPublisher = Timer
+            .publish(every: desiredInterval, on: .main, in: .default)
+
+        publisher.subscribe(tracking1)
+        publisher.subscribe(tracking2)
+        publisher.subscribe(tracking3)
+
+        XCTAssertEqual(tracking1.history, [.subscription("Timer")])
+
+        RunLoop.main.run(until: Date() + 1)
+
+        // Test that no output is produced until we connect
+        XCTAssertEqual(tracking1.history, [.subscription("Timer")])
+
+        let connection = publisher.connect()
+
+        RunLoop.main.run(until: Date() + 10)
+
+        assertCorrectIntervals(ticks: ticks,
+                               expectedNumberOfTicks: 10,
+                               desiredInterval: desiredInterval)
+
+        let fullHistory =
+            [TrackingSubscriberBase<Date, Never>.Event.subscription("Timer")] +
+                ticks.map { .value(Date(timeIntervalSinceReferenceDate: $0)) }
+
+        connection.cancel()
+        XCTAssert(connection is Subscription)
+
+        RunLoop.main.run(until: Date() + 1)
+
+        XCTAssertEqual(tracking1.history, fullHistory)
+        XCTAssertEqual(tracking2.history, fullHistory)
+        XCTAssertEqual(tracking3.history, fullHistory)
+    }
+
+    func testConnectAndCancelMultipleTimes() throws {
+        let publisher = TimerPublisher(interval: 0.25,
+                                       runLoop: .main,
+                                       mode: .default)
+
+        let tracking = TrackingSubscriberBase<Date, Never>()
+
+        publisher.subscribe(tracking)
+
+        let connection1 = publisher.connect()
+        let connection2 = publisher.connect()
+        XCTAssert((connection1 as AnyObject) === (connection2 as AnyObject))
+
+        connection1.cancel()
+        connection1.cancel()
+
+        let connection3 = try XCTUnwrap(publisher.connect() as? Subscription)
+        connection3.request(.max(1))
+
+        RunLoop.main.run(until: Date() + 0.3)
+
+        XCTAssertEqual(tracking.history, [.subscription("Timer")])
+    }
+
+    func testConnectionReflection() throws {
+        let publisher = TimerPublisher(interval: 0.25,
+                                       tolerance: 0.4,
+                                       runLoop: .main,
+                                       mode: .default,
+                                       options: nil)
+
+        let connection = publisher.connect()
+        defer { connection.cancel() }
+
+        XCTAssertEqual(
+            (connection as? CustomStringConvertible)?.description,
+            "Timer"
+        )
+        XCTAssertEqual(
+            (connection as? CustomPlaygroundDisplayConvertible)?
+                .playgroundDescription as? String,
+            "Timer"
+        )
+        XCTAssertFalse(connection is CustomDebugStringConvertible)
+
+        let connectionCombineID =
+            try XCTUnwrap(connection as? CustomCombineIdentifierConvertible)
+                .combineIdentifier
+
+        guard let inner = Mirror(reflecting: connection).descendant("some")
+        else {
+            XCTFail("Unexpected representation")
+            return
+        }
+
+        expectedChildren(
+            ("downstream", "Optional(Timer)"),
+            ("interval", "Optional(0.25)"),
+            ("tolerance", "Optional(0.4)")
+        )(Mirror(reflecting: inner))
+
+        connection.cancel()
+
+        expectedChildren(
+            ("downstream", "nil"),
+            ("interval", "nil"),
+            ("tolerance", "nil")
+        )(Mirror(reflecting: inner))
+
+        XCTAssert(inner is NSObject)
+
+        XCTAssertEqual(
+            (inner as? CustomStringConvertible)?.description,
+            "Timer"
+        )
+        XCTAssertEqual(
+            (inner as? CustomPlaygroundDisplayConvertible)?
+                .playgroundDescription as? String,
+            "Timer"
+        )
+        XCTAssertEqual(
+            (inner as? CustomDebugStringConvertible)?.debugDescription,
+            "Timer"
+        )
+
+        let innerCombineID =
+            try XCTUnwrap(inner as? CustomCombineIdentifierConvertible)
+                .combineIdentifier
+
+        XCTAssertEqual(connectionCombineID, innerCombineID)
+    }
+
+    private func assertCorrectIntervals(ticks: [TimeInterval],
+                                        expectedNumberOfTicks: Int,
+                                        desiredInterval: TimeInterval,
+                                        file: StaticString = #file,
+                                        line: UInt = #line) {
+        XCTAssertEqual(ticks.count, expectedNumberOfTicks, file: file, line: line)
+
+        if ticks.isEmpty { return }
+
+        let actualIntervals = zip(ticks.dropFirst(), ticks.dropLast()).map(-)
+        let averageInterval =
+            actualIntervals.reduce(0, +) / TimeInterval(actualIntervals.count)
+
+        XCTAssertEqual(averageInterval,
+                       desiredInterval,
+                       accuracy: desiredInterval / 2,
+                       """
+                       Actual average interval (\(averageInterval)) deviates from \
+                       desired interval (\(desiredInterval)) too much.
+
+                       Actual intervals: \(actualIntervals)
+                       """,
+                       file: file,
+                       line: line)
+    }
+}
+
+#if OPENCOMBINE_COMPATIBILITY_TEST || !canImport(Combine)
+@available(macOS 10.15, iOS 13.0, *)
+private typealias TimerPublisher = Timer.TimerPublisher
+#else
+private typealias TimerPublisher = Timer.OCombine.TimerPublisher
+#endif

Tests/OpenCombineTests/Helpers/CommonTests.swift

@@ -220,4 +220,8 @@ func unreachable<T>(_: T) -> Never {
     fatalError("unreachable")
 }
 
+func unreachable() -> Never {
+    fatalError("unreachable")
+}
+
 // swiftlint:enable generic_type_name

Tests/OpenCombineTests/Helpers/CustomPublisher.swift

@@ -44,10 +44,16 @@ class CustomPublisherBase<Output, Failure: Error>: Publisher, Cancellable {
 
     var didSubscribe: ((AnySubscriber<Output, Failure>) -> Void)?
 
+    var onDeinit: (() -> Void)?
+
     required init(subscription: Subscription?) {
         self.subscription = subscription
     }
 
+    deinit {
+        onDeinit?()
+    }
+
     func receive<Downstream: Subscriber>(subscriber: Downstream)
         where Failure == Downstream.Failure, Output == Downstream.Input
     {
@@ -81,7 +87,7 @@ class CustomPublisherBase<Output, Failure: Error>: Publisher, Cancellable {
 typealias CustomConnectablePublisher = CustomConnectablePublisherBase<Int, TestingError>
 
 @available(macOS 10.15, iOS 13.0, *)
-final class CustomConnectablePublisherBase<Output: Equatable, Failure: Error>
+final class CustomConnectablePublisherBase<Output, Failure: Error>
     : CustomPublisherBase<Output, Failure>,
       ConnectablePublisher
 {

Tests/OpenCombineTests/Helpers/CustomSubscription.swift

@@ -38,11 +38,18 @@ final class CustomSubscription: Subscription, CustomStringConvertible {
 
     var onRequest: ((Subscribers.Demand) -> Void)?
     var onCancel: (() -> Void)?
+    var onDeinit: (() -> Void)?
 
     init(onRequest: ((Subscribers.Demand) -> Void)? = nil,
-         onCancel: (() -> Void)? = nil) {
+         onCancel: (() -> Void)? = nil,
+         onDeinit: (() -> Void)? = nil) {
         self.onRequest = onRequest
         self.onCancel = onCancel
+        self.onDeinit = onDeinit
+    }
+
+    deinit {
+        onDeinit?()
     }
 
     var lastRequested: Subscribers.Demand? {

Tests/OpenCombineTests/Helpers/ExecuteOnBackgroundThread.swift

@@ -0,0 +1,83 @@
+//
+//  ExecuteOnBackgroundThread.swift
+//  
+//
+//  Created by Sergej Jaskiewicz on 04.02.2020.
+//
+
+#if canImport(Darwin)
+import Darwin
+#elseif canImport(Glibc)
+import Glibc
+#else
+#error("How to do threads on this platform?")
+#endif
+
+#if canImport(Darwin)
+private typealias ThreadPtr = UnsafeMutablePointer<pthread_t?>
+#else
+private typealias ThreadPtr = UnsafeMutablePointer<pthread_t>
+#endif
+
+func executeOnBackgroundThread<ResultType>(
+    _ body: () -> ResultType
+) -> ResultType {
+    return withoutActuallyEscaping(body) { body in
+
+        // We need this because @convention(c) closures can't capture generic params.
+        var typeErasedBody: () -> UnsafeMutableRawPointer = {
+            let resultPtr = UnsafeMutablePointer<ResultType>.allocate(capacity: 1)
+            resultPtr.initialize(to: body())
+            return UnsafeMutableRawPointer(resultPtr)
+        }
+
+        return withUnsafeMutablePointer(to: &typeErasedBody) { typeErasedBody in
+            let _backgroundThread = ThreadPtr.allocate(capacity: 1)
+
+            defer { _backgroundThread.deallocate() }
+
+            var status: Int32 = 0
+
+            // We could use Foundation's Thread, but it doesn't work on Linux for some
+            // reason.
+            status = pthread_create(
+                _backgroundThread,
+                nil,
+                { context in
+#if canImport(Darwin)
+                    let context = context
+#else
+                    let context = context!
+#endif
+                    return context
+                        .assumingMemoryBound(to: (() -> UnsafeMutableRawPointer).self)
+                        .pointee()
+                },
+                typeErasedBody
+            )
+
+            guard status == 0 else {
+                preconditionFailure("Could not create a background thread")
+            }
+
+#if canImport(Darwin)
+            guard let backgroundThread = _backgroundThread.pointee else {
+                preconditionFailure("Could not create a background thread")
+            }
+#else
+            let backgroundThread = _backgroundThread.pointee
+#endif
+
+            var _resultPtr: UnsafeMutableRawPointer?
+            status = pthread_join(backgroundThread, &_resultPtr)
+
+            guard status == 0, let resultPtr = _resultPtr else {
+                preconditionFailure("Could not join threads")
+            }
+
+            defer { resultPtr.deallocate() }
+
+            return resultPtr.assumingMemoryBound(to: ResultType.self).move()
+        }
+    }
+}

Tests/OpenCombineTests/Helpers/FairPriorityQueue.swift

@@ -26,8 +26,8 @@ struct FairPriorityQueue<Priority: Comparable, Element> {
         }
     }
 
-    func min() -> Element? {
-        return storage.first?.1
+    func min() -> (Priority, Element)? {
+        return storage.first.map { ($0.0.0, $0.1) }
     }
 
     @discardableResult

Tests/OpenCombineTests/Helpers/TestReflection.swift

@@ -68,7 +68,7 @@ func reduceLikeOperatorMirror(file: StaticString = #file,
         ("downstream", .contains("TrackingSubscriberBase")),
         ("result", .anything),
         ("initial", .anything),
-        ("status", .contains("awaitingSubscription")),
+        ("status", .anything),
         file: file,
         line: line
     )
@@ -83,6 +83,7 @@ internal func testReflection<Output, Failure: Error, Operator: Publisher>(
     description expectedDescription: String?,
     customMirror customMirrorPredicate: ((Mirror) -> Bool)?,
     playgroundDescription: String?,
+    subscriberIsAlsoSubscription: Bool = true,
     _ makeOperator: (CustomConnectablePublisherBase<Output, Failure>) -> Operator
 ) throws {
     let publisher = CustomConnectablePublisherBase<Output, Failure>(subscription: nil)
@@ -124,6 +125,43 @@ internal func testReflection<Output, Failure: Error, Operator: Publisher>(
         file: file,
         line: line
     )
+
+    if subscriberIsAlsoSubscription {
+        publisher.send(subscription: CustomSubscription())
+        let subscription = try XCTUnwrap(tracking.subscriptions.first?.underlying)
+
+        XCTAssertEqual((subscription as? CustomStringConvertible)?.description,
+                       expectedDescription,
+                       file: file,
+                       line: line)
+
+        if let customMirrorPredicate = customMirrorPredicate {
+            let customMirror =
+                try XCTUnwrap((subscription as? CustomReflectable)?.customMirror,
+                              file: file,
+                              line: line)
+            XCTAssert(customMirrorPredicate(customMirror),
+                      "customMirror doesn't satisfy the predicate",
+                      file: file,
+                      line: line)
+        } else {
+            XCTAssertFalse(subscription is CustomReflectable,
+                           "subscription shouldn't conform to CustomReflectable")
+        }
+
+        XCTAssertFalse(subscription is CustomDebugStringConvertible,
+                       "subscription shouldn't conform to CustomDebugStringConvertible",
+                       file: file,
+                       line: line)
+
+        XCTAssertEqual(
+            ((subscription as? CustomPlaygroundDisplayConvertible)?
+                .playgroundDescription as? String),
+            playgroundDescription,
+            file: file,
+            line: line
+        )
+    }
 }
 
 @available(macOS 10.15, iOS 13.0, *)
@@ -162,7 +200,7 @@ internal func testSubscriptionReflection<Sut: Publisher>(
     }
 
     XCTAssertFalse(subscription is CustomDebugStringConvertible,
-                   "subscriber shouldn't conform to CustomDebugStringConvertible",
+                   "Subscription shouldn't conform to CustomDebugStringConvertible",
                    file: file,
                    line: line)
 
@@ -174,3 +212,24 @@ internal func testSubscriptionReflection<Sut: Publisher>(
         line: line
     )
 }
+
+/// Prior to iOS 14 there was a bug in PassthroughSubject and
+/// CurrentValueSubject when after cancelling the subscription we couldn't
+/// reflect the subscription.
+@available(macOS, deprecated: 10.16, message: """
+If macOS 10.16/11.0 has already been released, this property should be removed
+""")
+@available(iOS, deprecated: 14, message: """
+If iOS 14  has already been released, this property should be removed
+""")
+var hasCustomMirrorUseAfterFreeBug: Bool { // swiftlint:disable:this let_var_whitespace
+#if OPENCOMBINE_COMPATIBILITY_TEST
+    if #available(macOS 10.16, iOS 14.0, *) {
+        return false
+    } else {
+        return true
+    }
+#else
+    return false
+#endif
+}

Tests/OpenCombineTests/Helpers/VirtualTimeScheduler.swift

@@ -11,6 +11,14 @@ import Combine
 import OpenCombine
 #endif
 
+@available(macOS 10.15, iOS 13.0, *)
+protocol CancellableTokenProtocol: Cancellable {
+
+    init(_ scheduler: VirtualTimeScheduler)
+
+    var isCancelled: Bool { get }
+}
+
 @available(macOS 10.15, iOS 13.0, *)
 final class VirtualTimeScheduler: Scheduler {
 
@@ -171,15 +179,42 @@ final class VirtualTimeScheduler: Scheduler {
         }
     }
 
-    private final class CancellableToken: Cancellable {
+    final class CancellableToken: CancellableTokenProtocol {
+
+        weak var scheduler: VirtualTimeScheduler?
 
         private(set) var isCancelled = false
 
+        init(_ scheduler: VirtualTimeScheduler) {
+            self.scheduler = scheduler
+        }
+
+        deinit {
+            scheduler?.cancellableTokenDeinitCount += 1
+        }
+
         func cancel() {
             isCancelled = true
         }
     }
 
+    final class NoopCancellableToken: CancellableTokenProtocol {
+
+        weak var scheduler: VirtualTimeScheduler?
+
+        init(_ scheduler: VirtualTimeScheduler) {
+            self.scheduler = scheduler
+        }
+
+        deinit {
+            scheduler?.cancellableTokenDeinitCount += 1
+        }
+
+        var isCancelled: Bool { return false }
+
+        func cancel() {}
+    }
+
     enum Event: Equatable, CustomStringConvertible {
         case now
         case minimumTolerance
@@ -221,7 +256,7 @@ final class VirtualTimeScheduler: Scheduler {
                 """
             case let .scheduleAfterDateWithInterval(date, interval, tolerance, options):
                 return """
-                .scheduleAfterDateWithInterval(\(describeDate(date)), \
+                .scheduleAfterDateWithInterval(\(describeDate(date))), \
                 interval: \(describeStride(interval)), \
                 tolerance: \(describeStride(tolerance)), \
                 options: \(describeOptions(options)))
@@ -239,6 +274,14 @@ final class VirtualTimeScheduler: Scheduler {
 
     private var workQueue = FairPriorityQueue<SchedulerTimeType, () -> Void>()
 
+    private let cancellableTokenType: CancellableTokenProtocol.Type
+
+    fileprivate(set) var cancellableTokenDeinitCount = 0
+
+    init(cancellableTokenType: CancellableTokenProtocol.Type = CancellableToken.self) {
+        self.cancellableTokenType = cancellableTokenType
+    }
+
     var scheduledDates: [SchedulerTimeType] {
         return workQueue.map { $0.0 }
     }
@@ -250,7 +293,7 @@ final class VirtualTimeScheduler: Scheduler {
 
     var minimumTolerance: SchedulerTimeType.Stride {
         history.append(.minimumTolerance)
-        return 0
+        return .nanoseconds(7)
     }
 
     func schedule(options: SchedulerOptions?, _ action: @escaping () -> Void) {
@@ -275,7 +318,7 @@ final class VirtualTimeScheduler: Scheduler {
                                                       interval: interval,
                                                       tolerance: tolerance,
                                                       options: options))
-        let cancellableToken = CancellableToken()
+        let cancellableToken = cancellableTokenType.init(self)
         repeatedlyExecute(after: date,
                           interval: interval,
                           cancellableToken: cancellableToken,
@@ -285,7 +328,7 @@ final class VirtualTimeScheduler: Scheduler {
 
     private func repeatedlyExecute(after date: SchedulerTimeType,
                                    interval: SchedulerTimeType.Stride,
-                                   cancellableToken: CancellableToken,
+                                   cancellableToken: CancellableTokenProtocol,
                                    action: @escaping () -> Void) {
         let enqueuedAction: () -> Void = { [unowned self] in
             if cancellableToken.isCancelled { return }
@@ -304,11 +347,20 @@ final class VirtualTimeScheduler: Scheduler {
     /// - Note: The actions that were already executed will not be executed again.
     ///   This function does **not** provide time machine-like functionality.
     func rewind(to time: SchedulerTimeType) {
+        if time > _now {
+            while let (nextActionTime, action) = workQueue.min(), nextActionTime <= time {
+                workQueue.extractMin()
+                _now = max(nextActionTime, _now)
+                action()
+            }
+        }
         _now = time
     }
 
-    func executeScheduledActions() {
-        while let (time, action) = workQueue.extractMin() {
+    func executeScheduledActions(until deadline: SchedulerTimeType = .nanoseconds(.max)) {
+        precondition(deadline >= _now)
+        while let (time, action) = workQueue.min(), time <= deadline {
+            workQueue.extractMin()
             _now = max(time, _now)
             action()
         }

Tests/OpenCombineTests/Helpers/XCTUnwrap.swift

@@ -7,7 +7,9 @@
 
 import XCTest
 
-// FIXME: XCTUnwrap is unavailable in Swift Package Manager yet.
+// XCTUnwrap is not available when using Swift Package Manager.
+// This has been fixed in Swift 5.2, but we want to maintain compatibility.
+#if swift(<5.2)
 
 private struct UnwrappingFailure: Error, LocalizedError {
 
@@ -52,3 +54,4 @@ public func XCTUnwrap<Result>(_ expression: @autoclosure () throws -> Result?,
         throw error
     }
 }
+#endif

Tests/OpenCombineTests/PassthroughSubjectTests.swift

@@ -105,18 +105,18 @@ final class PassthroughSubjectTests: XCTestCase {
     }
 
     func testSendFailureCompletion() {
-        let cvs = Sut()
+        let passthrough = Sut()
         let subscriber = TrackingSubscriber(
             receiveSubscription: { subscription in
                 subscription.request(.unlimited)
             }
         )
 
-        cvs.subscribe(subscriber)
+        passthrough.subscribe(subscriber)
 
         XCTAssertEqual(subscriber.history, [.subscription("PassthroughSubject")])
 
-        cvs.send(completion: .failure(.oops))
+        passthrough.send(completion: .failure(.oops))
 
         XCTAssertEqual(subscriber.history, [.subscription("PassthroughSubject"),
                                             .completion(.failure(.oops))])
@@ -173,6 +173,22 @@ final class PassthroughSubjectTests: XCTestCase {
         XCTAssertEqual(subscriber.subscriptions.count, 11)
         XCTAssertEqual(subscriber.inputs.count, 0)
         XCTAssertEqual(subscriber.completions.count, 0)
+
+        passthrough.send(0)
+
+        XCTAssertEqual(subscriber.inputs, [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0])
+
+        for (i, subscription) in subscriber.subscriptions.enumerated()
+            where i.isMultiple(of: 2)
+        {
+            subscription.cancel()
+        }
+        passthrough.send(1)
+
+        XCTAssertEqual(
+            subscriber.inputs,
+            [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1]
+        )
     }
 
     // Reactive Streams Spec: Rule #6
@@ -310,6 +326,97 @@ final class PassthroughSubjectTests: XCTestCase {
         XCTAssertEqual(subscription2.history, [.requested(.unlimited)])
     }
 
+    func testCompletion() throws {
+        let passthrough = Sut()
+        var downstreamSubscription: Subscription?
+        let tracking = TrackingSubscriber(
+            receiveSubscription: { downstreamSubscription = $0 }
+        )
+
+        passthrough.subscribe(tracking)
+
+        try XCTUnwrap(downstreamSubscription).request(.max(12))
+
+        passthrough.send(1)
+
+        expectedChildren(
+            ("parent", .contains("PassthroughSubject")),
+            ("downstream", .contains("TrackingSubscriberBase")),
+            ("demand", "max(11)"),
+            ("subject", .contains("PassthroughSubject"))
+        )(Mirror(reflecting: try XCTUnwrap(downstreamSubscription)))
+
+        passthrough.send(completion: .finished)
+
+        if !hasCustomMirrorUseAfterFreeBug {
+            expectedChildren(
+                ("parent", "nil"),
+                ("downstream", "nil"),
+                ("demand", "max(11)"),
+                ("subject", "nil")
+            )(Mirror(reflecting: try XCTUnwrap(downstreamSubscription)))
+        }
+
+        XCTAssertEqual(tracking.history, [.subscription("PassthroughSubject"),
+                                          .value(1),
+                                          .completion(.finished)])
+
+        passthrough.send(completion: .failure(.oops))
+        try XCTUnwrap(downstreamSubscription).cancel()
+        try XCTUnwrap(downstreamSubscription).request(.max(3))
+
+        if !hasCustomMirrorUseAfterFreeBug {
+            expectedChildren(
+                ("parent", "nil"),
+                ("downstream", "nil"),
+                ("demand", "max(11)"),
+                ("subject", "nil")
+            )(Mirror(reflecting: try XCTUnwrap(downstreamSubscription)))
+        }
+
+        XCTAssertEqual(tracking.history, [.subscription("PassthroughSubject"),
+                                          .value(1),
+                                          .completion(.finished)])
+    }
+
+    func testCancellation() throws {
+        let passthrough = Sut()
+        var downstreamSubscription: Subscription?
+        let tracking = TrackingSubscriber(
+            receiveSubscription: { downstreamSubscription = $0 }
+        )
+
+        passthrough.subscribe(tracking)
+
+        try XCTUnwrap(downstreamSubscription).request(.max(12))
+
+        passthrough.send(1)
+
+        expectedChildren(
+            ("parent", .contains("PassthroughSubject")),
+            ("downstream", .contains("TrackingSubscriberBase")),
+            ("demand", "max(11)"),
+            ("subject", .contains("PassthroughSubject"))
+        )(Mirror(reflecting: try XCTUnwrap(downstreamSubscription)))
+
+        try XCTUnwrap(downstreamSubscription).cancel()
+        try XCTUnwrap(downstreamSubscription).cancel()
+        try XCTUnwrap(downstreamSubscription).request(.max(3))
+        try XCTUnwrap(downstreamSubscription).request(.max(4))
+
+        if !hasCustomMirrorUseAfterFreeBug {
+            expectedChildren(
+                ("parent", "nil"),
+                ("downstream", "nil"),
+                ("demand", "max(11)"),
+                ("subject", "nil")
+            )(Mirror(reflecting: try XCTUnwrap(downstreamSubscription)))
+        }
+
+        XCTAssertEqual(tracking.history, [.subscription("PassthroughSubject"),
+                                          .value(1)])
+    }
+
     func testLifecycle() throws {
 
         var deinitCounter = 0
@@ -364,67 +471,76 @@ final class PassthroughSubjectTests: XCTestCase {
         XCTAssertEqual(deinitCounter, 2)
     }
 
