Bump the version to 0.11.0

Remove the note about the project being in early development
Enable URLSession tests on non-Darwin platforms since Swift 5.3
2020-10-30 00:53:48 +03:00 · 2020-10-30 00:53:48 +03:00 · 2020-10-29 19:49:34 +03:00 · 2020-10-29 19:49:34 +03:00 · 2020-10-29 19:49:34 +03:00 · 2020-10-29 19:49:34 +03:00
174 changed files with 12056 additions and 2533 deletions
@@ -1,3 +1,103 @@
+macOS_tests_steps: &macOS_tests_steps
+  steps:
+    - checkout
+    - run:
+        name: Building and running tests in debug mode with coverage
+        command: |
+          make test-debug \
+            SWIFT_BUILD_FLAGS="-Xswiftc -warnings-as-errors" \
+            SWIFT_TEST_FLAGS="--enable-code-coverage --build-path .build-test-debug"
+          xcrun llvm-cov show \
+            -instr-profile=.build-test-debug/debug/codecov/default.profdata \
+            .build-test-debug/debug/OpenCombinePackageTests.xctest/Contents/MacOS/OpenCombinePackageTests \
+            > coverage.txt
+    - run:
+        name: Building and running tests in debug mode with TSan
+        command: |
+          make test-debug-sanitize-thread \
+            SWIFT_BUILD_FLAGS="-Xswiftc -warnings-as-errors" \
+            SWIFT_TEST_FLAGS="--build-path .build-test-debug-sanitize-thread"
+    - run:
+        name: Building and running tests in release mode
+        command: |
+          make test-release \
+            SWIFT_BUILD_FLAGS="-Xswiftc -warnings-as-errors" \
+            SWIFT_TEST_FLAGS="--build-path .build-test-release"
+    - run:
+        name: Generating Xcode project
+        command: make generate-xcodeproj SWIFT_BUILD_FLAGS="-Xswiftc -warnings-as-errors"
+    - run:
+        name: Building for testing on macOS 10.15.0 with xcodebuild
+        command: |
+          set -o pipefail \
+            && xcodebuild build-for-testing \
+                  -scheme OpenCombine-Package \
+                  -sdk macosx10.15 \
+                  -derivedDataPath DerivedData \
+            | tee xcodebuild_build-for-testing.log \
+            | xcpretty
+    - store_artifacts:
+        path: xcodebuild_build-for-testing.log
+    - run:
+        name: Testing on macOS 10.15.0 with xcodebuild
+        command: |
+          set -o pipefail \
+            && xcodebuild test-without-building \
+                  -scheme OpenCombine-Package \
+                  -sdk macosx10.15 \
+                  -derivedDataPath DerivedData \
+            | tee xcodebuild_test-without-building.log \
+            | xcpretty --report junit -o build/reports/results.xml
+    - store_artifacts:
+        path: xcodebuild_test-without-building.log
+    - store_test_results:
+        path: build/reports
+    - run:
+        name: Uploading code coverage
+        command: |
+          bash <(curl -s https://codecov.io/bash) -D DerivedData
+
+ubuntu_tests_steps: &ubuntu_tests_steps
+  steps:
+    - checkout
+    - run:
+        name: Installing dependencies
+        command: |
+          apt update -y
+          apt upgrade -y
+          apt install -y curl python3.8
+    - run:
+        name: "Generating LinuxMain.swift"
+        command: python3.8 utils/discover_tests.py
+    - run:
+        name: Building and running tests in debug mode with coverage
+        command: |
+          make test-debug \
+            SWIFT_BUILD_FLAGS="-Xswiftc -warnings-as-errors" \
+            SWIFT_TEST_FLAGS="--enable-code-coverage \
+                              --disable-index-store \
+                              --build-path .build-test-debug"
+          llvm-cov show \
+            -instr-profile=.build-test-debug/debug/codecov/default.profdata \
+            .build-test-debug/debug/OpenCombinePackageTests.xctest \
+            > coverage.txt
+    - run:
+        name: Building and running tests in debug mode with TSan
+        command: |
+          make test-debug-sanitize-thread \
+            SWIFT_TEST_FLAGS="--disable-index-store \
+                              --build-path .build-test-debug-sanitize-thread" \
+    - run:
+        name: Building and running tests in release mode