-    func testSynchronization() {
-
-        let subscriptions = Atomic<[Subscription]>([])
-        let inputs =  Atomic<[Int]>([])
-        let completions = Atomic<[Subscribers.Completion<TestingError>]>([])
-
-        let passthrough = Sut()
-        let subscriber = AnySubscriber<Int, TestingError>(
-            receiveSubscription: { subscription in
-                subscriptions.do { $0.append(subscription) }
-                subscription.request(.unlimited)
-            },
-            receiveValue: { value in
-                inputs.do { $0.append(value) }
-                return .none
-            },
-            receiveCompletion: { completion in
-                completions.do { $0.append(completion) }
+    func testCancelsUpstreamSubscriptionsOnDeinit() {
+        let subscription = CustomSubscription()
+        do {
+            let passthrough = Sut()
+            for _ in 0 ..< 5 {
+                passthrough.send(subscription: subscription)
             }
-        )
+            XCTAssertEqual(subscription.history, [])
+        }
 
-        race(
-            {
-                passthrough.subscribe(subscriber)
-            },
-            {
-                passthrough.subscribe(subscriber)
+        XCTAssertEqual(subscription.history, [.cancelled,
+                                              .cancelled,
+                                              .cancelled,
+                                              .cancelled,
+                                              .cancelled])
+    }
+
+    func testReleasesEverythingOnTermination() {
+
+        enum TerminationReason: CaseIterable {
+            case cancelled
+            case finished
+            case failed
+        }
+
+        for reason in TerminationReason.allCases {
+            weak var weakSubscriber: TrackingSubscriber?
+            weak var weakSubject: Sut?
+            weak var weakSubscription: AnyObject?
+
+            do {
+                let subject = Sut()
+                do {
+                    let subscriber = TrackingSubscriber(receiveSubscription: {
+                        weakSubscription = $0 as AnyObject
+                    })
+                    weakSubscriber = subscriber
+                    weakSubject = subject
+
+                    subject.subscribe(subscriber)
+                }
+
+                switch reason {
+                case .cancelled:
+                    (weakSubscription as? Subscription)?.cancel()
+                case .finished:
+                    subject.send(completion: .finished)
+                case .failed:
+                    subject.send(completion: .failure(.oops))
+                }
+
+                XCTAssertNil(weakSubscriber, "Subscriber leaked - \(reason)")
+                XCTAssertNil(weakSubscription, "Subscription leaked - \(reason)")
             }
+
+            XCTAssertNil(weakSubject, "Subject leaked - \(reason)")
+        }
+    }
+
+    func testConduitReflection() throws {
+        try testSubscriptionReflection(
+            description: "PassthroughSubject",
+            customMirror: expectedChildren(
+                ("parent", .contains("PassthroughSubject")),
+                ("downstream", .contains("TrackingSubscriberBase")),
+                ("demand", "max(0)"),
+                ("subject", .contains("PassthroughSubject"))
+            ),
+            playgroundDescription:  "PassthroughSubject",
+            sut: PassthroughSubject<Int, Error>()
         )
-
-        XCTAssertEqual(subscriptions.value.count, 200)
-
-        race(
-            {
-                passthrough.send(31)
-            },
-            {
-                passthrough.send(42)
-            }
-        )
-
-        XCTAssertEqual(inputs.value.count, 40000)
-
-        race(
-            {
-                subscriptions.value[0].request(.max(4))
-            },
-            {
-                subscriptions.value[0].request(.max(10))
-            }
-        )
-
-        race(
-            {
-                passthrough.send(completion: .finished)
-            },
-            {
-                passthrough.send(completion: .failure(""))
-            }
-        )
-
-        XCTAssertEqual(completions.value.count, 200)
     }
 }

Tests/OpenCombineTests/PublisherTests/AssertNoFailureTests.swift

@@ -74,6 +74,7 @@ final class AssertNoFailureTests: XCTestCase {
                 ("prefix", "PREFIX")
             ),
             playgroundDescription: "AssertNoFailure",
+            subscriberIsAlsoSubscription: false,
             { $0.assertNoFailure("PREFIX", file: "SomeFile.swift", line: 1987) }
         )
     }

Tests/OpenCombineTests/PublisherTests/AutoconnectTests.swift

@@ -151,6 +151,7 @@ final class AutoconnectTests: XCTestCase {
                            description: "Autoconnect",
                            customMirror: customMirrorPredicate,
                            playgroundDescription: "Autoconnect",
+                           subscriberIsAlsoSubscription: false,
                            { $0.autoconnect() })
 
         let subscription = CustomSubscription()

Tests/OpenCombineTests/PublisherTests/BreakpointTests.swift

@@ -168,6 +168,7 @@ final class BreakpointTests: XCTestCase {
                                ("upstream", .contains("CustomConnectablePublisherBase"))
                            ),
                            playgroundDescription: "Breakpoint",
+                           subscriberIsAlsoSubscription: false,
                            { $0.breakpointOnError() })
     }
 }

Tests/OpenCombineTests/PublisherTests/BufferTests.swift

@@ -0,0 +1,953 @@
+//
+//  BufferTests.swift
+//  
+//
+//  Created by Sergej Jaskiewicz on 08.01.2020.
+//
+
+import XCTest
+
+#if OPENCOMBINE_COMPATIBILITY_TEST
+import Combine
+#else
+import OpenCombine
+#endif
+
+@available(macOS 10.15, iOS 13.0, *)
+final class BufferTests: XCTestCase {
+
+    func testInitialDemandWithKeepFullPrefetchStrategy() {
+        testInitialDemand(
+            withPrefetchStragety: .keepFull,
+            expectedSubscriptionHistory: [.requested(.max(42))]
+        )
+    }
+
+    func testInitialDemandWithByRequestPrefetchStrategy() {
+        testInitialDemand(
+            withPrefetchStragety: .byRequest,
+            expectedSubscriptionHistory: [.requested(.unlimited)]
+        )
+    }
+
+    func testBufferingInputDroppingNewest() throws {
+        try testBufferingInput(whenFull: .dropNewest)
+    }
+
+    func testBufferingInputDroppingOldest() throws {
+        try testBufferingInput(whenFull: .dropOldest)
+    }
+
+    func testBufferingInputFailingWhenBufferIsFull() throws {
+        try testBufferingInput(whenFull: .customError { .oops })
+    }
+
+    func testReceiveValueAfterFinishing() throws {
+        try testReceiveValueAfterCompleting(.finished)
+    }
+
+    func testReceiveValueAfterFailing() throws {
+        try testReceiveValueAfterCompleting(.failure(.oops))
+    }
+
+    func testDeadlockWhenErroringOnFullBuffer() {
+        var recursionCounter = 10
+        let helper = OperatorTestHelper(
+            publisherType: CustomPublisher.self,
+            initialDemand: .max(2),
+            receiveValueDemand: .none,
+            createSut: { publisher in
+                publisher.buffer(
+                    size: 0,
+                    prefetch: .keepFull,
+                    whenFull: .customError {
+                        if recursionCounter == 0 { return TestingError.oops }
+                        recursionCounter -= 1
+                        _ = publisher.send(1000)
+                        return TestingError.oops
+                    }
+                )
+            }
+        )
+
+        assertCrashes {
+            _ = helper.publisher.send(0)
+        }
+    }
+
+    func testRecursionByRequestDropNewest() {
+        testRecursion(prefetch: .byRequest, whenFull: .dropNewest)
+    }
+
+    func testRecursionByRequestDropOldest() {
+        testRecursion(prefetch: .byRequest, whenFull: .dropOldest)
+    }
+
+    func testRecursionByRequestCustomError() {
+        testRecursion(prefetch: .byRequest, whenFull: .customError { .oops })
+    }
+
+    func testRecursionKeepFullDropNewest() {
+        testRecursion(prefetch: .keepFull, whenFull: .dropNewest)
+    }
+
+    func testRecursionKeepFullDropOldest() {
+        testRecursion(prefetch: .keepFull, whenFull: .dropOldest)
+    }
+
+    func testRecursionKeepFullCustomError() {
+        testRecursion(prefetch: .keepFull, whenFull: .customError { .oops })
+    }
+
+    func testRequestingUnlimitedDemandByRequestDropNewest() {
+        testRequestingUnlimitedDemand(prefetch: .byRequest,
+                                      whenFull: .dropNewest)
+    }
+
+    func testRequestingUnlimitedDemandByRequestDropOldest() {
+        testRequestingUnlimitedDemand(prefetch: .byRequest,
+                                      whenFull: .dropOldest)
+    }
+
+    func testRequestingUnlimitedDemandByRequestCustomError() {
+        testRequestingUnlimitedDemand(prefetch: .byRequest,
+                                      whenFull: .customError { .oops })
+    }
+
+    func testRequestingUnlimitedDemandKeepFullDropNewest() {
+        testRequestingUnlimitedDemand(prefetch: .keepFull,
+                                      whenFull: .dropNewest)
+    }
+
+    func testRequestingUnlimitedDemandKeepFullDropOldest() {
+        testRequestingUnlimitedDemand(prefetch: .keepFull,
+                                      whenFull: .dropOldest)
+    }
+
+    func testRequestingUnlimitedDemandKeepFullCustomError() {
+        testRequestingUnlimitedDemand(prefetch: .keepFull,
+                                      whenFull: .customError { .oops })
+    }
+
+    func testRequestingFiniteDemandByRequestDropNewest() {
+        testRequestingFiniteDemand(prefetch: .byRequest, whenFull: .dropNewest)
+    }
+
+    func testRequestingFiniteDemandByRequestDropOldest() {
+        testRequestingFiniteDemand(prefetch: .byRequest, whenFull: .dropOldest)
+    }
+
+    func testRequestingFiniteDemandByRequestCustomError() {
+        testRequestingFiniteDemand(prefetch: .byRequest, whenFull: .customError { .oops })
+    }
+
+    func testRequestingFiniteDemandKeepFullDropNewest() {
+        testRequestingFiniteDemand(prefetch: .keepFull, whenFull: .dropNewest)
+    }
+
+    func testRequestingFiniteDemandKeepFullDropOldest() {
+        testRequestingFiniteDemand(prefetch: .keepFull, whenFull: .dropOldest)
+    }
+
+    func testRequestingFiniteDemandKeepFullCustomError() {
+        testRequestingFiniteDemand(prefetch: .keepFull, whenFull: .customError { .oops })
+    }
+
+    func testBufferByRequestDropNewestReceiveSubscriptionTwice() throws {
+        try testBufferReceiveSubscriptionTwice(prefetch: .byRequest,
+                                               whenFull: .dropNewest)
+    }
+
+    func testBufferByRequestDropOldestReceiveSubscriptionTwice() throws {
+        try testBufferReceiveSubscriptionTwice(prefetch: .byRequest,
+                                               whenFull: .dropOldest)
+    }
+
+    func testBufferByRequestCustomErrorReceiveSubscriptionTwice() throws {
+        try testBufferReceiveSubscriptionTwice(prefetch: .byRequest,
+                                               whenFull: .customError { .oops })
+    }
+
+    func testBufferKeepFullDropNewestReceiveSubscriptionTwice() throws {
+        try testBufferReceiveSubscriptionTwice(prefetch: .keepFull,
+                                               whenFull: .dropNewest)
+    }
+
+    func testBufferKeepFullDropOldestReceiveSubscriptionTwice() throws {
+        try testBufferReceiveSubscriptionTwice(prefetch: .keepFull,
+                                               whenFull: .dropOldest)
+    }
+
+    func testBufferKeepFullCustomErrorReceiveSubscriptionTwice() throws {
+        try testBufferReceiveSubscriptionTwice(prefetch: .keepFull,
+                                               whenFull: .customError { .oops })
+    }
+
+    func testBufferByRequestDropNewestReceiveValueBeforeSubscription() {
+        testBufferReceiveValueBeforeSubscription(prefetch: .byRequest,
+                                                 whenFull: .dropNewest)
+    }
+
+    func testBufferByRequestDropOldestReceiveValueBeforeSubscription() {
+        testBufferReceiveValueBeforeSubscription(prefetch: .byRequest,
+                                                 whenFull: .dropOldest)
+    }
+
+    func testBufferByRequestCustomErrorReceiveValueBeforeSubscription() {
+        testBufferReceiveValueBeforeSubscription(prefetch: .byRequest,
+                                                 whenFull: .customError(unreachable))
+    }
+
+    func testBufferKeepFullDropNewestReceiveValueBeforeSubscription() {
+        testBufferReceiveValueBeforeSubscription(prefetch: .keepFull,
+                                                 whenFull: .dropNewest)
+    }
+
+    func testBufferKeepFullDropOldestReceiveValueBeforeSubscription() {
+        testBufferReceiveValueBeforeSubscription(prefetch: .keepFull,
+                                                 whenFull: .dropOldest)
+    }
+
+    func testBufferKeepFullCustomErrorReceiveValueBeforeSubscription() {
+        testBufferReceiveValueBeforeSubscription(prefetch: .keepFull,
+                                                 whenFull: .customError(unreachable))
+    }
+
+    func testBufferByRequestDropNewestReceiveCompletionBeforeSubscription() {
+        testBufferReceiveCompletionBeforeSubscription(prefetch: .byRequest,
+                                                      whenFull: .dropNewest)
+    }
+
+    func testBufferByRequestDropOldestReceiveCompletionBeforeSubscription() {
+        testBufferReceiveCompletionBeforeSubscription(prefetch: .byRequest,
+                                                      whenFull: .dropOldest)
+    }
+
+    func testBufferByRequestCustomErrorReceiveCompletionBeforeSubscription() {
+        testBufferReceiveCompletionBeforeSubscription(prefetch: .byRequest,
+                                                      whenFull: .customError(unreachable))
+    }
+
+    func testBufferKeepFullDropNewestReceiveCompletionBeforeSubscription() {
+        testBufferReceiveCompletionBeforeSubscription(prefetch: .keepFull,
+                                                      whenFull: .dropNewest)
+    }
+
+    func testBufferKeepFullDropOldestReceiveCompletionBeforeSubscription() {
+        testBufferReceiveCompletionBeforeSubscription(prefetch: .keepFull,
+                                                      whenFull: .dropOldest)
+    }
+
+    func testBufferKeepFullCustomErrorReceiveCompletionBeforeSubscription() {
+        testBufferReceiveCompletionBeforeSubscription(prefetch: .keepFull,
+                                                      whenFull: .customError(unreachable))
+    }
+
+    func testBufferByRequestDropNewestRequestBeforeSubscription() {
+        testBufferRequestBeforeSubscription(prefetch: .byRequest,
+                                            whenFull: .dropNewest)
+    }
+
+    func testBufferByRequestDropOldestRequestBeforeSubscription() {
+        testBufferRequestBeforeSubscription(prefetch: .byRequest,
+                                            whenFull: .dropOldest)
+    }
+
+    func testBufferByRequestCustomErrorRequestBeforeSubscription() {
+        testBufferRequestBeforeSubscription(prefetch: .byRequest,
+                                            whenFull: .customError(unreachable))
+    }
+
+    func testBufferKeepFullDropNewestRequestBeforeSubscription() {
+        testBufferRequestBeforeSubscription(prefetch: .keepFull,
+                                            whenFull: .dropNewest)
+    }
+
+    func testBufferKeepFullDropOldestRequestBeforeSubscription() {
+        testBufferRequestBeforeSubscription(prefetch: .keepFull,
+                                            whenFull: .dropOldest)
+    }
+
+    func testBufferKeepFullCustomErrorRequestBeforeSubscription() {
+        testBufferRequestBeforeSubscription(prefetch: .keepFull,
+                                            whenFull: .customError(unreachable))
+    }
+
+    func testBufferByRequestDropNewestCancelBeforeSubscription() {
+        testBufferCancelBeforeSubscription(prefetch: .byRequest,
+                                           whenFull: .dropNewest)
+    }
+
+    func testBufferByRequestDropOldestCancelBeforeSubscription() {
+        testBufferCancelBeforeSubscription(prefetch: .byRequest,
+                                           whenFull: .dropOldest)
+    }
+
+    func testBufferByRequestCustomErrorCancelBeforeSubscription() {
+        testBufferCancelBeforeSubscription(prefetch: .byRequest,
+                                           whenFull: .customError(unreachable))
+    }
+
+    func testBufferKeepFullDropNewestCancelBeforeSubscription() {
+        testBufferCancelBeforeSubscription(prefetch: .keepFull,
+                                           whenFull: .dropNewest)
+    }
+
+    func testBufferKeepFullDropOldestCancelBeforeSubscription() {
+        testBufferCancelBeforeSubscription(prefetch: .keepFull,
+                                           whenFull: .dropOldest)
+    }
+
+    func testBufferKeepFullCustomErrorCancelBeforeSubscription() {
+        testBufferCancelBeforeSubscription(prefetch: .keepFull,
+                                           whenFull: .customError(unreachable))
+    }
+
+    func testFailWhileSendingValues() throws {
+        let helper = OperatorTestHelper(
+            publisherType: CustomPublisher.self,
+            initialDemand: nil,
+            receiveValueDemand: .none,
+            createSut: { $0.buffer(size: 5, prefetch: .byRequest, whenFull: .dropOldest) }
+        )
+
+        helper.tracking.onValue = { _ in
+            helper.publisher.send(completion: .failure(.oops))
+        }
+
+        XCTAssertEqual(helper.publisher.send(1), .none)
+        XCTAssertEqual(helper.publisher.send(2), .none)
+        XCTAssertEqual(helper.publisher.send(3), .none)
+        XCTAssertEqual(helper.publisher.send(4), .none)
+        XCTAssertEqual(helper.publisher.send(5), .none)
+
+        try XCTUnwrap(helper.downstreamSubscription).request(.max(3))
+
+        XCTAssertEqual(helper.tracking.history, [.subscription("Buffer"),
+                                                 .value(1),
+                                                 .completion(.failure(.oops)),
+                                                 .value(2),
+                                                 .value(3)])
+    }
+
+    func testBufferByRequestDropNewestLifecycle() {
+        testBufferLifecycle(prefetch: .byRequest,
+                            whenFull: .dropNewest)
+    }
+
+    func testBufferByRequestDropOldestLifecycle() {
+        testBufferLifecycle(prefetch: .byRequest,
+                            whenFull: .dropOldest)
+    }
+
+    func testBufferByRequestCustomErrorLifecycle() {
+        testBufferLifecycle(prefetch: .byRequest,
+                            whenFull: .customError { TestingError.oops })
+    }
+
+    func testBufferKeepFullDropNewestLifecycle() {
+        testBufferLifecycle(prefetch: .keepFull,
+                            whenFull: .dropNewest)
+    }
+
+    func testBufferKeepFullDropOldestLifecycle() {
+        testBufferLifecycle(prefetch: .keepFull,
+                            whenFull: .dropOldest)
+    }
+
+    func testBufferKeepFullCustomErrorLifecycle() {
+        testBufferLifecycle(prefetch: .keepFull,
+                            whenFull: .customError { TestingError.oops })
+    }
+
+    func testBufferReflection() throws {
+        try testReflection(
+            parentInput: Int.self,
+            parentFailure: TestingError.self,
+            description: "Buffer",
+            customMirror: expectedChildren(
+                ("values", "[]"),
+                ("state", .anything),
+                ("downstreamDemand", "max(0)"),
+                ("terminal", "nil")
+            ),
+            playgroundDescription: "Buffer",
+            { $0.buffer(size: 13, prefetch: .keepFull, whenFull: .dropNewest) }
+        )
+    }
+
+    // MARK: - Generic tests
+
+    private func testInitialDemand(
+        withPrefetchStragety prefetch: Publishers.PrefetchStrategy,
+        expectedSubscriptionHistory: [CustomSubscription.Event]
+    ) {
+        let subscription = CustomSubscription()
+        let publisher = CustomPublisher(subscription: nil)
+        let tracking = TrackingSubscriber()
+
+        subscription.onRequest = { _ in
+            XCTAssertEqual(tracking.history, [])
+        }
+
+        let buffer = publisher
+            .buffer(size: 42, prefetch: prefetch, whenFull: .dropOldest)
+
+        buffer.subscribe(tracking)
+
+        XCTAssertEqual(tracking.history, [])
+
+        publisher.send(subscription: subscription)
+
+        XCTAssertEqual(subscription.history, expectedSubscriptionHistory)
+        XCTAssertEqual(tracking.history, [.subscription("Buffer")])
+    }
+
+    private func testBufferingInput(
+        whenFull: Publishers.BufferingStrategy<TestingError>
+    ) throws {
+        let helper = OperatorTestHelper(
+            publisherType: CustomPublisher.self,
+            initialDemand: nil,
+            receiveValueDemand: .none,
+            createSut: { $0.buffer(size: 3, prefetch: .byRequest, whenFull: whenFull) }
+        )
+
+        XCTAssertEqual(helper.tracking.history, [.subscription("Buffer")])
+        XCTAssertEqual(helper.subscription.history, [.requested(.unlimited)])
+
+        XCTAssertEqual(helper.publisher.send(1), .none)
+        XCTAssertEqual(helper.publisher.send(2), .none)
+        XCTAssertEqual(helper.publisher.send(3), .none)
+        XCTAssertEqual(helper.publisher.send(4), .none)
+        XCTAssertEqual(helper.publisher.send(5), .none)
+
+        XCTAssertEqual(helper.tracking.history, [.subscription("Buffer")])
+        switch whenFull {
+        case .dropNewest, .dropOldest:
+            XCTAssertEqual(helper.subscription.history, [.requested(.unlimited)])
+        case .customError:
+            XCTAssertEqual(helper.subscription.history, [.requested(.unlimited),
+                                                         .cancelled])
+#if OPENCOMBINE_COMPATIBILITY_TEST
+        @unknown default:
+            unreachable()
+#endif
+        }
+
+        try XCTUnwrap(helper.downstreamSubscription).request(.max(3))
+
+        switch whenFull {
+        case .dropNewest:
+            XCTAssertEqual(helper.tracking.history, [.subscription("Buffer"),
+                                                     .value(1),
+                                                     .value(2),
+                                                     .value(3)])
+            XCTAssertEqual(helper.subscription.history, [.requested(.unlimited),
+                                                         .requested(.max(3))])
+        case .dropOldest:
+            XCTAssertEqual(helper.tracking.history, [.subscription("Buffer"),
+                                                     .value(3),
+                                                     .value(4),
+                                                     .value(5)])
+            XCTAssertEqual(helper.subscription.history, [.requested(.unlimited),
+                                                         .requested(.max(3))])
+        case .customError:
+            XCTAssertEqual(helper.tracking.history, [.subscription("Buffer"),
+                                                     .value(1),
+                                                     .value(2),
+                                                     .value(3),
+                                                     .completion(.failure(.oops))])
+            XCTAssertEqual(helper.subscription.history, [.requested(.unlimited),
+                                                         .cancelled,
+                                                         .requested(.max(3))])
+#if OPENCOMBINE_COMPATIBILITY_TEST
+        @unknown default:
+            unreachable()
+#endif
+        }
+
+        try XCTUnwrap(helper.downstreamSubscription).request(.max(1))
+
+        switch whenFull {
+        case .dropNewest:
+            XCTAssertEqual(helper.tracking.history, [.subscription("Buffer"),
+                                                     .value(1),
+                                                     .value(2),
+                                                     .value(3)])
+            XCTAssertEqual(helper.subscription.history, [.requested(.unlimited),
+                                                         .requested(.max(3)),
+                                                         .requested(.max(1))])
+        case .dropOldest:
+            XCTAssertEqual(helper.tracking.history, [.subscription("Buffer"),
+                                                     .value(3),
+                                                     .value(4),
+                                                     .value(5)])
+            XCTAssertEqual(helper.subscription.history, [.requested(.unlimited),
+                                                         .requested(.max(3)),
+                                                         .requested(.max(1))])
+        case .customError:
+            XCTAssertEqual(helper.tracking.history, [.subscription("Buffer"),
+                                                     .value(1),
+                                                     .value(2),
+                                                     .value(3),
+                                                     .completion(.failure(.oops))])
+            XCTAssertEqual(helper.subscription.history, [.requested(.unlimited),
+                                                         .cancelled,
+                                                         .requested(.max(3))])
+#if OPENCOMBINE_COMPATIBILITY_TEST
+        @unknown default:
+            unreachable()
+#endif
+        }
+    }
+
+    private func testRecursion(
+        prefetch: Publishers.PrefetchStrategy,
+        whenFull: Publishers.BufferingStrategy<TestingError>
+    ) {
+        let helper = OperatorTestHelper(
+            publisherType: CustomPublisher.self,
+            initialDemand: nil,
+            receiveValueDemand: .none,
+            createSut: {
+                $0.buffer(size: 5, prefetch: prefetch, whenFull: whenFull)
+            }
+        )
+
+        helper.tracking.onValue = { _ in
+            helper.downstreamSubscription?.request(.max(1))
+            helper.downstreamSubscription?.request(.none)
+        }
+
+        helper.downstreamSubscription?.request(.max(3))
+
+        for i in 0 ..< 10 {
+            switch prefetch {
+            case .byRequest:
+                XCTAssertEqual(helper.publisher.send(i), .none)
+            case .keepFull:
+                XCTAssertEqual(helper.publisher.send(i), .max(1))
+#if OPENCOMBINE_COMPATIBILITY_TEST
+            @unknown default:
+                unreachable()
+#endif
+            }
+        }
+
+        XCTAssertEqual(helper.tracking.history, [.subscription("Buffer"),
+                                                 .value(0),
+                                                 .value(1),
+                                                 .value(2),
+                                                 .value(3),
+                                                 .value(4),
+                                                 .value(5),
+                                                 .value(6),
+                                                 .value(7),
+                                                 .value(8),
+                                                 .value(9)])
+
+        switch prefetch {
+        case .byRequest:
+            XCTAssertEqual(helper.subscription.history, [.requested(.unlimited),
+                                                         .requested(.max(3))])
+        case .keepFull:
+            XCTAssertEqual(helper.subscription.history, [.requested(.max(5)),
+                                                         .requested(.max(3))])
+#if OPENCOMBINE_COMPATIBILITY_TEST
+        @unknown default:
+            unreachable()
+#endif
+        }
+    }
+
+    private func testRequestingFiniteDemand(
+        prefetch: Publishers.PrefetchStrategy,
+        whenFull: Publishers.BufferingStrategy<TestingError>
+    ) {
+        let helper = OperatorTestHelper(
+            publisherType: CustomPublisher.self,
+            initialDemand: Subscribers.Demand.none,
+            receiveValueDemand: .none,
+            createSut: {
+                $0.buffer(size: 5, prefetch: prefetch, whenFull: whenFull)
+            }
+        )
+
+        for i in 0 ..< 10 {
+            XCTAssertEqual(helper.publisher.send(i), .none)
+        }
+
+        XCTAssertEqual(helper.tracking.history, [.subscription("Buffer")])
+
+        switch (prefetch, whenFull) {
+        case (.byRequest, .customError):
+            XCTAssertEqual(helper.subscription.history, [.requested(.unlimited),
+                                                         .requested(.none),
+                                                         .cancelled])
+        case (.byRequest, _):
+            XCTAssertEqual(helper.subscription.history, [.requested(.unlimited),
+                                                         .requested(.none)])
+        case (.keepFull, .customError):
+            XCTAssertEqual(helper.subscription.history, [.requested(.max(5)),
+                                                         .requested(.none),
+                                                         .cancelled])
+        case (.keepFull, _):
+            XCTAssertEqual(helper.subscription.history, [.requested(.max(5)),
+                                                         .requested(.none)])
+#if OPENCOMBINE_COMPATIBILITY_TEST
+        @unknown default:
+            unreachable()
+#endif
+        }
+
+        helper.downstreamSubscription?.request(.max(3))
+        helper.downstreamSubscription?.request(.max(1))
+
+        switch whenFull {
+        case .dropNewest:
+            XCTAssertEqual(helper.tracking.history, [.subscription("Buffer"),
+                                                     .value(0),
+                                                     .value(1),
+                                                     .value(2),
+                                                     .value(3)])
+        case .dropOldest:
+            XCTAssertEqual(helper.tracking.history, [.subscription("Buffer"),
+                                                     .value(5),
+                                                     .value(6),
+                                                     .value(7),
+                                                     .value(8)])
+        case .customError:
+            XCTAssertEqual(helper.tracking.history, [.subscription("Buffer"),
+                                                     .value(0),
+                                                     .value(1),
+                                                     .value(2),
+                                                     .completion(.failure(.oops))])
+#if OPENCOMBINE_COMPATIBILITY_TEST
+        @unknown default:
+            unreachable()
+#endif
+        }
+
+        switch (prefetch, whenFull) {
+        case (.byRequest, .customError):
+            XCTAssertEqual(helper.subscription.history, [.requested(.unlimited),
+                                                         .requested(.none),
+                                                         .cancelled,
+                                                         .requested(.max(3))])
+        case (.byRequest, _):
+            XCTAssertEqual(helper.subscription.history, [.requested(.unlimited),
+                                                         .requested(.none),
+                                                         .requested(.max(3)),
+                                                         .requested(.max(1))])
+        case (.keepFull, .customError):
+            XCTAssertEqual(helper.subscription.history, [.requested(.max(5)),
+                                                         .requested(.none),
+                                                         .cancelled,
+                                                         .requested(.max(6))])
+        case (.keepFull, _):
+            XCTAssertEqual(helper.subscription.history, [.requested(.max(5)),
+                                                         .requested(.none),
+                                                         .requested(.max(6)),
+                                                         .requested(.max(2))])
+#if OPENCOMBINE_COMPATIBILITY_TEST
+        @unknown default:
+            unreachable()
+#endif
+        }
+    }
+
+    private func testRequestingUnlimitedDemand(
+        prefetch: Publishers.PrefetchStrategy,
+        whenFull: Publishers.BufferingStrategy<TestingError>
+    ) {
+        let helper = OperatorTestHelper(
+            publisherType: CustomPublisher.self,
+            initialDemand: .unlimited,
+            receiveValueDemand: .none,
+            createSut: {
+                $0.buffer(size: 5, prefetch: prefetch, whenFull: whenFull)
+            }
+        )
+
+        for i in 0 ..< 10 {
+            switch prefetch {
+            case .byRequest:
+                XCTAssertEqual(helper.publisher.send(i), .none)
+            case .keepFull:
+                XCTAssertEqual(helper.publisher.send(i), .max(1))
+#if OPENCOMBINE_COMPATIBILITY_TEST
+            @unknown default:
+                unreachable()
+#endif
+            }
+        }
+
+        XCTAssertEqual(helper.tracking.history, [.subscription("Buffer"),
+                                                 .value(0),
+                                                 .value(1),
+                                                 .value(2),
+                                                 .value(3),
+                                                 .value(4),
+                                                 .value(5),
+                                                 .value(6),
+                                                 .value(7),
+                                                 .value(8),
+                                                 .value(9)])
+
+        switch prefetch {
+        case .byRequest:
+            XCTAssertEqual(helper.subscription.history, [.requested(.unlimited),
+                                                         .requested(.unlimited)])
+        case .keepFull:
+            XCTAssertEqual(helper.subscription.history, [.requested(.max(5)),
+                                                         .requested(.unlimited)])
+#if OPENCOMBINE_COMPATIBILITY_TEST
+        @unknown default:
+            unreachable()
+#endif
+        }
+    }
+
+    private func testReceiveValueAfterCompleting(
+        _ completion: Subscribers.Completion<TestingError>
+    ) throws {
+        let helper = OperatorTestHelper(
+            publisherType: CustomPublisher.self,
+            initialDemand: nil,
+            receiveValueDemand: .none,
+            createSut: { $0.buffer(size: 3, prefetch: .byRequest, whenFull: .dropOldest) }
+        )
+
+        helper.publisher.send(completion: completion)
+        helper.publisher.send(completion: .finished) // Should be ignored
+        helper.publisher.send(completion: .failure(.oops))
+
+        switch completion {
+        case .finished:
+            XCTAssertEqual(helper.tracking.history, [.subscription("Buffer")])
+        case .failure:
+            XCTAssertEqual(helper.tracking.history, [.subscription("Buffer"),
+                                                     .completion(.failure(.oops))])
+        }
+        XCTAssertEqual(helper.subscription.history, [.requested(.unlimited)])
+
+        XCTAssertEqual(helper.publisher.send(1), .none)
+        XCTAssertEqual(helper.publisher.send(2), .none)
+
+        switch completion {
+        case .finished:
+            XCTAssertEqual(helper.tracking.history, [.subscription("Buffer")])
+        case .failure:
+            XCTAssertEqual(helper.tracking.history, [.subscription("Buffer"),
+                                                     .completion(.failure(.oops))])
+        }
+        XCTAssertEqual(helper.subscription.history, [.requested(.unlimited)])
+
+        try XCTUnwrap(helper.downstreamSubscription).request(.max(2))
+
+        switch completion {
+        case .finished:
+            XCTAssertEqual(helper.tracking.history, [.subscription("Buffer"),
+                                                     .value(1),
+                                                     .value(2)])
+            XCTAssertEqual(helper.subscription.history, [.requested(.unlimited),
+                                                         .requested(.max(2))])
+        case .failure:
+            XCTAssertEqual(helper.tracking.history, [.subscription("Buffer"),
+                                                     .completion(completion)])
+            XCTAssertEqual(helper.subscription.history, [.requested(.unlimited)])
+        }
+
+        try XCTUnwrap(helper.downstreamSubscription).request(.max(1))
+
+        switch completion {
+        case .finished:
+            XCTAssertEqual(helper.tracking.history, [.subscription("Buffer"),
+                                                     .value(1),
+                                                     .value(2),
+                                                     .completion(completion)])
+            XCTAssertEqual(helper.subscription.history, [.requested(.unlimited),
+                                                         .requested(.max(2)),
+                                                         .requested(.max(1))])
+        case .failure:
+            XCTAssertEqual(helper.tracking.history, [.subscription("Buffer"),
+                                                     .completion(completion)])
+            XCTAssertEqual(helper.subscription.history, [.requested(.unlimited)])
+        }
+    }
+
+    private func testBufferLifecycle(
+        prefetch: Publishers.PrefetchStrategy,
+        whenFull: Publishers.BufferingStrategy<TestingError>
+    ) {
+        var deinitCounter = 0
+        let onDeinit = { deinitCounter += 1 }
+
+        do {
+            let publisher = CustomPublisher(subscription: CustomSubscription())
+            publisher.onDeinit = onDeinit
+            let buffer = publisher.buffer(size: 1, prefetch: prefetch, whenFull: whenFull)
+            let tracking = TrackingSubscriber(
+                receiveSubscription: { $0.request(.unlimited) },
+                onDeinit: onDeinit
+            )
+            buffer.subscribe(tracking)
+            publisher.send(completion: .finished)
+        }
+
+        XCTAssertEqual(deinitCounter, 2)
+
+        do {
+            let publisher = CustomPublisher(subscription: CustomSubscription())
+            publisher.onDeinit = onDeinit
+            let buffer = publisher.buffer(size: 1, prefetch: prefetch, whenFull: whenFull)
+            let tracking = TrackingSubscriber(
+                receiveSubscription: { $0.request(.unlimited) },
+                onDeinit: onDeinit
+            )
+            buffer.subscribe(tracking)
+            publisher.send(completion: .failure(.oops))
+        }
+
+        XCTAssertEqual(deinitCounter, 4)
+
+        var downstreamSubscription: Subscription?
+
+        do {
+            let publisher = CustomPublisher(subscription: CustomSubscription())
+            publisher.onDeinit = onDeinit
+            let buffer = publisher.buffer(size: 1, prefetch: prefetch, whenFull: whenFull)
+            let tracking = TrackingSubscriber(
+                receiveSubscription: { downstreamSubscription = $0 },
+                onDeinit: onDeinit
+            )
+            buffer.subscribe(tracking)
+            publisher.send(completion: .failure(.oops))
+        }
+
+        XCTAssertEqual(deinitCounter, 6)
+        downstreamSubscription?.cancel()
+        XCTAssertEqual(deinitCounter, 6)
+
+        do {
+            let publisher = CustomPublisher(subscription: CustomSubscription())
+            publisher.onDeinit = onDeinit
+            let buffer = publisher.buffer(size: 1, prefetch: prefetch, whenFull: whenFull)
+            let tracking = TrackingSubscriber(
+                receiveSubscription: { $0.request(.unlimited) },
+                onDeinit: onDeinit
+            )
+            buffer.subscribe(tracking)
+        }
+
+        XCTAssertEqual(deinitCounter, 6)
+    }
+
+    private func testBufferReceiveSubscriptionTwice(
+        prefetch: Publishers.PrefetchStrategy,
+        whenFull: Publishers.BufferingStrategy<TestingError>
+    ) throws {
+        let helper = OperatorTestHelper(
+            publisherType: CustomPublisher.self,
+            initialDemand: nil,
+            receiveValueDemand: .none,
+            createSut: { $0.buffer(size: 2, prefetch: prefetch, whenFull: whenFull) }
+        )
+
+        switch prefetch {
+        case .keepFull:
+            XCTAssertEqual(helper.subscription.history, [.requested(.max(2))])
+        case .byRequest:
+            XCTAssertEqual(helper.subscription.history, [.requested(.unlimited)])
+#if OPENCOMBINE_COMPATIBILITY_TEST
+        @unknown default:
+            unreachable()
+#endif
+        }
+
+        let secondSubscription = CustomSubscription()
+
+        try XCTUnwrap(helper.publisher.subscriber)
+            .receive(subscription: secondSubscription)
+
+        XCTAssertEqual(secondSubscription.history, [.cancelled])
+
+        try XCTUnwrap(helper.publisher.subscriber)
+            .receive(subscription: helper.subscription)
+
+        switch prefetch {
+        case .keepFull:
+            XCTAssertEqual(helper.subscription.history, [.requested(.max(2)),
+                                                         .cancelled])
+        case .byRequest:
+            XCTAssertEqual(helper.subscription.history, [.requested(.unlimited),
+                                                         .cancelled])
+#if OPENCOMBINE_COMPATIBILITY_TEST
+        @unknown default:
+            unreachable()
+#endif
+        }
+
+        try XCTUnwrap(helper.downstreamSubscription).cancel()
+
+        switch prefetch {
+        case .keepFull:
+            XCTAssertEqual(helper.subscription.history, [.requested(.max(2)),
+                                                         .cancelled,
+                                                         .cancelled])
+        case .byRequest:
+            XCTAssertEqual(helper.subscription.history, [.requested(.unlimited),
+                                                         .cancelled,
+                                                         .cancelled])
+#if OPENCOMBINE_COMPATIBILITY_TEST
+        @unknown default:
+            unreachable()
+#endif
+        }
+    }
+
+    private func testBufferReceiveValueBeforeSubscription(
+        prefetch: Publishers.PrefetchStrategy,
+        whenFull: Publishers.BufferingStrategy<Never>
+    ) {
+        testReceiveValueBeforeSubscription(
+            value: 0,
+            expected: .history([], demand: .none),
+            { $0.buffer(size: 2, prefetch: prefetch, whenFull: whenFull) }
+        )
+    }
+
+    private func testBufferReceiveCompletionBeforeSubscription(
+        prefetch: Publishers.PrefetchStrategy,
+        whenFull: Publishers.BufferingStrategy<Never>
+    ) {
+        testReceiveCompletionBeforeSubscription(
+            inputType: Int.self,
+            expected: .history([]),
+            { $0.buffer(size: 2, prefetch: prefetch, whenFull: whenFull) }
+        )
+    }
+
+    private func testBufferRequestBeforeSubscription(
+        prefetch: Publishers.PrefetchStrategy,
+        whenFull: Publishers.BufferingStrategy<Never>
+    ) {
+        testRequestBeforeSubscription(
+            inputType: Int.self,
+            shouldCrash: false,
+            { $0.buffer(size: 2, prefetch: prefetch, whenFull: whenFull) }
+        )
+    }
+
+    private func testBufferCancelBeforeSubscription(
+        prefetch: Publishers.PrefetchStrategy,
+        whenFull: Publishers.BufferingStrategy<Never>
+    ) {
+        testCancelBeforeSubscription(
+            inputType: Int.self,
+            shouldCrash: false,
+            { $0.buffer(size: 2, prefetch: prefetch, whenFull: whenFull) }
+        )
+    }
+}

Tests/OpenCombineTests/PublisherTests/CatchTests.swift

@@ -0,0 +1,622 @@
+//
+//  CatchTests.swift
+//  
+//
+//  Created by Sergej Jaskiewicz on 25.12.2019.
+//
+
+import XCTest
+
+#if OPENCOMBINE_COMPATIBILITY_TEST
+import Combine
+#else
+import OpenCombine
+#endif
+
+@available(macOS 10.15, iOS 13.0, *)
+final class CatchTests: XCTestCase {
+
+    // MARK: - Catch
+
+    func testSimpleCatch() {
+        CatchTests
+            .testWithSequence(expectedSubscription: "Catch") { upstream, new in
+                upstream.catch { _ in new }
+            }
+    }
+
+    func testCatchReceiveSubscriptionTwice() throws {
+        try testReceiveSubscriptionTwice { $0.catch(Fail.init) }
+        try testReceiveSubscriptionTwice { publisher in
+            Fail(outputType: Int.self, failure: TestingError.oops)
+                .catch { _ in publisher }
+        }
+    }
+
+    func testCatchCrashesOnUnwantedInput() {
+        let helper = OperatorTestHelper(publisherType: CustomPublisher.self,
+                                        initialDemand: nil,
+                                        receiveValueDemand: .none,
+                                        createSut: { $0.catch { _ in Just(-1) } })
+        assertCrashes {
+            _ = helper.publisher.send(42)
+        }
+    }
+
+    func testCatchPreservesDemand() throws {
+        try CatchTests.testPreservesDemand(expectedSubscription: "Catch") {
+            $0.catch($1)
+        }
+    }
+
+    func testCatchUpstreamFinishes() {
+        CatchTests.testUpstreamFinishes(expectedSubscription: "Catch") {
+            $0.catch($1)
+        }
+    }
+
+    func testCatchRecursion() {
+        testRecursion(expectedSubscription: "Catch") {
+            $0.catch($1)
+        }
+    }
+
+    func testCatchReceiveValueBeforeSubscription() {
+        testReceiveValueBeforeSubscription(value: 1, expected: .crash) {
+            $0.catch { _ in Just(13) }
+        }
+    }
+
+    func testCatchReceiveCompletionBeforeSubscription() {
+        testReceiveCompletionBeforeSubscription(
+            inputType: Int.self,
+            expected: .history([]),
+            { $0.catch { _ in Just(13) } }
+        )
+    }
+
+    func testCatchRequestPendingPost() {
+        CatchTests.testRequestPendingPost(expectedSubscription: "Catch",
+                                          { $0.catch($1) })
+    }
+
+    func testCatchCancelPendingPost() {
+        CatchTests.testCancelPendingPost(expectedSubscription: "Catch",
+                                         { $0.catch($1) })
+    }
+
+    func testCatchRequestPost() throws {
+        try CatchTests.testRequestPost(expectedSubscription: "Catch",
+                                       { $0.catch($1) })
+    }
+
+    func testCatchCancellationBeforeRecovering() throws {
+        try CatchTests.testCancellationBeforeRecovering(expectedSubscription: "Catch",
+                                                        { $0.catch($1) })
+    }
+
+    func testCatchCancellationAfterRecovering() throws {
+        try CatchTests.testCancellationAfterRecovering(expectedSubscription: "Catch",
+                                                       { $0.catch($1) })
+    }
+
+    func testCatchUpstreamFailsTwice() {
+        testUpstreamFailsTwice(expectedSubscription: "Catch") { $0.catch($1) }
+    }
+
+    func testCatchReflection() throws {
+        try testReflection(parentInput: Int.self,
+                           parentFailure: TestingError.self,
+                           description: "Catch",
+                           customMirror: expectedChildren(
+                               ("downstream", .contains("TrackingSubscriberBase")),
+                               ("demand", "max(0)")
+                           ),
+                           playgroundDescription: "Catch",
+                           { $0.catch(Fail.init) })
+
+        try testReflection(
+            parentInput: Int.self,
+            parentFailure: TestingError.self,
+            description: "Catch",
+            customMirror: expectedChildren(
+                ("downstream", .contains("TrackingSubscriberBase")),
+                ("demand", "max(0)")
+            ),
+            playgroundDescription: "Catch",
+            { publisher in
+                Fail(outputType: Int.self, failure: TestingError.oops)
+                    .catch { _ in publisher }
+            }
+        )
+    }
+
+    // MARK: - TryCatch
+
+    func testSimpleTryCatch() {
+        CatchTests
+            .testWithSequence(expectedSubscription: "TryCatch") { upstream, new in
+                upstream.tryCatch { _ in new }
+            }
+    }
+
+    func testTryCatchReceiveSubscriptionTwice() throws {
+
+        try testReceiveSubscriptionTwice { $0.tryCatch(Fail.init) }
+
+        try testReceiveSubscriptionTwice { publisher in
+            Fail(outputType: Int.self, failure: TestingError.oops)
+                .tryCatch { _ in publisher }
+        }
+    }
+
+    func testTryCatchCrashesOnUnwantedInput() {
+        let helper = OperatorTestHelper(publisherType: CustomPublisher.self,
+                                        initialDemand: nil,
+                                        receiveValueDemand: .none,
+                                        createSut: { $0.tryCatch { _ in Just(-1) } })
+        assertCrashes {
+            _ = helper.publisher.send(42)
+        }
+    }
+
+    func testTryCatchPreservesDemand() throws {
+        try CatchTests.testPreservesDemand(expectedSubscription: "TryCatch") {
+            $0.tryCatch($1)
+        }
+    }
+
+    func testTryCatchUpstreamFinishes() {
+        CatchTests.testUpstreamFinishes(expectedSubscription: "TryCatch") {
+            $0.tryCatch($1)
+        }
+    }
+
+    func testTryCatchHandlerThrows() {
+
+        var handledErrors = [TestingError]()
+
+        func handler(_ error: TestingError) throws -> Just<Int> {
+            handledErrors.append(error)
+            throw "oops2" as TestingError
+        }
+
+        let helper = OperatorTestHelper(publisherType: CustomPublisher.self,
+                                        initialDemand: .unlimited,
+                                        receiveValueDemand: .none,
+                                        createSut: { $0.tryCatch(handler) })
+
+        XCTAssertEqual(helper.publisher.send(1), .none)
+        XCTAssertEqual(helper.publisher.send(2), .none)
+        helper.publisher.send(completion: .failure(.oops))
+
+        XCTAssertEqual(helper.tracking.history,
+                       [.subscription("TryCatch"),
+                        .value(1),
+                        .value(2),
+                        .completion(.failure("oops2" as TestingError))])
+        XCTAssertEqual(helper.subscription.history, [.requested(.unlimited)])
+        XCTAssertEqual(handledErrors, [.oops])
+
+        XCTAssertEqual(helper.publisher.send(-1), .none)
+        helper.publisher.send(completion: .failure(.oops))
+        helper.publisher.send(completion: .finished)
+
+        XCTAssertEqual(helper.tracking.history,
+                       [.subscription("TryCatch"),
+                        .value(1),
+                        .value(2),
+                        .completion(.failure("oops2" as TestingError)),
+                        .value(-1)])
+        XCTAssertEqual(helper.subscription.history, [.requested(.unlimited)])
+        XCTAssertEqual(handledErrors, [.oops])
+    }
+
+    func testTryCatchRecursion() {
+        testRecursion(expectedSubscription: "TryCatch") {
+            $0.tryCatch($1)
+        }
+    }
+
+    func testTryCatchReceiveValueBeforeSubscription() {
+        testReceiveValueBeforeSubscription(value: 1, expected: .crash) {
+            $0.tryCatch { _ in Just(13) }
+        }
+    }
+
+    func testTryCatchReceiveCompletionBeforeSubscription() {
+        testReceiveCompletionBeforeSubscription(
+            inputType: Int.self,
+            expected: .history([]),
+            { $0.tryCatch { _ in Just(13) } }
+        )
+    }
+
+    func testTryCatchRequestPendingPost() {
+        CatchTests.testRequestPendingPost(expectedSubscription: "TryCatch",
+                                          { $0.tryCatch($1) })
+    }
+
+    func testTryCatchCancelPendingPost() {
+        CatchTests.testCancelPendingPost(expectedSubscription: "TryCatch",
+                                         { $0.tryCatch($1) })
+    }
+
+    func testTryCatchRequestPost() throws {
+        try CatchTests.testRequestPost(expectedSubscription: "TryCatch",
+                                       { $0.tryCatch($1) })
+    }
+
+    func testTryCatchCancellationBeforeRecovering() throws {
+        try CatchTests.testCancellationBeforeRecovering(expectedSubscription: "TryCatch",
+                                                        { $0.tryCatch($1) })
+    }
+
+    func testTryCatchCancellationAfterRecovering() throws {
+        try CatchTests.testCancellationAfterRecovering(expectedSubscription: "TryCatch",
+                                                       { $0.tryCatch($1) })
+    }
+
+    func testTryCatchUpstreamFailsTwice() {
+        testUpstreamFailsTwice(expectedSubscription: "TryCatch") { $0.tryCatch($1) }
+    }
+
+    func testTryCatchReflection() throws {
+        try testReflection(parentInput: Int.self,
+                           parentFailure: TestingError.self,
+                           description: "TryCatch",
+                           customMirror: expectedChildren(
+                               ("downstream", .contains("TrackingSubscriberBase")),
+                               ("demand", "max(0)")
+                           ),
+                           playgroundDescription: "TryCatch",
+                           { $0.tryCatch(Fail.init) })
+
+        try testReflection(
+            parentInput: Int.self,
+            parentFailure: TestingError.self,
+            description: "TryCatch",
+            customMirror: expectedChildren(
+                ("downstream", .contains("TrackingSubscriberBase")),
+                ("demand", "max(0)")
+            ),
+            playgroundDescription: "TryCatch",
+            { publisher in
+                Fail<Int, TestingError>(error: .oops).tryCatch { _ in publisher }
+            }
+        )
+    }
+
+    // MARK: - Generic tests
+
+    private typealias TestSequence = Publishers.Sequence<[Int], Never>
+
+    private static func testWithSequence<Operator: Publisher>(
+        expectedSubscription: StringSubscription,
+        _ makeCatch: (Publishers.TryMap<TestSequence, Int>, TestSequence) -> Operator
+    ) where Operator.Output == Int {
+        let throwingSequence = TestSequence(sequence: Array(0 ..< 10))
+            .tryMap { v -> Int in
+                if v < 5 {
+                    return v
+                } else {
+                    throw TestingError.oops
+                }
+            }
+
+        let `catch` = makeCatch(throwingSequence, [3, 2, 1, 0].publisher)
+
+        let tracking = TrackingSubscriberBase<Int, Operator.Failure>(
+            receiveSubscription: { $0.request(.max(1)) },
+            receiveValue: { _ in .max(1) }
+        )
+        `catch`.subscribe(tracking)
+
+        XCTAssertEqual(tracking.history, [.subscription(expectedSubscription),
+                                          .value(0),
+                                          .value(1),
+                                          .value(2),
+                                          .value(3),
+                                          .value(4),
+                                          .value(3),
+                                          .value(2),
+                                          .value(1),
+                                          .value(0),
+                                          .completion(.finished)])
+    }
+
+    private static func testPreservesDemand<Operator: Publisher>(
+        expectedSubscription: StringSubscription,
+        _ makeCatch:
+            (CustomPublisher,
+             @escaping (TestingError) -> CustomPublisherBase<Int, Error>) -> Operator
+    ) throws where Operator.Output == Int {
+
+        let errorHandlerSubscription = CustomSubscription()
+        let errorHandlerPublisher = CustomPublisherBase<Int, Error>(
+            subscription: errorHandlerSubscription
+        )
+
+        let helper = OperatorTestHelper(
+            publisherType: CustomPublisher.self,
+            initialDemand: .max(12),
+            receiveValueDemand: .max(3),
+            createSut: { makeCatch($0) { _ in errorHandlerPublisher } }
+        )
+
+        XCTAssertEqual(helper.tracking.history, [.subscription(expectedSubscription)])
+        XCTAssertEqual(helper.subscription.history, [.requested(.max(12))])
+
+        try XCTUnwrap(helper.downstreamSubscription).request(.max(5))
+
+        XCTAssertEqual(helper.subscription.history, [.requested(.max(12)),
+                                                     .requested(.max(5))])
+
+        for i in 1 ... 8 {
+            XCTAssertEqual(helper.publisher.send(i), .max(3))
+        }
+
+        helper.publisher.send(completion: .failure(.oops))
+
+        XCTAssertEqual(helper.tracking.history, [.subscription(expectedSubscription),
+                                                 .value(1),
+                                                 .value(2),
+                                                 .value(3),
+                                                 .value(4),
+                                                 .value(5),
+                                                 .value(6),
+                                                 .value(7),
+                                                 .value(8)])
+        XCTAssertEqual(helper.subscription.history, [.requested(.max(12)),
+                                                     .requested(.max(5))])
+        XCTAssertEqual(errorHandlerSubscription.history, [.requested(.max(33))])
+    }
+
+    private static func testUpstreamFinishes<Operator: Publisher>(
+        expectedSubscription: StringSubscription,
+        _ makeCatch: (CustomPublisher,
+                      @escaping (TestingError) -> Fail<Int, Error>) -> Operator
+    ) where Operator.Output == Int {
+        var counter = 0
+        let helper = OperatorTestHelper(
+            publisherType: CustomPublisher.self,
+            initialDemand: .max(2),
+            receiveValueDemand: .none,
+            createSut: { makeCatch($0) { counter += 1; return Fail(error: $0) } }
+        )
+
+        XCTAssertEqual(helper.publisher.send(1), .none)
+        XCTAssertEqual(helper.publisher.send(2), .none)
+        helper.publisher.send(completion: .finished)
+        helper.publisher.send(completion: .finished)
+        helper.publisher.send(completion: .failure(.oops))
+
+        XCTAssertEqual(helper.tracking.history, [.subscription(expectedSubscription),
+                                                 .value(1),
+                                                 .value(2),
+                                                 .completion(.finished)])
+        XCTAssertEqual(helper.subscription.history, [.requested(.max(2))])
+        XCTAssertEqual(counter, 0)
+    }
+
+    private func testRecursion<Operator: Publisher>(
+        expectedSubscription: StringSubscription,
+        _ makeCatch: @escaping (CustomPublisher,
+                                @escaping (TestingError) -> Just<Int>) -> Operator
+    ) where Operator.Output == Int {
+
+        func createSut(_ publisher: CustomPublisher) -> Operator {
+            return makeCatch(publisher) { _ in
+                publisher.send(completion: .failure(.oops))
+                return Just(13)
+            }
+        }
+
+        let helper = OperatorTestHelper(
+            publisherType: CustomPublisher.self,
+            initialDemand: .unlimited,
+            receiveValueDemand: .none,
+            createSut: createSut
+        )
+
+        XCTAssertEqual(helper.publisher.send(1), .none)
+        XCTAssertEqual(helper.publisher.send(2), .none)
+
+        assertCrashes {
+            helper.publisher.send(completion: .failure(.oops))
+        }
+    }
+
+    private static func testRequestPendingPost<Operator: Publisher>(
+        expectedSubscription: StringSubscription,
+        _ makeCatch: (CustomPublisher,
+                      @escaping (TestingError) -> CustomPublisher) -> Operator
+    ) where Operator.Output == Int {
+
+        let handlerSubscription = CustomSubscription()
+        let handlerPublisher = CustomPublisher(subscription: handlerSubscription)
+
+        let helper = OperatorTestHelper(
+            publisherType: CustomPublisher.self,
+            initialDemand: .max(3),
+            receiveValueDemand: .none,
+            createSut: { makeCatch($0) { _ in handlerPublisher } }
+        )
+
+        handlerPublisher.willSubscribe = { _ in
+            guard let downstreamSubscription = helper.downstreamSubscription else {
+                XCTFail("missing downstream subscription")
+                return
+            }
+            downstreamSubscription.request(.max(10))
+            XCTAssertEqual(handlerSubscription.history, [])
+        }
+
+        XCTAssertEqual(helper.publisher.send(1), .none)
+        helper.publisher.send(completion: .failure(.oops))
+
+        XCTAssertEqual(helper.tracking.history, [.subscription(expectedSubscription),
+                                                 .value(1)])
+        XCTAssertEqual(helper.subscription.history, [.requested(.max(3))])
+        XCTAssertEqual(handlerSubscription.history, [.requested(.max(12))])
+    }
+
+    private static func testCancelPendingPost<Operator: Publisher>(
+        expectedSubscription: StringSubscription,
+        _ makeCatch: (CustomPublisher,
+                      @escaping (TestingError) -> CustomPublisher) -> Operator
+    ) where Operator.Output == Int {
+
+        let handlerSubscription = CustomSubscription()
+        let handlerPublisher = CustomPublisher(subscription: handlerSubscription)
+
+        let helper = OperatorTestHelper(
+            publisherType: CustomPublisher.self,
+            initialDemand: .max(3),
+            receiveValueDemand: .none,
+            createSut: { makeCatch($0) { _ in handlerPublisher } }
+        )
+
+        handlerPublisher.willSubscribe = { _ in
+            guard let downstreamSubscription = helper.downstreamSubscription else {
+                XCTFail("missing downstream subscription")
+                return
+            }
+            downstreamSubscription.cancel()
+            XCTAssertEqual(handlerSubscription.history, [])
+        }
+
+        helper.publisher.send(completion: .failure(.oops))
+
+        XCTAssertEqual(helper.tracking.history, [.subscription(expectedSubscription)])
+        XCTAssertEqual(helper.subscription.history, [.requested(.max(3))])
+        XCTAssertEqual(handlerSubscription.history, [.requested(.max(3))])
+
+        handlerPublisher.send(completion: .finished)
+
+        XCTAssertEqual(helper.tracking.history, [.subscription(expectedSubscription),
+                                                 .completion(.finished)])
+        XCTAssertEqual(handlerSubscription.history, [.requested(.max(3))])
+    }
+
+    private static func testRequestPost<Operator: Publisher>(
+        expectedSubscription: StringSubscription,
+        _ makeCatch: (CustomPublisher,
+                      @escaping (TestingError) -> CustomPublisher) -> Operator
+    ) throws where Operator.Output == Int {
+
+        let handlerSubscription = CustomSubscription()
+        let handlerPublisher = CustomPublisher(subscription: handlerSubscription)
+
+        let helper = OperatorTestHelper(
+            publisherType: CustomPublisher.self,
+            initialDemand: nil,
+            receiveValueDemand: .none,
+            createSut: { makeCatch($0) { _ in handlerPublisher } }
+        )
+
+        helper.publisher.send(completion: .failure(.oops))
+
+        XCTAssertEqual(helper.tracking.history, [.subscription(expectedSubscription)])
+        XCTAssertEqual(helper.subscription.history, [])
+        XCTAssertNotNil(handlerPublisher.subscriber)
+
+        try XCTUnwrap(helper.downstreamSubscription).request(.max(12))
+        XCTAssertEqual(handlerSubscription.history, [.requested(.max(12))])
+        XCTAssertEqual(helper.tracking.history, [.subscription(expectedSubscription)])
+
+        XCTAssertEqual(handlerPublisher.send(100), .none)
+
+        XCTAssertEqual(helper.tracking.history, [.subscription(expectedSubscription),
+                                                 .value(100)])
+    }
+
+    private static func testCancellationBeforeRecovering<Operator: Publisher>(
+        expectedSubscription: StringSubscription,
+        _ makeCatch: (CustomPublisher,
+                      @escaping (TestingError) -> CustomPublisher) -> Operator
+    ) throws where Operator.Output == Int {
+
+        func handler(_ error: TestingError) -> CustomPublisher {
+            XCTFail("Should not be called")
+            return CustomPublisher(subscription: CustomSubscription())
+        }
+
+        let helper = OperatorTestHelper(
+            publisherType: CustomPublisher.self,
+            initialDemand: .max(1),
+            receiveValueDemand: .max(3),
+            createSut: { makeCatch($0, handler) }
+        )
+
+        let downstreamSubscription = try XCTUnwrap(helper.downstreamSubscription)
+
+        downstreamSubscription.cancel()
+        downstreamSubscription.request(.max(199))
+
+        XCTAssertEqual(helper.publisher.send(1), .max(3))
+        helper.publisher.send(completion: .failure(.oops))
+
+        XCTAssertEqual(helper.subscription.history, [.requested(.max(1)), .cancelled])
+        XCTAssertEqual(helper.tracking.history, [.subscription(expectedSubscription),
+                                                 .value(1)])
+    }
+
+    private static func testCancellationAfterRecovering<Operator: Publisher>(
+        expectedSubscription: StringSubscription,
+        _ makeCatch: (CustomPublisher,
+                      @escaping (TestingError) -> CustomPublisher) -> Operator
+    ) throws where Operator.Output == Int {
+
+        let handlerSubscription = CustomSubscription()
+        let handlerPublisher = CustomPublisher(subscription: handlerSubscription)
+
+        let helper = OperatorTestHelper(
+            publisherType: CustomPublisher.self,
+            initialDemand: nil,
+            receiveValueDemand: .max(3),
+            createSut: { makeCatch($0) { _ in handlerPublisher } }
+        )
+
+        helper.publisher.send(completion: .failure(.oops))
+
+        let downstreamSubscription = try XCTUnwrap(helper.downstreamSubscription)
+        downstreamSubscription.cancel()
+
+        XCTAssertEqual(handlerPublisher.send(1), .max(3))
+        handlerPublisher.send(completion: .finished)
+        handlerPublisher.send(completion: .failure(.oops))
+
+        XCTAssertEqual(helper.tracking.history, [.subscription(expectedSubscription),
+                                                 .value(1)])
+        XCTAssertEqual(helper.subscription.history, [])
+        XCTAssertEqual(handlerSubscription.history, [.cancelled])
+
+        let extraSubscription = CustomSubscription()
+        handlerPublisher.send(subscription: extraSubscription)
+        XCTAssertEqual(extraSubscription.history, [.cancelled])
+    }
+
+    private func testUpstreamFailsTwice<Operator: Publisher>(
+        expectedSubscription: StringSubscription,
+        _ makeCatch: (CustomPublisher,
+                      @escaping (TestingError) -> CustomPublisher) -> Operator
+    ) where Operator.Output == Int {
+
+        let handlerSubscription = CustomSubscription()
+        let handlerPublisher = CustomPublisher(subscription: handlerSubscription)
+
+        let helper = OperatorTestHelper(
+            publisherType: CustomPublisher.self,
+            initialDemand: nil,
+            receiveValueDemand: .max(3),
+            createSut: { makeCatch($0) { _ in handlerPublisher } }
+        )
+
+        helper.publisher.send(completion: .failure(.oops))
+
+        assertCrashes {
+            helper.publisher.send(completion: .failure(.oops))
+        }
+    }
+}

Tests/OpenCombineTests/PublisherTests/CollectByCountTests.swift

@@ -217,7 +217,7 @@ final class CollectByCountTests: XCTestCase {
                            description: "CollectByCount",
                            customMirror: expectedChildren(
                                ("downstream", .contains("TrackingSubscriberBase")),
-                               ("upstreamSubscription", "nil"),
+                               ("upstreamSubscription", .anything),
                                ("buffer", "[]"),
                                ("count", "53")
                            ),

Tests/OpenCombineTests/PublisherTests/ConcatenateTests.swift

@@ -196,7 +196,7 @@ final class ConcatenateTests: XCTestCase {
         var didSubscribe = false
 
         publisher.didSubscribe = { _ in
-            XCTAssertEqual(tracking.history, [])
+            XCTAssertEqual(tracking.history, [.subscription("Concatenate")])
             didSubscribe = true
         }
 
@@ -430,7 +430,7 @@ final class ConcatenateTests: XCTestCase {
                            description: "Concatenate",
                            customMirror: expectedChildren(
                                ("downstream", .contains("TrackingSubscriberBase")),
-                               ("upstreamSubscription", "nil"),
+                               ("upstreamSubscription", .anything),
                                ("suffix", .contains("(sequence: [2.0, 3.0, 5.0, 7.0])")),
                                ("demand", "max(0)")
                            ),

Tests/OpenCombineTests/PublisherTests/DebounceTests.swift

@@ -0,0 +1,630 @@
+//
+//  DebounceTests.swift
+//  
+//
+//  Created by Sergej Jaskiewicz on 28.06.2020.
+//
+
+import XCTest
+
+#if OPENCOMBINE_COMPATIBILITY_TEST
+import Combine
+#else
+import OpenCombine
+#endif
+
+@available(macOS 10.15, iOS 13.0, *)
+final class DebounceTests: XCTestCase {
+
+    func testBasicBehavior() {
+        let scheduler = VirtualTimeScheduler()
+        let helper = OperatorTestHelper(
+            publisherType: CustomPublisher.self,
+            initialDemand: .max(2),
+            receiveValueDemand: .max(1),
+            createSut: {
+                $0.debounce(for: .nanoseconds(13),
+                            scheduler: scheduler,
+                            options: .nontrivialOptions)
+            }
+        )
+
+        XCTAssertEqual(helper.tracking.history, [.subscription("Debounce")])
+        XCTAssertEqual(helper.subscription.history, [.requested(.unlimited)])
+        XCTAssertEqual(scheduler.history, [])
+
+        XCTAssertEqual(helper.publisher.send(1), .none)
+
+        XCTAssertEqual(helper.tracking.history, [.subscription("Debounce")])
+        XCTAssertEqual(helper.subscription.history, [.requested(.unlimited)])
+        XCTAssertEqual(scheduler.history,
+                       [.now,
+                        .minimumTolerance,
+                        .scheduleAfterDateWithInterval(.nanoseconds(13),
+                                                       interval: .nanoseconds(13),
+                                                       tolerance: .nanoseconds(7),
+                                                       options: .nontrivialOptions)])
+
+        scheduler.rewind(to: .nanoseconds(4))
+        XCTAssertEqual(helper.publisher.send(2), .none)
+        scheduler.rewind(to: .nanoseconds(9))
+        XCTAssertEqual(helper.publisher.send(3), .none)
+
+        scheduler.rewind(to: .nanoseconds(200))
+
+        XCTAssertEqual(helper.tracking.history, [.subscription("Debounce"),
+                                                 .value(3)])
+        XCTAssertEqual(helper.subscription.history, [.requested(.unlimited)])
+        XCTAssertEqual(scheduler.history,
+                       [.now,
+                        .minimumTolerance,
+                        .scheduleAfterDateWithInterval(.nanoseconds(13),
+                                                       interval: .nanoseconds(13),
+                                                       tolerance: .nanoseconds(7),
+                                                       options: .nontrivialOptions),
+                        .now,
+                        .minimumTolerance,
+                        .scheduleAfterDateWithInterval(.nanoseconds(17),
+                                                       interval: .nanoseconds(13),
+                                                       tolerance: .nanoseconds(7),
+                                                       options: .nontrivialOptions),
+                        .now,
+                        .minimumTolerance,
+                        .scheduleAfterDateWithInterval(.nanoseconds(22),
+                                                       interval: .nanoseconds(13),
+                                                       tolerance: .nanoseconds(7),
+                                                       options: .nontrivialOptions)])
+
+        helper.publisher.send(completion: .finished)
+        helper.publisher.send(completion: .failure(.oops)) // ignored
+        XCTAssertEqual(helper.publisher.send(-1), .none) // ignored
+
+        XCTAssertEqual(helper.tracking.history, [.subscription("Debounce"),
+                                                 .value(3)])
+        XCTAssertEqual(helper.subscription.history, [.requested(.unlimited)])
+        XCTAssertEqual(scheduler.history,
+                       [.now,
+                        .minimumTolerance,
+                        .scheduleAfterDateWithInterval(.nanoseconds(13),
+                                                       interval: .nanoseconds(13),
+                                                       tolerance: .nanoseconds(7),
+                                                       options: .nontrivialOptions),
+                        .now,
+                        .minimumTolerance,
+                        .scheduleAfterDateWithInterval(.nanoseconds(17),
+                                                       interval: .nanoseconds(13),
+                                                       tolerance: .nanoseconds(7),
+                                                       options: .nontrivialOptions),
+                        .now,
+                        .minimumTolerance,
+                        .scheduleAfterDateWithInterval(.nanoseconds(22),
+                                                       interval: .nanoseconds(13),
+                                                       tolerance: .nanoseconds(7),
+                                                       options: .nontrivialOptions),
+                        .schedule(options: nil)])
+
+        scheduler.rewind(to: .nanoseconds(300))
+
+        XCTAssertEqual(helper.tracking.history, [.subscription("Debounce"),
+                                                 .value(3),
+                                                 .completion(.finished)])
+        XCTAssertEqual(helper.subscription.history, [.requested(.unlimited)])
+        XCTAssertEqual(scheduler.history,
+                       [.now,
+                        .minimumTolerance,
+                        .scheduleAfterDateWithInterval(.nanoseconds(13),
+                                                       interval: .nanoseconds(13),
+                                                       tolerance: .nanoseconds(7),
+                                                       options: .nontrivialOptions),
+                        .now,
+                        .minimumTolerance,
+                        .scheduleAfterDateWithInterval(.nanoseconds(17),
+                                                       interval: .nanoseconds(13),
+                                                       tolerance: .nanoseconds(7),
+                                                       options: .nontrivialOptions),
+                        .now,
+                        .minimumTolerance,
+                        .scheduleAfterDateWithInterval(.nanoseconds(22),
+                                                       interval: .nanoseconds(13),
+                                                       tolerance: .nanoseconds(7),
+                                                       options: .nontrivialOptions),
+                        .schedule(options: nil)])
+        XCTAssertEqual(scheduler.cancellableTokenDeinitCount, 2)
+    }
+
+    func testFinishBeforeDue() {
+        let scheduler = VirtualTimeScheduler()
+        let helper = OperatorTestHelper(
+            publisherType: CustomPublisher.self,
+            initialDemand: .max(1),
+            receiveValueDemand: .none,
+            createSut: {
+                $0.debounce(for: .nanoseconds(13),
+                            scheduler: scheduler,
+                            options: .nontrivialOptions)
+            }
+        )
+
+        XCTAssertEqual(helper.publisher.send(1), .none)
+        scheduler.rewind(to: .nanoseconds(4))
+        helper.publisher.send(completion: .failure(.oops))
+
+        XCTAssertEqual(helper.tracking.history, [.subscription("Debounce")])
+        XCTAssertEqual(helper.subscription.history, [.requested(.unlimited)])
+        XCTAssertEqual(scheduler.history,
+                       [
+                        .now,
+                        .minimumTolerance,
+                        .scheduleAfterDateWithInterval(.nanoseconds(13),
+                                                       interval: .nanoseconds(13),
+                                                       tolerance: .nanoseconds(7),
+                                                       options: .nontrivialOptions),
+                        .schedule(options: nil)])
+
+        scheduler.rewind(to: .nanoseconds(100))
+
+        XCTAssertEqual(helper.tracking.history, [.subscription("Debounce"),
+                                                 .completion(.failure(.oops))])
+        XCTAssertEqual(helper.subscription.history, [.requested(.unlimited)])
+        XCTAssertEqual(scheduler.history,
+                       [
+                        .now,
+                        .minimumTolerance,
+                        .scheduleAfterDateWithInterval(.nanoseconds(13),
+                                                       interval: .nanoseconds(13),
+                                                       tolerance: .nanoseconds(7),
+                                                       options: .nontrivialOptions),
+                        .schedule(options: nil)])
+        XCTAssertEqual(scheduler.cancellableTokenDeinitCount, 0)
+    }
+
+    func testFailBeforeDue() {
+        let scheduler = VirtualTimeScheduler()
+        let helper = OperatorTestHelper(
+            publisherType: CustomPublisher.self,
+            initialDemand: .max(1),
+            receiveValueDemand: .none,
+            createSut: {
+                $0.debounce(for: .nanoseconds(13),
+                            scheduler: scheduler,
+                            options: .nontrivialOptions)
+            }
+        )
+
+        XCTAssertEqual(helper.publisher.send(1), .none)
+        scheduler.rewind(to: .nanoseconds(4))
+        helper.publisher.send(completion: .finished)
+
+        XCTAssertEqual(helper.tracking.history, [.subscription("Debounce")])
+        XCTAssertEqual(helper.subscription.history, [.requested(.unlimited)])
+        XCTAssertEqual(scheduler.history,
+                       [
+                        .now,
+                        .minimumTolerance,
+                        .scheduleAfterDateWithInterval(.nanoseconds(13),
+                                                       interval: .nanoseconds(13),
+                                                       tolerance: .nanoseconds(7),
+                                                       options: .nontrivialOptions),
+                        .schedule(options: nil)])
+
+        scheduler.rewind(to: .nanoseconds(100))
+
+        XCTAssertEqual(helper.tracking.history, [.subscription("Debounce"),
+                                                 .completion(.finished)])
+        XCTAssertEqual(helper.subscription.history, [.requested(.unlimited)])
+        XCTAssertEqual(scheduler.history,
+                       [
+                        .now,
+                        .minimumTolerance,
+                        .scheduleAfterDateWithInterval(.nanoseconds(13),
+                                                       interval: .nanoseconds(13),
+                                                       tolerance: .nanoseconds(7),
+                                                       options: .nontrivialOptions),
+                        .schedule(options: nil)])
+        XCTAssertEqual(scheduler.cancellableTokenDeinitCount, 0)
+    }
+
+    func testCancelBeforeDue() throws {
+        let scheduler = VirtualTimeScheduler()
+        let helper = OperatorTestHelper(
+            publisherType: CustomPublisher.self,
+            initialDemand: .max(1),
+            receiveValueDemand: .none,
+            createSut: {
+                $0.debounce(for: .nanoseconds(13),
+                            scheduler: scheduler,
+                            options: .nontrivialOptions)
+            }
+        )
+
+        XCTAssertEqual(helper.publisher.send(1), .none)
+        scheduler.rewind(to: .nanoseconds(4))
+        try XCTUnwrap(helper.downstreamSubscription).cancel()
+
+        XCTAssertEqual(helper.tracking.history, [.subscription("Debounce")])
+        XCTAssertEqual(helper.subscription.history, [.requested(.unlimited), .cancelled])
+        XCTAssertEqual(scheduler.history,
+                       [.now,
+                        .minimumTolerance,
+                        .scheduleAfterDateWithInterval(.nanoseconds(13),
+                                                       interval: .nanoseconds(13),
+                                                       tolerance: .nanoseconds(7),
+                                                       options: .nontrivialOptions)])
+
+        scheduler.rewind(to: .nanoseconds(100))
+
+        XCTAssertEqual(helper.tracking.history, [.subscription("Debounce")])
+        XCTAssertEqual(helper.subscription.history, [.requested(.unlimited), .cancelled])
+        XCTAssertEqual(scheduler.history,
+                       [.now,
+                        .minimumTolerance,
+                        .scheduleAfterDateWithInterval(.nanoseconds(13),
+                                                       interval: .nanoseconds(13),
+                                                       tolerance: .nanoseconds(7),
+                                                       options: .nontrivialOptions)])
+        XCTAssertEqual(scheduler.cancellableTokenDeinitCount, 0)
+    }
+
+    func testDemand() throws {
+        let scheduler = VirtualTimeScheduler()
+        let helper = OperatorTestHelper(
+            publisherType: CustomPublisher.self,
+            initialDemand: nil,
+            receiveValueDemand: .none,
+            createSut: {
+                $0.debounce(for: .nanoseconds(13),
+                            scheduler: scheduler,
+                            options: .nontrivialOptions)
+            }
+        )
+
+        XCTAssertEqual(helper.tracking.history, [.subscription("Debounce")])
+        XCTAssertEqual(helper.subscription.history, [.requested(.unlimited)])
+        XCTAssertEqual(scheduler.history, [])
+
+        XCTAssertEqual(helper.publisher.send(1), .none)
+
+        XCTAssertEqual(helper.tracking.history, [.subscription("Debounce")])
+        XCTAssertEqual(helper.subscription.history, [.requested(.unlimited)])
+        XCTAssertEqual(scheduler.history,
+                       [.now,
+                        .minimumTolerance,
+                        .scheduleAfterDateWithInterval(.nanoseconds(13),
+                                                       interval: .nanoseconds(13),
+                                                       tolerance: .nanoseconds(7),
+                                                       options: .nontrivialOptions)])
+
+        scheduler.rewind(to: .nanoseconds(100))
+
+        XCTAssertEqual(helper.tracking.history, [.subscription("Debounce")])
+        XCTAssertEqual(helper.subscription.history, [.requested(.unlimited)])
+        XCTAssertEqual(scheduler.history,
+                       [.now,
+                       .minimumTolerance,
+                       .scheduleAfterDateWithInterval(.nanoseconds(13),
+                                                      interval: .nanoseconds(13),
+                                                      tolerance: .nanoseconds(7),
+                                                      options: .nontrivialOptions)])
+
+        try XCTUnwrap(helper.downstreamSubscription).request(.max(3))
+
+        XCTAssertEqual(helper.tracking.history, [.subscription("Debounce")])
+        XCTAssertEqual(helper.subscription.history, [.requested(.unlimited)])
+        XCTAssertEqual(scheduler.history,
+                       [.now,
+                       .minimumTolerance,
+                       .scheduleAfterDateWithInterval(.nanoseconds(13),
+                                                      interval: .nanoseconds(13),
+                                                      tolerance: .nanoseconds(7),
+                                                      options: .nontrivialOptions)])
+
+        scheduler.rewind(to: .nanoseconds(200))
+
+        XCTAssertEqual(helper.tracking.history, [.subscription("Debounce")])
+        XCTAssertEqual(helper.subscription.history, [.requested(.unlimited)])
+        XCTAssertEqual(scheduler.history,
+                       [.now,
+                       .minimumTolerance,
+                       .scheduleAfterDateWithInterval(.nanoseconds(13),
+                                                      interval: .nanoseconds(13),
+                                                      tolerance: .nanoseconds(7),
+                                                      options: .nontrivialOptions)])
+
+        XCTAssertEqual(helper.publisher.send(2), .none)
+        scheduler.rewind(to: .nanoseconds(250))
+        XCTAssertEqual(helper.publisher.send(3), .none)
+        scheduler.rewind(to: .nanoseconds(300))
+        XCTAssertEqual(helper.publisher.send(4), .none)
+        scheduler.rewind(to: .nanoseconds(350))
+        XCTAssertEqual(helper.publisher.send(5), .none)
+        scheduler.rewind(to: .nanoseconds(400))
+
+        XCTAssertEqual(helper.tracking.history, [.subscription("Debounce"),
+                                                 .value(2),
+                                                 .value(3),
+                                                 .value(4)])
+        XCTAssertEqual(helper.subscription.history, [.requested(.unlimited)])
+        XCTAssertEqual(scheduler.history,
+                       [.now,
+                        .minimumTolerance,
+                        .scheduleAfterDateWithInterval(.nanoseconds(13),
+                                                       interval: .nanoseconds(13),
+                                                       tolerance: .nanoseconds(7),
+                                                       options: .nontrivialOptions),
+                        .now,
+                        .minimumTolerance,
+                        .scheduleAfterDateWithInterval(.nanoseconds(213),
+                                                       interval: .nanoseconds(13),
+                                                       tolerance: .nanoseconds(7),
+                                                       options: .nontrivialOptions),
+                        .now,
+                        .minimumTolerance,
+                        .scheduleAfterDateWithInterval(.nanoseconds(263),
+                                                       interval: .nanoseconds(13),
+                                                       tolerance: .nanoseconds(7),
+                                                       options: .nontrivialOptions),
+                        .now,
+                        .minimumTolerance,
+                        .scheduleAfterDateWithInterval(.nanoseconds(313),
+                                                       interval: .nanoseconds(13),
+                                                       tolerance: .nanoseconds(7),
+                                                       options: .nontrivialOptions),
+                        .now,
+                        .minimumTolerance,
+                        .scheduleAfterDateWithInterval(.nanoseconds(363),
+                                                       interval: .nanoseconds(13),
+                                                       tolerance: .nanoseconds(7),
+                                                       options: .nontrivialOptions)])
+
+        try XCTUnwrap(helper.downstreamSubscription).cancel()
+        try XCTUnwrap(helper.downstreamSubscription).request(.max(1))
+
+        XCTAssertEqual(helper.publisher.send(6), .none)
+        scheduler.rewind(to: .nanoseconds(450))
+
+        XCTAssertEqual(helper.tracking.history, [.subscription("Debounce"),
+                                                 .value(2),
+                                                 .value(3),
+                                                 .value(4)])
+        XCTAssertEqual(helper.subscription.history, [.requested(.unlimited), .cancelled])
+        XCTAssertEqual(scheduler.history,
+                       [.now,
+                        .minimumTolerance,
+                        .scheduleAfterDateWithInterval(.nanoseconds(13),
+                                                       interval: .nanoseconds(13),
+                                                       tolerance: .nanoseconds(7),
+                                                       options: .nontrivialOptions),
+                        .now,
+                        .minimumTolerance,
+                        .scheduleAfterDateWithInterval(.nanoseconds(213),
+                                                       interval: .nanoseconds(13),
+                                                       tolerance: .nanoseconds(7),
+                                                       options: .nontrivialOptions),
+                        .now,
+                        .minimumTolerance,
+                        .scheduleAfterDateWithInterval(.nanoseconds(263),
+                                                       interval: .nanoseconds(13),
+                                                       tolerance: .nanoseconds(7),
+                                                       options: .nontrivialOptions),
+                        .now,
+                        .minimumTolerance,
+                        .scheduleAfterDateWithInterval(.nanoseconds(313),
+                                                       interval: .nanoseconds(13),
+                                                       tolerance: .nanoseconds(7),
+                                                       options: .nontrivialOptions),
+                        .now,
+                        .minimumTolerance,
+                        .scheduleAfterDateWithInterval(.nanoseconds(363),
+                                                       interval: .nanoseconds(13),
+                                                       tolerance: .nanoseconds(7),
+                                                       options: .nontrivialOptions)])
+        XCTAssertEqual(scheduler.cancellableTokenDeinitCount, 4)
+    }
+
+    func testBadScheduler() {
+        // What if the scheduler returns a cancellable that does nothing at all?
+
+        let scheduler = VirtualTimeScheduler(
+            cancellableTokenType: VirtualTimeScheduler.NoopCancellableToken.self
+        )
+
+        let helper = OperatorTestHelper(
+            publisherType: CustomPublisher.self,
+            initialDemand: .unlimited,
+            receiveValueDemand: .none,
+            createSut: {
+                $0.debounce(for: .nanoseconds(13),
+                            scheduler: scheduler,
+                            options: .nontrivialOptions)
+            }
+        )
+
+        XCTAssertEqual(helper.publisher.send(1), .none)
+        scheduler.rewind(to: .nanoseconds(2))
+        XCTAssertEqual(helper.publisher.send(2), .none)
+        scheduler.rewind(to: .nanoseconds(4))
+        XCTAssertEqual(helper.publisher.send(3), .none)
+        scheduler.rewind(to: .nanoseconds(6))
+
+        XCTAssertEqual(helper.publisher.send(42), .none)
+        scheduler.rewind(to: .nanoseconds(50))
+
+        XCTAssertEqual(helper.tracking.history, [.subscription("Debounce"),
+                                                 .value(42),
+                                                 .value(42),
+                                                 .value(42)])
+        XCTAssertEqual(helper.subscription.history, [.requested(.unlimited)])
+        XCTAssertEqual(scheduler.history,
+                       [.now,
+                        .minimumTolerance,
+                        .scheduleAfterDateWithInterval(.nanoseconds(13),
+                                                       interval: .nanoseconds(13),
+                                                       tolerance: .nanoseconds(7),
+                                                       options: .nontrivialOptions),
+                        .now,
+                        .minimumTolerance,
+                        .scheduleAfterDateWithInterval(.nanoseconds(15),
+                                                       interval: .nanoseconds(13),
+                                                       tolerance: .nanoseconds(7),
+                                                       options: .nontrivialOptions),
+                        .now,
+                        .minimumTolerance,
+                        .scheduleAfterDateWithInterval(.nanoseconds(17),
+                                                       interval: .nanoseconds(13),
+                                                       tolerance: .nanoseconds(7),
+                                                       options: .nontrivialOptions),
+                        .now,
+                        .minimumTolerance,
+                        .scheduleAfterDateWithInterval(.nanoseconds(19),
+                                                       interval: .nanoseconds(13),
+                                                       tolerance: .nanoseconds(7),
+                                                       options: .nontrivialOptions)])
+
+        XCTAssertEqual(scheduler.cancellableTokenDeinitCount, 0)
+    }
+
+    func testSetupTimerWeakCapture() {
+        let scheduler = VirtualTimeScheduler()
+        var subscriptionDestroyed = false
+        do {
+            let helper = OperatorTestHelper(
+                publisherType: CustomPublisher.self,
+                initialDemand: .unlimited,
+                receiveValueDemand: .none,
+                createSut: {
+                    $0.debounce(for: .nanoseconds(13), scheduler: scheduler)
+                }
+            )
+
+            helper.tracking.onDeinit = { subscriptionDestroyed = true }
+
+            XCTAssertEqual(helper.publisher.send(1), .none)
+        }
+
+        XCTAssertTrue(subscriptionDestroyed)
+    }
+
+    func testCrashesWithImmediateScheduler() {
+        let helper = OperatorTestHelper(
+            publisherType: CustomPublisher.self,
+            initialDemand: .max(2),
+            receiveValueDemand: .max(1),
+            createSut: {
+                $0.debounce(for: .nanoseconds(13),
+                            scheduler: ImmediateScheduler.shared)
+            }
+        )
+
+        assertCrashes {
+            _ = helper.publisher.send(1)
+        }
+    }
+
+    func testTimeoutReceiveValueBeforeSubscription() {
+        let scheduler = VirtualTimeScheduler()
+        testReceiveValueBeforeSubscription(
+            value: 42,
+            expected: .crash,
+            { $0.debounce(for: .nanoseconds(13), scheduler: scheduler) }
+        )
+    }
+
+    func testTimeoutReceiveCompletionBeforeSubscription() {
+        let scheduler = VirtualTimeScheduler()
+        testReceiveCompletionBeforeSubscription(
+            inputType: Int.self,
+            expected: .crash,
+            { $0.debounce(for: .nanoseconds(13), scheduler: scheduler) }
+        )
+    }
+
+    func testTimeoutRequestBeforeSubscription() {
+        let scheduler = VirtualTimeScheduler()
+        testRequestBeforeSubscription(
+            inputType: Int.self,
+            shouldCrash: true,
+            { $0.debounce(for: .nanoseconds(13), scheduler: scheduler) }
+        )
+    }
+
+    func testTimeoutReceiveSubscriptionTwice() throws {
+        let scheduler = VirtualTimeScheduler()
+
+        let helper = OperatorTestHelper(
+            publisherType: CustomPublisher.self,
+            initialDemand: nil,
+            receiveValueDemand: .none,
+            createSut: { $0.debounce(for: .nanoseconds(13), scheduler: scheduler) }
+        )
+
+        XCTAssertEqual(helper.subscription.history, [.requested(.unlimited)])
+
+        let secondSubscription = CustomSubscription()
+
+        try XCTUnwrap(helper.publisher.subscriber)
+            .receive(subscription: secondSubscription)
+
+        XCTAssertEqual(secondSubscription.history, [.cancelled])
+
+        try XCTUnwrap(helper.publisher.subscriber)
+            .receive(subscription: helper.subscription)
+
+        XCTAssertEqual(helper.subscription.history, [.requested(.unlimited),
+                                                     .cancelled])
+
+        try XCTUnwrap(helper.downstreamSubscription).cancel()
+
+        XCTAssertEqual(helper.subscription.history, [.requested(.unlimited),
+                                                     .cancelled,
+                                                     .cancelled])
+    }
+
+    func testTimeoutCancelBeforeSubscription() {
+        let scheduler = VirtualTimeScheduler()
+        testCancelBeforeSubscription(
+            inputType: Int.self,
+            shouldCrash: false,
+            { $0.timeout(.nanoseconds(13), scheduler: scheduler) }
+        )
+    }
+
+    func testDebounceReflection() throws {
+        let scheduler = VirtualTimeScheduler()
+
+        let customMirror = hasUpdatedReflection
+            ? expectedChildren(
+                  ("downstream", .contains("TrackingSubscriberBase")),
+                  ("downstreamDemand", "max(0)"),
+                  ("currentValue", "nil")
+              )
+            : expectedChildren(
+                  ("upstream", .contains("CustomConnectablePublisherBase")),
+                  ("downstream", .contains("TrackingSubscriberBase")),
+                  ("upstreamSubscription", .anything),
+                  ("downstreamDemand", "max(0)"),
+                  ("currentValue", "nil")
+              )
+
+        try testReflection(
+            parentInput: Int.self,
+            parentFailure: Error.self,
+            description: "Debounce",
+            customMirror: customMirror,
+            playgroundDescription: "Debounce",
+            { $0.debounce(for: .nanoseconds(13), scheduler: scheduler) }
+        )
+    }
+}
+
+// FIXME: Remove this as soon as we switch our CI to the latest OS releases
+private var hasUpdatedReflection: Bool {
+#if OPENCOMBINE_COMPATIBILITY_TEST
+    if #available(macOS 10.16, iOS 14.0, *) {
+        return true
+    } else {
+        return false
+    }
+#else
+    return true
+#endif
+}

Tests/OpenCombineTests/PublisherTests/DelayTests.swift

@@ -372,7 +372,7 @@ final class DelayTests: XCTestCase {
                            [.minimumTolerance,
                             .now,
                             .scheduleAfterDate(.seconds(0.35),
-                                               tolerance: 0,
+                                               tolerance: .nanoseconds(7),
                                                options: nil)])
             tracking.cancel()
             publisher.cancel()
@@ -401,7 +401,7 @@ final class DelayTests: XCTestCase {
                            [.minimumTolerance,
                             .now,
                             .scheduleAfterDate(.seconds(0.35),
-                                               tolerance: 0,
+                                               tolerance: .nanoseconds(7),
                                                options: nil)])
             tracking.cancel()
             publisher.cancel()
@@ -445,7 +445,7 @@ final class DelayTests: XCTestCase {
     }
 
     func testDelayReflection() throws {
-        /// Delay's Inner doesn't customize its reflection
+        // Delay's Inner doesn't customize its reflection
         try testReflection(parentInput: Int.self,
                            parentFailure: Error.self,
                            description: nil,

Tests/OpenCombineTests/PublisherTests/DropUntilOutputTests.swift

@@ -167,10 +167,10 @@ final class DropUntilOutputTests: XCTestCase {
         let dropUntilOutput = publisher.drop(untilOutputFrom: otherPublisher)
         let tracking = TrackingSubscriber(
             receiveSubscription: { _ in
-                XCTAssertNotNil(publisher.subscriber)
-                XCTAssertNotNil(otherPublisher.subscriber)
+                XCTAssertNil(publisher.subscriber)
+                XCTAssertNil(otherPublisher.subscriber)
                 XCTAssertEqual(subscription.history, [])
-                XCTAssertEqual(otherSubscription.history, [.requested(.max(1))])
+                XCTAssertEqual(otherSubscription.history, [])
             }
         )
 
@@ -220,15 +220,15 @@ final class DropUntilOutputTests: XCTestCase {
 
         dropUntilOutput.subscribe(tracking)
 
-        XCTAssertEqual(tracking.history, [.completion(.finished),
-                                          .subscription("DropUntilOutput")])
+        XCTAssertEqual(tracking.history, [.subscription("DropUntilOutput"),
+                                          .completion(.finished)])
 
         let subscription = CustomSubscription()
         try XCTUnwrap(publisher.subscriber).receive(subscription: subscription)
 
         XCTAssertEqual(subscription.history, [.requested(.max(14))])
-        XCTAssertEqual(tracking.history, [.completion(.finished),
-                                          .subscription("DropUntilOutput")])
+        XCTAssertEqual(tracking.history, [.subscription("DropUntilOutput"),
+                                          .completion(.finished)])
     }
 
     func testReusableOtherSubscriber() throws {
@@ -285,7 +285,7 @@ final class DropUntilOutputTests: XCTestCase {
     func testDropUntilOutputOtherReceiveValueBeforeSubscription() {
         testReceiveValueBeforeSubscription(
             value: 42,
-            expected: .history([.completion(.finished), .subscription("DropUntilOutput")],
+            expected: .history([.subscription("DropUntilOutput"), .completion(.finished)],
                                demand: .none),
             { Empty<Int, Never>().drop(untilOutputFrom: $0) }
         )
@@ -294,8 +294,8 @@ final class DropUntilOutputTests: XCTestCase {
     func testDropUntilOutputReceiveCompletionBeforeSubscription() {
         testReceiveCompletionBeforeSubscription(
             inputType: Int.self,
-            expected: .history([.completion(.finished),
-                                .subscription("DropUntilOutput"),
+            expected: .history([.subscription("DropUntilOutput"),
+                                .completion(.finished),
                                 .completion(.finished)]),
             { $0.drop(untilOutputFrom: Empty<Int, Never>()) }
         )
@@ -304,8 +304,8 @@ final class DropUntilOutputTests: XCTestCase {
     func testDropUntilOutputOtherReceiveCompletionBeforeSubscription() {
         testReceiveCompletionBeforeSubscription(
             inputType: Int.self,
-            expected: .history([.completion(.finished),
-                                .subscription("DropUntilOutput"),
+            expected: .history([.subscription("DropUntilOutput"),
+                                .completion(.finished),
                                 .completion(.finished)]),
             { Empty<Int, Never>().drop(untilOutputFrom: $0) }
         )

Tests/OpenCombineTests/PublisherTests/EncodeTests.swift

@@ -155,7 +155,7 @@ final class EncodeTests: XCTestCase {
                            customMirror: expectedChildren(
                                ("downstream", .contains("TrackingSubscriberBase")),
                                ("finished", "false"),
-                               ("upstreamSubscription", "nil")
+                               ("upstreamSubscription", .anything)
                            ),
                            playgroundDescription: "Encode",
                            { $0.encode(encoder: encoder) })
@@ -275,7 +275,7 @@ final class EncodeTests: XCTestCase {
                            customMirror: expectedChildren(
                                ("downstream", .contains("TrackingSubscriberBase")),
                                ("finished", "false"),
-                               ("upstreamSubscription", "nil")
+                               ("upstreamSubscription", .anything)
                            ),
                            playgroundDescription: "Decode",
                            { $0.decode(type: Int.self, decoder: decoder) })

Tests/OpenCombineTests/PublisherTests/FlatMapTests.swift

@@ -142,32 +142,43 @@ final class FlatMapTests: XCTestCase {
         // Simply making it here shows that there's no dealock
     }
 
-    func testCancelCancels() {
+    func testCancelCancels() throws {
         let upstreamSubscription = CustomSubscription()
         let upstreamPublisher = CustomPublisherBase<Int, Never>(
-            subscription: upstreamSubscription)
+            subscription: upstreamSubscription
+        )
 
         let childSubscription = CustomSubscription()
         let childPublisher = CustomPublisherBase<Int, Never>(
-            subscription: childSubscription)
+            subscription: childSubscription
+        )
 
         let flatMap = upstreamPublisher.flatMap { _ in childPublisher }
 
         var downstreamSubscription: Subscription?
-        let downstreamSubscriber = TrackingSubscriberBase<Int, Never>(receiveSubscription:
-        {
-            downstreamSubscription = $0
-            $0.request(.unlimited)
-        })
+        let downstreamSubscriber = TrackingSubscriberBase<Int, Never>(
+            receiveSubscription: {
+                downstreamSubscription = $0
+                $0.request(.max(42))
+            }
+        )
+
+        upstreamSubscription.onCancel = {
+            XCTAssertEqual(childSubscription.history, [.requested(.max(1)), .cancelled])
+        }
+
+        childSubscription.onCancel = {
+            XCTAssertEqual(upstreamSubscription.history, [.requested(.unlimited)])
+        }
 
         flatMap.subscribe(downstreamSubscriber)
 
         XCTAssertEqual(upstreamPublisher.send(1), .none)
 
-        downstreamSubscription?.cancel()
+        try XCTUnwrap(downstreamSubscription).cancel()
 
-        XCTAssertEqual(upstreamSubscription.history.last, .cancelled)
-        XCTAssertEqual(childSubscription.history.last, .cancelled)
+        XCTAssertEqual(upstreamSubscription.history, [.requested(.unlimited), .cancelled])
+        XCTAssertEqual(childSubscription.history, [.requested(.max(1)), .cancelled])
     }
 
     func testCancelTwice() throws {
@@ -436,10 +447,7 @@ final class FlatMapTests: XCTestCase {
         let childSubscription = CustomSubscription()
         let child = CustomPublisher(subscription: childSubscription)
 
-        // If Apple changes the implementation to use recursive lock,
-        // we must make sure no stack overflow occurs here,
-        // which will also be detected as a crash, which is not what we want.
-        var recursionDepth = 10
+        var recursionDepth = 5
         helper.subscription.onRequest = { _ in
             if recursionDepth <= 0 {
                 return
@@ -450,9 +458,17 @@ final class FlatMapTests: XCTestCase {
 
         XCTAssertEqual(helper.publisher.send(child), .none)
 
-        assertCrashes {
-            child.send(completion: .finished)
-        }
+        child.send(completion: .finished)
+
+        XCTAssertEqual(helper.tracking.history, [.subscription("FlatMap")])
+        XCTAssertEqual(helper.subscription.history, [.requested(.max(1)),
+                                                     .requested(.max(1)),
+                                                     .requested(.max(1)),
+                                                     .requested(.max(1)),
+                                                     .requested(.max(1)),
+                                                     .requested(.max(1)),
+                                                     .requested(.max(1))])
+        XCTAssertEqual(childSubscription.history, [.requested(.max(1))])
     }
 
     func testDownstreamLockReentrance() throws {

Tests/OpenCombineTests/PublisherTests/FutureTests.swift

@@ -98,6 +98,8 @@ final class FutureTests: XCTestCase {
         future.subscribe(subscriber)
 
         subscriber.subscriptions.forEach { $0.cancel() }
+        subscriber.subscriptions.forEach { $0.cancel() }
+        subscriber.subscriptions.forEach { $0.request(.max(1)) }
 
         promise?(.success(42))
 
@@ -106,7 +108,7 @@ final class FutureTests: XCTestCase {
         ])
     }
 
-    func testSubscribeAfterResolution() {
+    func testSubscribeAfterSuccessfulResolution() {
         var promise: Sut.Promise?
 
         let future = Sut { promise = $0 }
@@ -124,15 +126,36 @@ final class FutureTests: XCTestCase {
         ])
     }
 
-    func testCrashesOnZeroDemand() {
+    func testSubscribeAfterFailure() {
+        // TODO: Remove this `if` as soon as iOS 14 is released.
+        if hasMissingFailureAfterLateSubscriptionBug { return }
+
+        var promise: Sut.Promise?
+
+        let future = Sut { promise = $0 }
+        promise?(.failure(.oops))
+
+        let subscriber = TrackingSubscriber()
+        future.subscribe(subscriber)
+
+        XCTAssertEqual(subscriber.history, [.subscription("Future"),
+                                            .completion(.failure(.oops))])
+    }
+
+    func testCrashesOnZeroDemand() throws {
         let future = Sut { _ in }
 
-        let subscriber = TrackingSubscriber(receiveSubscription: { subscription in
-            self.assertCrashes {
-                subscription.request(.none)
+        var downstreamSubscription: Subscription?
+        let subscriber = TrackingSubscriber(
+            receiveSubscription: {
+                downstreamSubscription = $0
             }
-        })
+        )
         future.subscribe(subscriber)
+
+        try self.assertCrashes {
+            try XCTUnwrap(downstreamSubscription).request(.none)
+        }
     }
 
     func testValueRecursion() {
@@ -211,7 +234,7 @@ final class FutureTests: XCTestCase {
         XCTAssertTrue(hasStarted)
     }
 
-    func testWaitsForRequest() {
+    func testWaitsForDemandSuccess() {
         var promise: Sut.Promise?
 
         let future = Sut { promise = $0 }
@@ -236,4 +259,105 @@ final class FutureTests: XCTestCase {
             .completion(.finished)
         ])
     }
+
+    func testReleasesEverythingOnTermination() {
+
+        enum TerminationReason: CaseIterable {
+            case cancelled
+            case finished
+            case failed
+        }
+
+        for reason in TerminationReason.allCases {
+            weak var weakSubscriber: TrackingSubscriber?
+            weak var weakFuture: Sut?
+            weak var weakSubscription: AnyObject?
+
+            do {
+                var promise: Sut.Promise?
+                let future = Sut { promise = $0 }
+                do {
+                    let subscriber = TrackingSubscriber(
+                        receiveSubscription: {
+                            weakSubscription = $0 as AnyObject
+                            $0.request(.max(1))
+                        }
+                    )
+                    weakSubscriber = subscriber
+                    weakFuture = future
+
+                    future.subscribe(subscriber)
+                }
+
+                switch reason {
+                case .cancelled:
+                    (weakSubscription as? Subscription)?.cancel()
+                case .finished:
+                    promise?(.success(1))
+                case .failed:
+                    promise?(.failure(.oops))
+                }
+
+                XCTAssertNil(weakSubscriber, "Subscriber leaked - \(reason)")
+                if !leaksSubscription {
+                    // This leak has been fixed in the betas.
+                    // TODO: Remove this `if` as soon as iOS 14 is released.
+                    XCTAssertNil(weakSubscription, "Subscription leaked - \(reason)")
+                }
+            }
+
+            XCTAssertNil(weakFuture, "Future leaked - \(reason)")
+        }
+    }
+
+    func testConduitReflection() throws {
+        try testSubscriptionReflection(
+            description: "Future",
+            customMirror: expectedChildren(
+                ("parent", .contains("Future")),
+                ("downstream", .contains("TrackingSubscriberBase")),
+                ("hasAnyDemand", "false"),
+                ("subject", .contains("Future"))
+            ),
+            playgroundDescription: "Future",
+            sut: Sut { _ in }
+        )
+    }
+}
+
+@available(macOS, deprecated: 10.16, message: """
+If macOS 10.16/11.0 has already been released, this property should be removed
+""")
+@available(iOS, deprecated: 14, message: """
+If iOS 14  has already been released, this property should be removed
+""")
+private var leaksSubscription: Bool { // swiftlint:disable:this let_var_whitespace
+#if OPENCOMBINE_COMPATIBILITY_TEST
+    if #available(macOS 10.16, iOS 14.0, *) {
+        return false
+    } else {
+        return true
+    }
+#else
+    return false
+#endif
+}
+
+@available(macOS, deprecated: 10.16, message: """
+If macOS 10.16/11.0 has already been released, this property should be removed
+""")
+@available(iOS, deprecated: 14, message: """
+If iOS 14  has already been released, this property should be removed
+""")
+private var hasMissingFailureAfterLateSubscriptionBug: Bool {
+// swiftlint:disable:previous let_var_whitespace
+#if OPENCOMBINE_COMPATIBILITY_TEST
+    if #available(macOS 10.16, iOS 14.0, *) {
+        return false
+    } else {
+        return true
+    }
+#else
+    return false
+#endif
 }

Tests/OpenCombineTests/PublisherTests/IgnoreOutputTests.swift

@@ -208,7 +208,7 @@ final class IgnoreOutputTests: XCTestCase {
                            description: "IgnoreOutput",
                            customMirror: expectedChildren(
                                ("downstream", .contains("TrackingSubscriberBase")),
-                               ("status", .contains("awaitingSubscription"))
+                               ("status", .anything)
                            ),
                            playgroundDescription: "IgnoreOutput",
                            { $0.ignoreOutput() })

Tests/OpenCombineTests/PublisherTests/MapErrorTests.swift

@@ -185,6 +185,7 @@ final class MapErrorTests: XCTestCase {
                            description: "MapError",
                            customMirror: childrenIsEmpty,
                            playgroundDescription: "MapError",
+                           subscriberIsAlsoSubscription: false,
                            { $0.mapError { $0 } })
     }
 

Tests/OpenCombineTests/PublisherTests/MapKeyPathTests.swift

@@ -126,6 +126,7 @@ final class MapKeyPathTests: XCTestCase {
                                ("keyPath", .contains("KeyPath"))
                            ),
                            playgroundDescription: "ValueForKey",
+                           subscriberIsAlsoSubscription: false,
                            { $0.map(\.doubled) })
 
         try testReflection(parentInput: Int.self,
@@ -136,6 +137,7 @@ final class MapKeyPathTests: XCTestCase {
                                ("keyPath1", .contains("KeyPath"))
                            ),
                            playgroundDescription: "ValueForKeys",
+                           subscriberIsAlsoSubscription: false,
                            { $0.map(\.doubled, \.tripled) })
 
         try testReflection(parentInput: Int.self,
@@ -147,6 +149,7 @@ final class MapKeyPathTests: XCTestCase {
                                ("keyPath2", .contains("KeyPath"))
                            ),
                            playgroundDescription: "ValueForKeys",
+                           subscriberIsAlsoSubscription: false,
                            { $0.map(\.doubled, \.tripled, \.quadripled) })
     }
 

Tests/OpenCombineTests/PublisherTests/MapTests.swift

@@ -221,6 +221,7 @@ final class MapTests: XCTestCase {
                            description: "Map",
                            customMirror: childrenIsEmpty,
                            playgroundDescription: "Map",
+                           subscriberIsAlsoSubscription: false,
                            { $0.map { $0 * 2 } })
     }
 

Tests/OpenCombineTests/PublisherTests/ReceiveOnTests.swift

@@ -256,7 +256,7 @@ final class ReceiveOnTests: XCTestCase {
         helper.publisher.send(completion: .finished)
     }
 
-    func testWeakCaptureWhenSchedulingValue() {
+    func testStrongCaptureWhenSchedulingValue() {
         let scheduler = VirtualTimeScheduler()
         var value: Int?
         var subscriberReleased = false
@@ -276,11 +276,11 @@ final class ReceiveOnTests: XCTestCase {
         }
         XCTAssertFalse(subscriberReleased)
         scheduler.executeScheduledActions()
-        XCTAssertNil(value)
+        XCTAssertEqual(value, 42)
         XCTAssertTrue(subscriberReleased)
     }
 
-    func testWeakCaptureWhenSchedulingCompletion() {
+    func testStrongCaptureWhenSchedulingCompletion() {
         let scheduler = VirtualTimeScheduler()
         var completion: Subscribers.Completion<TestingError>?
         var subscriberReleased = false
@@ -300,7 +300,7 @@ final class ReceiveOnTests: XCTestCase {
         }
         XCTAssertFalse(subscriberReleased)
         scheduler.executeScheduledActions()
-        XCTAssertNil(completion)
+        XCTAssertEqual(completion, .finished)
         XCTAssertTrue(subscriberReleased)
     }
 

Tests/OpenCombineTests/PublisherTests/ReplaceNilTests.swift

@@ -108,6 +108,7 @@ final class ReplaceNilTests: XCTestCase {
                            description: "Map",
                            customMirror: childrenIsEmpty,
                            playgroundDescription: "Map",
+                           subscriberIsAlsoSubscription: false,
                            { $0.replaceNil(with: 0) })
     }
 }

Tests/OpenCombineTests/PublisherTests/ScanTests.swift

@@ -152,6 +152,7 @@ final class ScanTests: XCTestCase {
                                ("result", "0")
                            ),
                            playgroundDescription: "Scan",
+                           subscriberIsAlsoSubscription: false,
                            { $0.scan(0, +) })
     }
 
@@ -313,7 +314,7 @@ final class ScanTests: XCTestCase {
                            description: "TryScan",
                            customMirror: expectedChildren(
                                ("downstream", .contains("TrackingSubscriber")),
-                               ("status", .contains("awaitingSubscription")),
+                               ("status", .anything),
                                ("result", "0")
                            ),
                            playgroundDescription: "TryScan",

Tests/OpenCombineTests/PublisherTests/SetFailureTypeTests.swift

@@ -180,6 +180,7 @@ final class SetFailureTypeTests: XCTestCase {
                            description: "SetFailureType",
                            customMirror: childrenIsEmpty,
                            playgroundDescription: "SetFailureType",
+                           subscriberIsAlsoSubscription: false,
                            { $0.setFailureType(to: Error.self) })
     }
 }

Tests/OpenCombineTests/PublisherTests/ShareTests.swift

@@ -134,15 +134,17 @@ final class ShareTests: XCTestCase {
     }
 
     func testShareReceiveValueBeforeSubscription() {
-        testReceiveValueBeforeSubscription(value: 0,
-                                           expected: .crash,
-                                           { $0.share() })
+        testReceiveValueBeforeSubscription(
+            value: 0,
+            expected: .history([.subscription("Multicast")], demand: .none),
+            { $0.share() }
+        )
     }
 
     func testShareCompletionBeforeSubscription() {
         testReceiveCompletionBeforeSubscription(
             inputType: Int.self,
-            expected: .crash,
+            expected: .history([.subscription("Multicast")]),
             { $0.share() }
         )
     }

Tests/OpenCombineTests/PublisherTests/SwitchToLatestTests.swift

@@ -0,0 +1,821 @@
+//
+//  SwitchToLatestTests.swift
+//  
+//
+//  Created by Sergej Jaskiewicz on 07.01.2020.
+//
+
+import XCTest
+
+#if OPENCOMBINE_COMPATIBILITY_TEST
+import Combine
+#else
+import OpenCombine
+#endif
+
+@available(macOS 10.15, iOS 13.0, *)
+final class SwitchToLatestTests: XCTestCase {
+
+    var cancellables = [AnyCancellable]()
+
+    override func tearDown() {
+        cancellables = []
+        super.tearDown()
+    }
+
+    func testSwitchToLatestSequenceWithSink() {
+        var history = [Int]()
+        (1 ..< 5)
+            .publisher
+            .map {
+                ($0 ..< $0 + 4).publisher
+            }
+            .switchToLatest()
+            .sink {
+                history.append($0)
+            }.store(in: &cancellables)
+
+        XCTAssertEqual(history, [1, 2, 3, 4, 2, 3, 4, 5, 3, 4, 5, 6, 4, 5, 6, 7])
+    }
+
+    func testRequestOneByOne() {
+        let tracking = TrackingSubscriberBase<Int, Never>(
+            receiveSubscription: { $0.request(.max(1)) },
+            receiveValue: { _ in .max(1) }
+        )
+        tracking.store(in: &cancellables)
+
+        (1 ..< 5)
+            .publisher
+            .map {
+                ($0 ..< $0 + 4).publisher
+            }
+            .switchToLatest()
+            .subscribe(tracking)
+
+        XCTAssertEqual(tracking.history, [.subscription("SwitchToLatest"),
+                                          .value(1),
+                                          .value(2),
+                                          .value(3),
+                                          .value(4),
+                                          .value(2),
+                                          .value(3),
+                                          .value(4),
+                                          .value(5),
+                                          .value(3),
+                                          .value(4),
+                                          .value(5),
+                                          .value(6),
+                                          .value(4),
+                                          .value(5),
+                                          .value(6),
+                                          .value(7),
+                                          .completion(.finished)])
+    }
+
+    func testSendsChildValuesFromLatestOuterPublisher() {
+        let upstreamPublisher =
+            PassthroughSubject<PassthroughSubject<Int, TestingError>, TestingError>()
+        let childPublisher1 = PassthroughSubject<Int, TestingError>()
+        let childPublisher2 = PassthroughSubject<Int, TestingError>()
+
+        let switchToLatest = upstreamPublisher.switchToLatest()
+
+        let downstreamSubscriber = TrackingSubscriber(receiveSubscription: {
+            $0.request(.unlimited)
+        })
+
+        switchToLatest.subscribe(downstreamSubscriber)
+
+        upstreamPublisher.send(childPublisher1)
+        upstreamPublisher.send(childPublisher2)
+
+        childPublisher1.send(666)
+        childPublisher2.send(777)
+        childPublisher1.send(888)
+        childPublisher2.send(999)
+        XCTAssertEqual(downstreamSubscriber.history, [.subscription("SwitchToLatest"),
+                                                      .value(777),
+                                                      .value(999)])
+    }
+
+    func testDemand() throws {
+        let helper = OperatorTestHelper(
+            publisherType: CustomPublisherBase<CustomPublisher, TestingError>.self,
+            initialDemand: nil,
+            receiveValueDemand: .max(5),
+            createSut: { $0.switchToLatest() }
+        )
+
+        XCTAssertEqual(helper.tracking.history, [.subscription("SwitchToLatest")])
+        XCTAssertEqual(helper.subscription.history, [.requested(.unlimited)])
+
+        let subscription1 = CustomSubscription()
+        let nestedPublisher1 = CustomPublisher(subscription: subscription1)
+        XCTAssertEqual(helper.publisher.send(nestedPublisher1), .none)
+        XCTAssertNotNil(nestedPublisher1.subscriber)
+        XCTAssertEqual(helper.subscription.history, [.requested(.unlimited)])
+        XCTAssertEqual(subscription1.history, [])
+
+        try XCTUnwrap(helper.downstreamSubscription).request(.max(1)) // demand == 1
+        XCTAssertEqual(subscription1.history, [.requested(.max(1))])
+
+        nestedPublisher1.send(completion: .finished)
+
+        try XCTUnwrap(helper.downstreamSubscription).request(.max(41)) // demand == 42
+        try XCTUnwrap(helper.downstreamSubscription).request(.max(1)) // demand == 43
+
+        XCTAssertEqual(subscription1.history, [.requested(.max(1))])
+        XCTAssertEqual(helper.subscription.history, [.requested(.unlimited)])
+
+        let subscription2 = CustomSubscription()
+        let nestedPublisher2 = CustomPublisher(subscription: subscription2)
+
+        XCTAssertEqual(helper.publisher.send(nestedPublisher2), .none)
+        XCTAssertEqual(subscription1.history, [.requested(.max(1))])
+        XCTAssertEqual(subscription2.history, [.requested(.max(43))])
+
+        XCTAssertEqual(nestedPublisher2.send(1), .max(5)) // demand == 42
+        XCTAssertEqual(nestedPublisher2.send(2), .max(5)) // demand == 41
+        XCTAssertEqual(subscription2.history, [.requested(.max(43))])
+
+        let subscription3 = CustomSubscription()
+        let nestedPublisher3 = CustomPublisher(subscription: subscription3)
+
+        XCTAssertEqual(helper.publisher.send(nestedPublisher3), .none)
+        XCTAssertEqual(subscription2.history, [.requested(.max(43)), .cancelled])
+        XCTAssertEqual(subscription3.history, [.requested(.max(51))])
+
+        helper.publisher.send(completion: .finished)
+
+        try XCTUnwrap(helper.downstreamSubscription).request(.max(9)) // demand == 50
+        XCTAssertEqual(subscription3.history, [.requested(.max(51)),
+                                               .requested(.max(9))])
+    }
+
+    func testCrashesWhenRequestingZeroFromOuterDemand() {
+        let helper = OperatorTestHelper(
+            publisherType: CustomPublisherBase<CustomPublisher, TestingError>.self,
+            initialDemand: nil,
+            receiveValueDemand: .max(5),
+            createSut: { $0.switchToLatest() }
+        )
+
+        assertCrashes {
+            helper.downstreamSubscription?.request(.none)
+        }
+    }
+
+    func testCrashesWhenReceivingUnwantedValueFromNestedPublisher() {
+        let helper = OperatorTestHelper(
+            publisherType: CustomPublisherBase<CustomPublisher, TestingError>.self,
+            initialDemand: nil,
+            receiveValueDemand: .none,
+            createSut: { $0.switchToLatest() }
+        )
+
+        let nestedSubscription = CustomSubscription()
+        let nestedPublisher = CustomPublisher(subscription: nestedSubscription)
+        XCTAssertEqual(helper.publisher.send(nestedPublisher), .none)
+
+        XCTAssertEqual(helper.tracking.history, [.subscription("SwitchToLatest")])
+        XCTAssertEqual(helper.subscription.history, [.requested(.unlimited)])
+
+        assertCrashes {
+            _ = nestedPublisher.send(-1)
+        }
+    }
+
+    func testCancelCancels() throws {
+        typealias NestedPublisher = CustomPublisherBase<Int, Never>
+        let upstreamSubscription = CustomSubscription()
+        let upstreamPublisher = CustomPublisherBase<NestedPublisher, Never>(
+            subscription: upstreamSubscription
+        )
+
+        let childSubscription = CustomSubscription()
+        let childPublisher = NestedPublisher(subscription: childSubscription)
+
+        let switchToLatest = upstreamPublisher.switchToLatest()
+
+        var downstreamSubscription: Subscription?
+        let tracking = TrackingSubscriberBase<Int, Never>(
+            receiveSubscription: {
+                downstreamSubscription = $0
+                $0.request(.max(42))
+            }
+        )
+
+        upstreamSubscription.onCancel = {
+            XCTAssertEqual(childSubscription.history, [.requested(.max(42)),
+                                                       .cancelled])
+        }
+
+        childSubscription.onCancel = {
+            XCTAssertEqual(upstreamSubscription.history, [.requested(.unlimited)])
+        }
+
+        switchToLatest.subscribe(tracking)
+
+        XCTAssertEqual(upstreamPublisher.send(childPublisher), .none)
+
+        try XCTUnwrap(downstreamSubscription).cancel()
+        try XCTUnwrap(downstreamSubscription).cancel()
+
+        XCTAssertEqual(upstreamSubscription.history, [.requested(.unlimited), .cancelled])
+        XCTAssertEqual(childSubscription.history, [.requested(.max(42)), .cancelled])
+    }
+
+    func testInnerFails() {
+        let helper = OperatorTestHelper(
+            publisherType: CustomPublisherBase<CustomPublisher, TestingError>.self,
+            initialDemand: .max(1),
+            receiveValueDemand: .max(1),
+            createSut: { $0.switchToLatest() }
+        )
+
+        let subscription = CustomSubscription()
+        let nestedPublisher = CustomPublisher(subscription: subscription)
+
+        XCTAssertEqual(helper.publisher.send(nestedPublisher), .none)
+
+        XCTAssertNotNil(nestedPublisher.subscriber)
+
+        XCTAssertEqual(nestedPublisher.send(1), .max(1))
+        XCTAssertEqual(nestedPublisher.send(2), .max(1))
+        nestedPublisher.send(completion: .failure(.oops))
+        nestedPublisher.send(completion: .failure(.oops))
+        nestedPublisher.send(completion: .finished)
+        XCTAssertEqual(nestedPublisher.send(-1), .none)
+
+        XCTAssertEqual(helper.tracking.history, [.subscription("SwitchToLatest"),
+                                                 .value(1),
+                                                 .value(2),
+                                                 .completion(.failure(.oops))])
+        XCTAssertEqual(helper.subscription.history, [.requested(.unlimited), .cancelled])
+        XCTAssertEqual(subscription.history, [.requested(.max(1))])
+    }
+
+    func testSwitchToLatestOuterReceiveSubscriptionTwice() throws {
+        let subscription1 = CustomSubscription()
+        let publisher =
+            CustomPublisherBase<CustomPublisher, TestingError>(subscription: nil)
+        var downstreamSubscription: Subscription?
+        let tracking = TrackingSubscriber(
+            receiveSubscription: { downstreamSubscription = $0 }
+        )
+        tracking.store(in: &cancellables)
+        publisher.switchToLatest().subscribe(tracking)
+
+        XCTAssertEqual(tracking.history, [.subscription("SwitchToLatest")])
+
+        publisher.send(subscription: subscription1)
+
+        XCTAssertEqual(tracking.history, [.subscription("SwitchToLatest")])
+        XCTAssertEqual(subscription1.history, [.requested(.unlimited)])
+
+        let subscription2 = CustomSubscription()
+        publisher.send(subscription: subscription2)
+
+        XCTAssertEqual(tracking.history, [.subscription("SwitchToLatest")])
+        XCTAssertEqual(subscription1.history, [.requested(.unlimited)])
+        XCTAssertEqual(subscription2.history, [.cancelled])
+
+        try XCTUnwrap(downstreamSubscription).cancel()
+        XCTAssertEqual(subscription1.history, [.requested(.unlimited), .cancelled])
+
+        let subscription3 = CustomSubscription()
+        publisher.send(subscription: subscription3)
+        XCTAssertEqual(subscription3.history, [.cancelled])
+    }
+
+    func testSwitchToLatestInnerReceiveSubscriptionTwice() throws {
+        let publisher = CustomPublisherBase<CustomPublisher, TestingError>(
+            subscription: CustomSubscription()
+        )
+        let tracking = TrackingSubscriber()
+        tracking.store(in: &cancellables)
+        publisher.switchToLatest().subscribe(tracking)
+
+        XCTAssertEqual(tracking.history, [.subscription("SwitchToLatest")])
+
+        let subscription1 = CustomSubscription()
+        let nestedPublisher1 = CustomPublisher(subscription: subscription1)
+        XCTAssertEqual(publisher.send(nestedPublisher1), .none)
+
+        XCTAssertEqual(subscription1.history, [])
+
+        let subscription2 = CustomSubscription()
+        nestedPublisher1.send(subscription: subscription2)
+
+        XCTAssertEqual(subscription1.history, [])
+        XCTAssertEqual(subscription2.history, [.cancelled])
+
+        let subscription3 = CustomSubscription()
+        nestedPublisher1.send(subscription: subscription3)
+        XCTAssertEqual(subscription3.history, [.cancelled])
+
+        let nestedPublisher2 = CustomPublisher(subscription: nil)
+
+        var subscription4Destroyed = false
+        do {
+            let subscription4 = CustomSubscription(
+                onDeinit: { subscription4Destroyed = true }
+            )
+            XCTAssertEqual(publisher.send(nestedPublisher2), .none)
+            nestedPublisher2.send(subscription: subscription4)
+
+            try XCTUnwrap(nestedPublisher2.subscriber).receive(completion: .finished)
+            XCTAssertEqual(subscription4.history, [])
+        }
+        XCTAssert(subscription4Destroyed)
+
+        let subscription5 = CustomSubscription()
+        nestedPublisher2.send(subscription: subscription5)
+        XCTAssertEqual(subscription5.history, [])
+    }
+
+    func testSwitchToLatestOuterReceiveValueBeforeSubscription() {
+        testReceiveValueBeforeSubscription(
+            value: CustomPublisherBase<Int, Never>(subscription: CustomSubscription()),
+            expected: .history([.subscription("SwitchToLatest")], demand: .none),
+            { $0.switchToLatest() }
+        )
+    }
+
+    func testSwitchToLatestInnerReceiveValueBeforeSubscription() {
+        let publisher = CustomPublisherBase<CustomPublisher, TestingError>(
+            subscription: CustomSubscription()
+        )
+        let tracking = TrackingSubscriber(receiveSubscription: { $0.request(.unlimited) },
+                                          receiveValue: { _ in .max(42) })
+        tracking.store(in: &cancellables)
+        publisher.switchToLatest().subscribe(tracking)
+        XCTAssertEqual(tracking.history, [.subscription("SwitchToLatest")])
+
+        let nestedPublisher = CustomPublisher(subscription: nil)
+        XCTAssertEqual(publisher.send(nestedPublisher), .none)
+        XCTAssertEqual(nestedPublisher.send(1), .max(42))
+        XCTAssertEqual(tracking.history, [.subscription("SwitchToLatest"),
+                                          .value(1)])
+    }
+
+    func testSwitchToLatestOuterReceiveCompletionBeforeSubscription() {
+        testReceiveCompletionBeforeSubscription(
+            inputType: CustomPublisherBase<Int, Never>.self,
+            expected: .history([.subscription("SwitchToLatest"),
+                                .completion(.finished)]),
+            { $0.switchToLatest() }
+        )
+    }
+
+    func testSwitchToLatestInnerReceiveCompletionBeforeSubscription() {
+        let publisher = CustomPublisherBase<CustomPublisher, TestingError>(
+            subscription: CustomSubscription()
+        )
+        let tracking = TrackingSubscriber(receiveSubscription: { $0.request(.unlimited) },
+                                          receiveValue: { _ in .max(42) })
+        tracking.store(in: &cancellables)
+        publisher.switchToLatest().subscribe(tracking)
+        XCTAssertEqual(tracking.history, [.subscription("SwitchToLatest")])
+
+        let nestedPublisher = CustomPublisher(subscription: nil)
+        XCTAssertEqual(publisher.send(nestedPublisher), .none)
+
+        assertCrashes {
+            nestedPublisher.send(completion: .finished)
+        }
+    }
+
+    func testSwitchToLatestOuterReceiveCompletionWhileWaitingForInnerSubscription() {
+
+        let completions: [Subscribers.Completion<TestingError>] =
+            [.finished, .failure(.oops)]
+
+        for completion in completions {
+            let publisher = CustomPublisherBase<CustomPublisher, TestingError>(
+                subscription: CustomSubscription()
+            )
+            let tracking = TrackingSubscriber(
+                receiveSubscription: { $0.request(.unlimited) },
+                receiveValue: { _ in .max(42) }
+            )
+            tracking.store(in: &cancellables)
+            publisher.switchToLatest().subscribe(tracking)
+            XCTAssertEqual(tracking.history, [.subscription("SwitchToLatest")])
+
+            let nestedPublisher = CustomPublisher(subscription: nil)
+            XCTAssertEqual(publisher.send(nestedPublisher), .none)
+
+            publisher.send(completion: completion)
+
+            switch completion {
+            case .finished:
+                XCTAssertEqual(tracking.history, [.subscription("SwitchToLatest")])
+            case .failure:
+                XCTAssertEqual(tracking.history, [.subscription("SwitchToLatest"),
+                                                  .completion(.failure(.oops))])
+            }
+
+            let subscription = CustomSubscription()
+            nestedPublisher.send(subscription: subscription)
+
+            switch completion {
+            case .finished:
+                XCTAssertEqual(tracking.history, [.subscription("SwitchToLatest")])
+            case .failure:
+                XCTAssertEqual(tracking.history, [.subscription("SwitchToLatest"),
+                                                  .completion(.failure(.oops))])
+            }
+            XCTAssertEqual(subscription.history, [.requested(.unlimited)])
+        }
+    }
+
+    func testSwitchToLatestInnerReceiveFailureBeforeSubscription() {
+        let publisher = CustomPublisherBase<CustomPublisher, TestingError>(
+            subscription: CustomSubscription()
+        )
+        let tracking = TrackingSubscriber(receiveSubscription: { $0.request(.unlimited) },
+                                          receiveValue: { _ in .max(42) })
+        tracking.store(in: &cancellables)
+        publisher.switchToLatest().subscribe(tracking)
+        XCTAssertEqual(tracking.history, [.subscription("SwitchToLatest")])
+
+        let nestedPublisher = CustomPublisher(subscription: nil)
+        XCTAssertEqual(publisher.send(nestedPublisher), .none)
+
+        assertCrashes {
+            nestedPublisher.send(completion: .failure(.oops))
+        }
+    }
+
+    func testOuterIgnoresInputAfterCancelling() throws {
+        let helper = OperatorTestHelper(
+            publisherType: CustomPublisherBase<CustomPublisher, TestingError>.self,
+            initialDemand: nil,
+            receiveValueDemand: .none,
+            createSut: { $0.switchToLatest() }
+        )
+
+        XCTAssertEqual(helper.tracking.history, [.subscription("SwitchToLatest")])
+        XCTAssertEqual(helper.subscription.history, [.requested(.unlimited)])
+
+        try XCTUnwrap(helper.downstreamSubscription).cancel()
+
+        XCTAssertEqual(helper.subscription.history, [.requested(.unlimited), .cancelled])
+
+        let nestedSubscription = CustomSubscription()
+        let nestedPublisher = CustomPublisher(subscription: nestedSubscription)
+
+        XCTAssertEqual(helper.publisher.send(nestedPublisher), .none)
+        XCTAssertEqual(nestedSubscription.history, [])
+        XCTAssertNil(nestedPublisher.subscriber)
+    }
+
+    func testOuterIgnoresInputAfterFinishing() throws {
+        let helper = OperatorTestHelper(
+            publisherType: CustomPublisherBase<CustomPublisher, TestingError>.self,
+            initialDemand: nil,
+            receiveValueDemand: .none,
+            createSut: { $0.switchToLatest() }
+        )
+
+        XCTAssertEqual(helper.tracking.history, [.subscription("SwitchToLatest")])
+        XCTAssertEqual(helper.subscription.history, [.requested(.unlimited)])
+
+        helper.publisher.send(completion: .finished)
+
+        let nestedSubscription = CustomSubscription()
+        let nestedPublisher = CustomPublisher(subscription: nestedSubscription)
+
+        XCTAssertEqual(helper.publisher.send(nestedPublisher), .none)
+        XCTAssertEqual(nestedSubscription.history, [])
+        XCTAssertNil(nestedPublisher.subscriber)
+    }
+
+    func testInnerIgnoresInputAfterCancelling() throws {
+        let helper = OperatorTestHelper(
+            publisherType: CustomPublisherBase<CustomPublisher, TestingError>.self,
+            initialDemand: .max(1),
+            receiveValueDemand: .max(100),
+            createSut: { $0.switchToLatest() }
+        )
+
+        XCTAssertEqual(helper.tracking.history, [.subscription("SwitchToLatest")])
+        XCTAssertEqual(helper.subscription.history, [.requested(.unlimited)])
+
+        let nestedSubscription = CustomSubscription()
+        let nestedPublisher = CustomPublisher(subscription: nestedSubscription)
+
+        XCTAssertEqual(helper.publisher.send(nestedPublisher), .none)
+        XCTAssertEqual(nestedSubscription.history, [.requested(.max(1))])
+        XCTAssertNotNil(nestedPublisher.subscriber)
+
+        try XCTUnwrap(helper.downstreamSubscription).cancel()
+
+        XCTAssertEqual(nestedSubscription.history, [.requested(.max(1)), .cancelled])
+        XCTAssertEqual(helper.subscription.history, [.requested(.unlimited), .cancelled])
+
+        XCTAssertEqual(nestedPublisher.send(1), .none)
+        XCTAssertEqual(helper.tracking.history, [.subscription("SwitchToLatest")])
+    }
+
+    func testInnerReceivesInputAfterOuterFinishes() throws {
+        let helper = OperatorTestHelper(
+            publisherType: CustomPublisherBase<CustomPublisher, TestingError>.self,
+            initialDemand: .max(1),
+            receiveValueDemand: .max(100),
+            createSut: { $0.switchToLatest() }
+        )
+
+        XCTAssertEqual(helper.tracking.history, [.subscription("SwitchToLatest")])
+        XCTAssertEqual(helper.subscription.history, [.requested(.unlimited)])
+
+        let nestedSubscription = CustomSubscription()
+        let nestedPublisher = CustomPublisher(subscription: nestedSubscription)
+
+        XCTAssertEqual(helper.publisher.send(nestedPublisher), .none)
+        XCTAssertEqual(nestedSubscription.history, [.requested(.max(1))])
+        XCTAssertNotNil(nestedPublisher.subscriber)
+
+        helper.publisher.send(completion: .finished)
+
+        XCTAssertEqual(nestedSubscription.history, [.requested(.max(1))])
+        XCTAssertEqual(helper.subscription.history, [.requested(.unlimited)])
+
+        XCTAssertEqual(nestedPublisher.send(1), .max(100))
+        XCTAssertEqual(helper.tracking.history, [.subscription("SwitchToLatest"),
+                                                 .value(1)])
+    }
+
+    func testOuterIgnoresCompletionAfterCancelling() throws {
+        let helper = OperatorTestHelper(
+            publisherType: CustomPublisherBase<CustomPublisher, TestingError>.self,
+            initialDemand: nil,
+            receiveValueDemand: .none,
+            createSut: { $0.switchToLatest() }
+        )
+
+        XCTAssertEqual(helper.tracking.history, [.subscription("SwitchToLatest")])
+        XCTAssertEqual(helper.subscription.history, [.requested(.unlimited)])
+
+        try XCTUnwrap(helper.downstreamSubscription).cancel()
+
+        XCTAssertEqual(helper.subscription.history, [.requested(.unlimited), .cancelled])
+
+        helper.publisher.send(completion: .failure(.oops))
+        XCTAssertEqual(helper.tracking.history, [.subscription("SwitchToLatest")])
+    }
+
+    func testOuterReceiveCompletionTwice() throws {
+        let helper = OperatorTestHelper(
+            publisherType: CustomPublisherBase<CustomPublisher, TestingError>.self,
+            initialDemand: nil,
+            receiveValueDemand: .none,
+            createSut: { $0.switchToLatest() }
+        )
+
+        XCTAssertEqual(helper.tracking.history, [.subscription("SwitchToLatest")])
+        XCTAssertEqual(helper.subscription.history, [.requested(.unlimited)])
+
+        helper.publisher.send(completion: .finished)
+        helper.publisher.send(completion: .failure(.oops))
+        helper.publisher.send(completion: .finished)
+        helper.publisher.send(completion: .failure(.oops))
+        XCTAssertEqual(helper.tracking.history, [.subscription("SwitchToLatest"),
+                                                 .completion(.finished),
+                                                 .completion(.failure(.oops)),
+                                                 .completion(.finished),
+                                                 .completion(.failure(.oops))])
+    }
+
+    func testInnerIgnoresCompletionAfterCancelling() throws {
+        let helper = OperatorTestHelper(
+            publisherType: CustomPublisherBase<CustomPublisher, TestingError>.self,
+            initialDemand: .max(1),
+            receiveValueDemand: .max(100),
+            createSut: { $0.switchToLatest() }
+        )
+
+        XCTAssertEqual(helper.tracking.history, [.subscription("SwitchToLatest")])
+        XCTAssertEqual(helper.subscription.history, [.requested(.unlimited)])
+
+        let nestedSubscription = CustomSubscription()
+        let nestedPublisher = CustomPublisher(subscription: nestedSubscription)
+
+        XCTAssertEqual(helper.publisher.send(nestedPublisher), .none)
+        XCTAssertEqual(nestedSubscription.history, [.requested(.max(1))])
+        XCTAssertNotNil(nestedPublisher.subscriber)
+
+        try XCTUnwrap(helper.downstreamSubscription).cancel()
+
+        XCTAssertEqual(nestedSubscription.history, [.requested(.max(1)), .cancelled])
+        XCTAssertEqual(helper.subscription.history, [.requested(.unlimited), .cancelled])
+
+        nestedPublisher.send(completion: .failure(.oops))
+        XCTAssertEqual(helper.tracking.history, [.subscription("SwitchToLatest")])
+    }
+
+    func testInnerIgnoresEventsAfterOuterFails() throws {
+        let helper = OperatorTestHelper(
+            publisherType: CustomPublisherBase<CustomPublisher, TestingError>.self,
+            initialDemand: .max(1),
+            receiveValueDemand: .max(100),
+            createSut: { $0.switchToLatest() }
+        )
+
+        XCTAssertEqual(helper.tracking.history, [.subscription("SwitchToLatest")])
+        XCTAssertEqual(helper.subscription.history, [.requested(.unlimited)])
+
+        let nestedSubscription = CustomSubscription()
+        let nestedPublisher = CustomPublisher(subscription: nestedSubscription)
+
+        XCTAssertEqual(helper.publisher.send(nestedPublisher), .none)
+        XCTAssertEqual(nestedSubscription.history, [.requested(.max(1))])
+        XCTAssertNotNil(nestedPublisher.subscriber)
+
+        helper.publisher.send(completion: .failure(.oops))
+
+        XCTAssertEqual(nestedSubscription.history, [.requested(.max(1)), .cancelled])
+        XCTAssertEqual(helper.subscription.history, [.requested(.unlimited)])
+
+        XCTAssertEqual(nestedPublisher.send(1), .max(100))
+        nestedPublisher.send(completion: .finished)
+        nestedPublisher.send(completion: .failure(.oops))
+        XCTAssertEqual(helper.tracking.history, [.subscription("SwitchToLatest"),
+                                                 .completion(.failure(.oops)),
+                                                 .value(1)])
+    }
+
+    func testInnerIgnoresEventsAfterOuterFinishes() throws {
+        let helper = OperatorTestHelper(
+            publisherType: CustomPublisherBase<CustomPublisher, TestingError>.self,
+            initialDemand: .max(1),
+            receiveValueDemand: .max(100),
+            createSut: { $0.switchToLatest() }
+        )
+
+        XCTAssertEqual(helper.tracking.history, [.subscription("SwitchToLatest")])
+        XCTAssertEqual(helper.subscription.history, [.requested(.unlimited)])
+
+        let nestedSubscription = CustomSubscription()
+        let nestedPublisher = CustomPublisher(subscription: nestedSubscription)
+
+        XCTAssertEqual(helper.publisher.send(nestedPublisher), .none)
+        XCTAssertEqual(nestedSubscription.history, [.requested(.max(1))])
+        XCTAssertNotNil(nestedPublisher.subscriber)
+
+        helper.publisher.send(completion: .finished)
+
+        XCTAssertEqual(nestedSubscription.history, [.requested(.max(1))])
+        XCTAssertEqual(helper.subscription.history, [.requested(.unlimited)])
+
+        XCTAssertEqual(nestedPublisher.send(1), .max(100))
+        nestedPublisher.send(completion: .finished)
+        nestedPublisher.send(completion: .failure(.oops))
+        XCTAssertEqual(helper.tracking.history, [.subscription("SwitchToLatest"),
+                                                 .value(1),
+                                                 .completion(.finished)])
+    }
+
+    func testOuterFinishesThenInnerFinishes() {
+        let helper = OperatorTestHelper(
+            publisherType: CustomPublisherBase<CustomPublisher, TestingError>.self,
+            initialDemand: .max(1),
+            receiveValueDemand: .max(100),
+            createSut: { $0.switchToLatest() }
+        )
+
+        XCTAssertEqual(helper.tracking.history, [.subscription("SwitchToLatest")])
+        XCTAssertEqual(helper.subscription.history, [.requested(.unlimited)])
+
+        let nestedSubscription = CustomSubscription()
+        let nestedPublisher = CustomPublisher(subscription: nestedSubscription)
+
+        XCTAssertEqual(helper.publisher.send(nestedPublisher), .none)
+        helper.publisher.send(completion: .finished)
+
+        XCTAssertEqual(helper.tracking.history, [.subscription("SwitchToLatest")])
+        XCTAssertEqual(helper.subscription.history, [.requested(.unlimited)])
+        XCTAssertEqual(nestedSubscription.history, [.requested(.max(1))])
+
+        nestedPublisher.send(completion: .finished)
+
+        XCTAssertEqual(helper.tracking.history, [.subscription("SwitchToLatest"),
+                                                 .completion(.finished)])
+        XCTAssertEqual(helper.subscription.history, [.requested(.unlimited)])
+        XCTAssertEqual(nestedSubscription.history, [.requested(.max(1))])
+    }
+
+    func testInnerFinishesThenOuterFinishes() {
+        let helper = OperatorTestHelper(
+            publisherType: CustomPublisherBase<CustomPublisher, TestingError>.self,
+            initialDemand: .max(1),
+            receiveValueDemand: .max(100),
+            createSut: { $0.switchToLatest() }
+        )
+
+        XCTAssertEqual(helper.tracking.history, [.subscription("SwitchToLatest")])
+        XCTAssertEqual(helper.subscription.history, [.requested(.unlimited)])
+
+        let nestedSubscription = CustomSubscription()
+        let nestedPublisher = CustomPublisher(subscription: nestedSubscription)
+
+        XCTAssertEqual(helper.publisher.send(nestedPublisher), .none)
+        nestedPublisher.send(completion: .finished)
+
+        XCTAssertEqual(helper.tracking.history, [.subscription("SwitchToLatest")])
+        XCTAssertEqual(helper.subscription.history, [.requested(.unlimited)])
+        XCTAssertEqual(nestedSubscription.history, [.requested(.max(1))])
+
+        helper.publisher.send(completion: .finished)
+
+        XCTAssertEqual(helper.tracking.history, [.subscription("SwitchToLatest"),
+                                                 .completion(.finished)])
+        XCTAssertEqual(helper.subscription.history, [.requested(.unlimited)])
+        XCTAssertEqual(nestedSubscription.history, [.requested(.max(1))])
+    }
+
+    func testSwitchToLatestLifecycle() throws {
+        try testLifecycle(sendValue: CustomPublisher(subscription: CustomSubscription()),
+                          cancellingSubscriptionReleasesSubscriber: false,
+                          { $0.switchToLatest() })
+    }
+
+    func testSwitchToLatestReflection() throws {
+        let publisher = CustomPublisherBase<CustomPublisher, TestingError>(
+            subscription: CustomSubscription()
+        )
+        let tracking = TrackingSubscriber()
+        tracking.store(in: &cancellables)
+        publisher.switchToLatest().subscribe(tracking)
+        XCTAssert(publisher.erasedSubscriber is Subscription)
+
+        guard let outer = publisher.erasedSubscriber,
+              let downstreamSubscription = tracking.subscriptions.first?.underlying else {
+            XCTFail("Missing subscriber/subscription")
+            return
+        }
+
+        XCTAssert(type(of: outer) == type(of: downstreamSubscription))
+
+        let outerCombineID = try XCTUnwrap((outer as? Subscription)?.combineIdentifier)
+
+        XCTAssertEqual(downstreamSubscription.combineIdentifier, outerCombineID)
+
+        func testReflections(_ subject: Any,
+                             hasChildren: Bool,
+                             file: StaticString = #file,
+                             line: UInt = #line) {
+            XCTAssertEqual((subject as? CustomStringConvertible)?.description,
+                           "SwitchToLatest",
+                           file: file,
+                           line: line)
+            XCTAssertFalse(subject is CustomDebugStringConvertible,
+                           file: file,
+                           line: line)
+            XCTAssertEqual(
+                (subject as? CustomPlaygroundDisplayConvertible)?
+                    .playgroundDescription as? String,
+                "SwitchToLatest",
+                file: file,
+                line: line
+            )
+            if let mirror = (subject as? CustomReflectable)?.customMirror {
+                if hasChildren {
+                    XCTAssert(expectedChildren(
+                                  ("parentSubscription",
+                                   .matches(String(describing: outerCombineID))),
+                                  file: file,
+                                  line: line
+                              )(mirror),
+                              file: file,
+                              line: line)
+                }
+            } else {
+                XCTFail("subject should conform to CustomReflectable",
+                        file: file,
+                        line: line)
+            }
+        }
+
+        testReflections(outer, hasChildren: false)
+        testReflections(downstreamSubscription, hasChildren: false)
+
+        let nestedPublisher = CustomPublisher(subscription: CustomSubscription())
+        _ = publisher.send(nestedPublisher)
+
+        guard let side = nestedPublisher
+            .erasedSubscriber as? CustomCombineIdentifierConvertible else {
+
+            XCTFail("Missing Side")
+            return
+        }
+
+        XCTAssertFalse(side is Subscription)
+        XCTAssert(type(of: side) != type(of: outer))
+        XCTAssert(type(of: side) != type(of: downstreamSubscription))
+        XCTAssertNotEqual(side.combineIdentifier, outerCombineID)
+        testReflections(side, hasChildren: true)
+    }
+}

Tests/OpenCombineTests/PublisherTests/TimeoutTests.swift

@@ -0,0 +1,395 @@
+//
+//  TimeoutTests.swift
+//  
+//
+//  Created by Sergej Jaskiewicz on 14.06.2020.
+//
+
+import XCTest
+
+#if OPENCOMBINE_COMPATIBILITY_TEST
+import Combine
+#else
+import OpenCombine
+#endif
+
+@available(macOS 10.15, iOS 13.0, *)
+final class TimeoutTests: XCTestCase {
+
+    func testBasicBehaviorWithoutCustomError() {
+        testBasicBehavior(customError: nil)
+    }
+
+    func testBasicBehaviorWithCustomError() {
+        testBasicBehavior(customError: "timeout")
+    }
+
+    private func testBasicBehavior(customError: TestingError?) {
+
+        let expectedCompletion =
+            customError.map(Subscribers.Completion.failure) ?? .finished
+
+        let scheduler = VirtualTimeScheduler()
+        let helper = OperatorTestHelper(
+            publisherType: CustomPublisher.self,
+            initialDemand: .max(2),
+            receiveValueDemand: .max(1),
+            createSut: {
+                $0.timeout(.nanoseconds(13),
+                           scheduler: scheduler,
+                           options: .nontrivialOptions,
+                           customError: customError.map { e in { e } })
+            }
+        )
+        XCTAssertEqual(helper.tracking.history, [.subscription("Timeout")])
+        XCTAssertEqual(helper.subscription.history, [.requested(.max(2)),
+                                                     .requested(.unlimited)])
+        XCTAssertEqual(scheduler.history,
+                       [.now,
+                        .minimumTolerance,
+                        .scheduleAfterDateWithInterval(.nanoseconds(13),
+                                                       interval: .nanoseconds(13),
+                                                       tolerance: .nanoseconds(7),
+                                                       options: .nontrivialOptions)])
+
+        scheduler.rewind(to: .nanoseconds(10))
+        XCTAssertEqual(helper.publisher.send(1), .unlimited)
+
+        XCTAssertEqual(helper.tracking.history, [.subscription("Timeout")])
+
+        scheduler.rewind(to: .nanoseconds(11))
+        XCTAssertEqual(helper.publisher.send(2), .unlimited)
+
+        XCTAssertEqual(helper.tracking.history, [.subscription("Timeout"),
+                                                 .value(1)])
+
+        scheduler.rewind(to: .nanoseconds(12))
+
+        XCTAssertEqual(helper.tracking.history, [.subscription("Timeout"),
+                                                 .value(1),
+                                                 .value(2)])
+
+        XCTAssertEqual(scheduler.history,
+                       [.now,
+                        .minimumTolerance,
+                        .scheduleAfterDateWithInterval(.nanoseconds(13),
+                                                       interval: .nanoseconds(13),
+                                                       tolerance: .nanoseconds(7),
+                                                       options: .nontrivialOptions),
+                        .now,
+                        .minimumTolerance,
+                        .scheduleAfterDateWithInterval(.nanoseconds(23),
+                                                       interval: .nanoseconds(13),
+                                                       tolerance: .nanoseconds(7),
+                                                       options: .nontrivialOptions),
+                        .schedule(options: .nontrivialOptions),
+                        .now,
+                        .minimumTolerance,
+                        .scheduleAfterDateWithInterval(.nanoseconds(24),
+                                                       interval: .nanoseconds(13),
+                                                       tolerance: .nanoseconds(7),
+                                                       options: .nontrivialOptions),
+                        .schedule(options: .nontrivialOptions)]
+        )
+
+        scheduler.executeScheduledActions(until: .nanoseconds(200))
+
+        XCTAssertEqual(helper.tracking.history, [.subscription("Timeout"),
+                                                 .value(1),
+                                                 .value(2),
+                                                 .completion(expectedCompletion)])
+        XCTAssertEqual(helper.subscription.history, [.requested(.max(2)),
+                                                     .requested(.unlimited),
+                                                     .cancelled])
+
+        XCTAssertEqual(scheduler.history,
+                       [.now,
+                        .minimumTolerance,
+                        .scheduleAfterDateWithInterval(.nanoseconds(13),
+                                                       interval: .nanoseconds(13),
+                                                       tolerance: .nanoseconds(7),
+                                                       options: .nontrivialOptions),
+                        .now,
+                        .minimumTolerance,
+                        .scheduleAfterDateWithInterval(.nanoseconds(23),
+                                                       interval: .nanoseconds(13),
+                                                       tolerance: .nanoseconds(7),
+                                                       options: .nontrivialOptions),
+                        .schedule(options: .nontrivialOptions),
+                        .now,
+                        .minimumTolerance,
+                        .scheduleAfterDateWithInterval(.nanoseconds(24),
+                                                       interval: .nanoseconds(13),
+                                                       tolerance: .nanoseconds(7),
+                                                       options: .nontrivialOptions),
+                        .schedule(options: .nontrivialOptions)]
+        )
+
+        scheduler.rewind(to: .nanoseconds(210))
+        helper.publisher.send(completion: .finished)
+        scheduler.rewind(to: .nanoseconds(220))
+        helper.publisher.send(completion: .failure(.oops))
+
+        scheduler.executeScheduledActions(until: .nanoseconds(400))
+
+        XCTAssertEqual(helper.tracking.history, [.subscription("Timeout"),
+                                                 .value(1),
+                                                 .value(2),
+                                                 .completion(expectedCompletion),
+                                                 .completion(.finished),
+                                                 .completion(.failure(.oops))])
+        XCTAssertEqual(helper.subscription.history, [.requested(.max(2)),
+                                                     .requested(.unlimited),
+                                                     .cancelled])
+
+        XCTAssertEqual(scheduler.history,
+                       [.now,
+                        .minimumTolerance,
+                        .scheduleAfterDateWithInterval(.nanoseconds(13),
+                                                       interval: .nanoseconds(13),
+                                                       tolerance: .nanoseconds(7),
+                                                       options: .nontrivialOptions),
+                        .now,
+                        .minimumTolerance,
+                        .scheduleAfterDateWithInterval(.nanoseconds(23),
+                                                       interval: .nanoseconds(13),
+                                                       tolerance: .nanoseconds(7),
+                                                       options: .nontrivialOptions),
+                        .schedule(options: .nontrivialOptions),
+                        .now,
+                        .minimumTolerance,
+                        .scheduleAfterDateWithInterval(.nanoseconds(24),
+                                                       interval: .nanoseconds(13),
+                                                       tolerance: .nanoseconds(7),
+                                                       options: .nontrivialOptions),
+                        .schedule(options: .nontrivialOptions),
+                        .schedule(options: .nontrivialOptions),
+                        .schedule(options: .nontrivialOptions)]
+        )
+    }
+
+    func testRequestWithoutCustomError() throws {
+        try testRequest(customError: nil)
+    }
+
+    func testRequestWithCustomError() throws {
+        try testRequest(customError: "timeout")
+    }
+
+    private func testRequest(customError: TestingError?) throws {
+        let scheduler = VirtualTimeScheduler()
+        let helper = OperatorTestHelper(
+            publisherType: CustomPublisher.self,
+            initialDemand: nil,
+            receiveValueDemand: .max(1),
+            createSut: {
+                $0.timeout(.nanoseconds(13),
+                           scheduler: scheduler,
+                           options: .nontrivialOptions,
+                           customError: customError.map { e in { e } })
+            }
+        )
+
+        XCTAssertEqual(helper.subscription.history, [.requested(.unlimited)])
+        let downstreamSubscription = try XCTUnwrap(helper.downstreamSubscription)
+
+        downstreamSubscription.request(.none)
+        downstreamSubscription.request(.max(2))
+        downstreamSubscription.request(.max(42))
+
+        XCTAssertEqual(helper.subscription.history, [.requested(.unlimited),
+                                                     .requested(.none),
+                                                     .requested(.max(2)),
+                                                     .requested(.max(42))])
+
+        scheduler.rewind(to: .nanoseconds(14))
+        downstreamSubscription.request(.max(13))
+
+        XCTAssertEqual(helper.subscription.history, [.requested(.unlimited),
+                                                     .requested(.none),
+                                                     .requested(.max(2)),
+                                                     .requested(.max(42)),
+                                                     .cancelled])
+    }
+
+    func testCancellationWithoutCustomError() throws {
+        try testCancellation(customError: nil)
+    }
+
+    func testCancellationWithCustomError() throws {
+        try testCancellation(customError: "timeout")
+    }
+
+    private func testCancellation(customError: TestingError?) throws {
+        let scheduler = VirtualTimeScheduler()
+        let helper = OperatorTestHelper(
+            publisherType: CustomPublisher.self,
+            initialDemand: nil,
+            receiveValueDemand: .max(1),
+            createSut: {
+                $0.timeout(.nanoseconds(13),
+                           scheduler: scheduler,
+                           options: .nontrivialOptions,
+                           customError: customError.map { e in { e } })
+            }
+        )
+
+        XCTAssertEqual(helper.subscription.history, [.requested(.unlimited)])
+        let downstreamSubscription = try XCTUnwrap(helper.downstreamSubscription)
+
+        downstreamSubscription.cancel()
+
+        XCTAssertEqual(helper.subscription.history, [.requested(.unlimited),
+                                                     .cancelled])
+
+        scheduler.rewind(to: .nanoseconds(14))
+
+        XCTAssertEqual(helper.subscription.history, [.requested(.unlimited),
+                                                     .cancelled])
+    }
+
+    func testCrashesWithImmediateScheduler() {
+        let publisher = CustomPublisher(subscription: CustomSubscription())
+        let tracking = TrackingSubscriber()
+        let timeout = publisher
+            .timeout(.nanoseconds(10), scheduler: ImmediateScheduler.shared)
+
+        assertCrashes {
+            timeout.subscribe(tracking)
+        }
+    }
+
+    func testTimeoutReceiveValueBeforeSubscription() {
+        let scheduler = VirtualTimeScheduler()
+        testReceiveValueBeforeSubscription(
+            value: 42,
+            expected: .history([], demand: .none),
+            { $0.timeout(.nanoseconds(13), scheduler: scheduler) }
+        )
+    }
+
+    func testTimeoutReceiveCompletionBeforeSubscription() {
+        let scheduler = VirtualTimeScheduler()
+        testReceiveCompletionBeforeSubscription(
+            inputType: Int.self,
+            expected: .history([]),
+            { $0.timeout(.nanoseconds(13), scheduler: scheduler) }
+        )
+    }
+
+    func testTimeoutRequestBeforeSubscription() {
+        let scheduler = VirtualTimeScheduler()
+        testRequestBeforeSubscription(
+            inputType: Int.self,
+            shouldCrash: false,
+            { $0.timeout(.nanoseconds(13), scheduler: scheduler) }
+        )
+    }
+
+    func testTimeoutReceiveSubscriptionTwice() throws {
+        let scheduler = VirtualTimeScheduler()
+
+        let helper = OperatorTestHelper(
+            publisherType: CustomPublisher.self,
+            initialDemand: nil,
+            receiveValueDemand: .none,
+            createSut: { $0.timeout(.nanoseconds(13), scheduler: scheduler) }
+        )
+
+        XCTAssertEqual(helper.subscription.history, [.requested(.unlimited)])
+
+        let secondSubscription = CustomSubscription()
+
+        try XCTUnwrap(helper.publisher.subscriber)
+            .receive(subscription: secondSubscription)
+
+        XCTAssertEqual(secondSubscription.history, [.cancelled])
+
+        try XCTUnwrap(helper.publisher.subscriber)
+            .receive(subscription: helper.subscription)
+
+        XCTAssertEqual(helper.subscription.history, [.requested(.unlimited),
+                                                     .cancelled])
+
+        try XCTUnwrap(helper.downstreamSubscription).cancel()
+
+        XCTAssertEqual(helper.subscription.history, [.requested(.unlimited),
+                                                     .cancelled,
+                                                     .cancelled])
+    }
+
+    func testTimeoutCancelBeforeSubscription() {
+        let scheduler = VirtualTimeScheduler()
+        testCancelBeforeSubscription(
+            inputType: Int.self,
+            shouldCrash: false,
+            { $0.timeout(.nanoseconds(13), scheduler: scheduler) }
+        )
+    }
+
+    func testSetupTimerWeakCapture() {
+        let scheduler = VirtualTimeScheduler()
+        var subscriptionDestroyed = false
+        do {
+            let helper = OperatorTestHelper(
+                publisherType: CustomPublisher.self,
+                initialDemand: nil,
+                receiveValueDemand: .none,
+                createSut: { $0.timeout(.nanoseconds(13), scheduler: scheduler) }
+            )
+
+            helper.tracking.onDeinit = { subscriptionDestroyed = true }
+        }
+
+        XCTAssertTrue(subscriptionDestroyed)
+    }
+
+    func testReceiveValueScheduleStrongCapture() {
+        let scheduler = VirtualTimeScheduler()
+        var subscriptionDestroyed = false
+        do {
+            let helper = OperatorTestHelper(
+                publisherType: CustomPublisher.self,
+                initialDemand: nil,
+                receiveValueDemand: .none,
+                createSut: { $0.timeout(.nanoseconds(13), scheduler: scheduler) }
+            )
+
+            helper.tracking.onDeinit = { subscriptionDestroyed = true }
+
+            XCTAssertEqual(helper.publisher.send(1), .unlimited)
+        }
+
+        XCTAssertFalse(subscriptionDestroyed)
+    }
+
+    func testReceiveCompletionScheduleStrongCapture() {
+        let scheduler = VirtualTimeScheduler()
+        var subscriptionDestroyed = false
+        do {
+            let helper = OperatorTestHelper(
+                publisherType: CustomPublisher.self,
+                initialDemand: nil,
+                receiveValueDemand: .none,
+                createSut: { $0.timeout(.nanoseconds(13), scheduler: scheduler) }
+            )
+
+            helper.tracking.onDeinit = { subscriptionDestroyed = true }
+
+            helper.publisher.send(completion: .finished)
+        }
+
+        XCTAssertFalse(subscriptionDestroyed)
+    }
+
+    func testTimeoutReflection() throws {
+        try testReflection(
+            parentInput: Double.self,
+            parentFailure: Error.self,
+            description: "Timeout",
+            customMirror: childrenIsEmpty,
+            playgroundDescription: "Timeout",
+            { $0.timeout(.nanoseconds(13), scheduler: VirtualTimeScheduler()) }
+        )
+    }
+}

Tests/OpenCombineTests/VirtualTimeSchedulerTests.swift

@@ -43,20 +43,20 @@ final class VirtualTimeSchedulerTests: XCTestCase {
         XCTAssertEqual(scheduler.history, [.now,
                                            .minimumTolerance,
                                            .scheduleAfterDate(.nanoseconds(10),
-                                                              tolerance: 0,
+                                                              tolerance: .nanoseconds(7),
                                                               options: nil),
                                            .schedule(options: nil),
                                            .now,
                                            .minimumTolerance,
                                            .scheduleAfterDate(.nanoseconds(5),
-                                                              tolerance: 0,
+                                                              tolerance: .nanoseconds(7),
                                                               options: nil),
                                            .schedule(options: nil),
                                            .now,
                                            .now,
                                            .minimumTolerance,
                                            .scheduleAfterDate(.nanoseconds(7),
-                                                              tolerance: 0,
+                                                              tolerance: .nanoseconds(7),
                                                               options: nil),
                                            .now])
     }
@@ -88,12 +88,12 @@ final class VirtualTimeSchedulerTests: XCTestCase {
                         .minimumTolerance,
                         .scheduleAfterDateWithInterval(.microseconds(2),
                                                        interval: .milliseconds(40),
-                                                       tolerance: 0,
+                                                       tolerance: .nanoseconds(7),
                                                        options: nil),
                         .now,
                         .minimumTolerance,
                         .scheduleAfterDate(.milliseconds(300),
-                                           tolerance: .nanoseconds(0),
+                                           tolerance: .nanoseconds(7),
                                            options: nil),
                         .now,
                         .now,
@@ -105,4 +105,45 @@ final class VirtualTimeSchedulerTests: XCTestCase {
                         .now,
                         .now])
     }
+
+    func testRewindForward() {
+        let scheduler = VirtualTimeScheduler()
+        var history = [Int]()
+        let cancellable = scheduler.schedule(after: scheduler.now + .microseconds(2),
+                                             interval: .milliseconds(40)) {
+            history.append(Int(scheduler.now.time))
+        }
+        scheduler.schedule(after: scheduler.now + .milliseconds(300)) {
+            cancellable.cancel()
+        }
+        XCTAssertEqual(scheduler.scheduledDates, [.microseconds(2), .milliseconds(300)])
+        scheduler.rewind(to: .milliseconds(81))
+
+        XCTAssertEqual(history, [2000,
+                                 40002000,
+                                 80002000])
+
+        scheduler.executeScheduledActions()
+
+        XCTAssertEqual(history, [2000,
+                                 40002000,
+                                 80002000,
+                                 120002000,
+                                 160002000,
+                                 200002000,
+                                 240002000,
+                                 280002000])
+
+        scheduler.rewind(to: .milliseconds(0))
+        scheduler.executeScheduledActions()
+
+        XCTAssertEqual(history, [2000,
+                                 40002000,
+                                 80002000,
+                                 120002000,
+                                 160002000,
+                                 200002000,
+                                 240002000,
+                                 280002000])
+    }
 }

opencombine_lldb.py

@@ -0,0 +1,29 @@
+# To use `opencombine_lldb.py`, figure out its full path.
+# Let's say the full path is `~/projects/OpenCombine/opencombine_lldb.py`.
+# Then add the following statement to your `~/.lldbinit` file:
+#
+#     command script import ~/projects/OpenCombine/opencombine_lldb.py
+
+import lldb
+
+# Show a Demand as either `max(N)` or `unlimited`.
+def summary_Demand(sb_value, internal_dict):
+    child = sb_value.GetChildAtIndex(0)
+    if not child.IsValid():
+        return 'failed to get child of Demand'
+    number = child.GetValueAsUnsigned()
+
+    # .unlimited is represented by a rawValue of UInt(Int.max) + 1.
+    # Int.max is either 2**31 - 1 or 2**63 - 1 depending on the
+    # target platform. So .unlimited is either 2**31 or 2**63.
+    #     31 = 4 * 8 - 1
+    #     63 = 8 * 8 - 1
+    unlimited = 2**(child.GetByteSize() * 8 - 1)
+
+    if number == unlimited:
+        return 'unlimited'
+    else:
+        return 'max(%d)' % number
+
+def __lldb_init_module(debugger, internal_dict):
+    debugger.HandleCommand('type summary add -w swift OpenCombine.Subscribers.Demand -F "' + __name__ + '.summary_Demand"')