+        command: |
+          make test-release \
+            SWIFT_BUILD_FLAGS="-Xswiftc -warnings-as-errors" \
+            SWIFT_TEST_FLAGS="--build-path .build-test-release"
+    - run:
+        name: Uploading code coverage
+        command: |
+          bash <(curl -s https://codecov.io/bash)
+
 version: 2
 jobs:
  "Execute tests on macOS 10.15.0 (Xcode 11.3.0, Swift 5.1.3)":
@@ -5,93 +105,44 @@ jobs:
      xcode: "11.3.0"
    environment:
      SWIFT_VERSION: "5.1.3"
-    steps:
-      - checkout
-      - run:
-          name: Building and running tests in debug mode with coverage
-          command: |
-            make test-debug \
-              SWIFT_BUILD_FLAGS="-Xswiftc -warnings-as-errors" \
-              SWIFT_TEST_FLAGS="--enable-code-coverage --build-path .build-test-debug"
-            xcrun llvm-cov show \
-              -instr-profile=.build-test-debug/debug/codecov/default.profdata \
-              .build-test-debug/debug/OpenCombinePackageTests.xctest/Contents/MacOS/OpenCombinePackageTests \
-              > coverage.txt
-      - run:
-          name: Building and running tests in debug mode with TSan
-          command: |
-            make test-debug-sanitize-thread \
-              SWIFT_BUILD_FLAGS="-Xswiftc -warnings-as-errors" \
-              SWIFT_TEST_FLAGS="--build-path .build-test-debug-sanitize-thread"
-      - run:
-          name: Building and running tests in release mode
-          command: |
-            make test-release \
-              SWIFT_BUILD_FLAGS="-Xswiftc -warnings-as-errors" \
-              SWIFT_TEST_FLAGS="--build-path .build-test-release"
-      - run:
-          name: Generating Xcode project
-          command: make generate-xcodeproj SWIFT_BUILD_FLAGS="-Xswiftc -warnings-as-errors"
-      - run:
-          name: Building for testing on macOS 10.15.0 with xcodebuild
-          command: |
-            set -o pipefail \
-              && xcodebuild build-for-testing \
-                   -scheme OpenCombine-Package \
-                   -sdk macosx10.15 \
-                   -derivedDataPath DerivedData \
-              | tee xcodebuild_build-for-testing.log \
-              | xcpretty
-      - store_artifacts:
-          path: xcodebuild_build-for-testing.log
-      - run:
-          name: Testing on macOS 10.15.0 with xcodebuild
-          command: |
-            set -o pipefail \
-              && xcodebuild test-without-building \
-                   -scheme OpenCombine-Package \
-                   -sdk macosx10.15 \
-                   -derivedDataPath DerivedData \
-              | tee xcodebuild_test-without-building.log \
-              | xcpretty --report junit -o build/reports/results.xml
-      - store_artifacts:
-          path: xcodebuild_test-without-building.log
-      - store_test_results:
-          path: build/reports
-      - run:
-          name: Uploading code coverage
-          command: |
-            bash <(curl -s https://codecov.io/bash) -D DerivedData
-
-  "Execute compatibility tests on iOS 13.4 (Xcode 11.4.0, Swift 5.2.0)":
+    <<: *macOS_tests_steps
+  
+  "Execute tests on macOS 10.15.0 (Xcode 12.1.0, Swift 5.3.0)":
    macos:
-      xcode: "11.4.0"
+      xcode: "12.1.0"
    environment:
-      SWIFT_VERSION: "5.2.0"
+      SWIFT_VERSION: "5.3.0"
+    <<: *macOS_tests_steps
+
+  "Execute compatibility tests on iOS 14.1 (Xcode 12.1.0, Swift 5.3.0)":
+    macos:
+      xcode: "12.1.0"
+    environment:
+      SWIFT_VERSION: "5.3.0"
    steps:
      - checkout
      - run:
          name: Generating Xcode project
          command: make generate-compatibility-xcodeproj
      - run:
-          name: Building for testing on iOS 13.4 with xcodebuild
+          name: Building for testing on iOS 14.1 with xcodebuild
          command: |
            set -o pipefail \
              && xcodebuild build-for-testing \
                   -scheme OpenCombine-Package \
-                   -destination "platform=iOS Simulator,name=iPhone 11,OS=13.4" \
+                   -destination "platform=iOS Simulator,name=iPhone 11,OS=14.1" \
                   -derivedDataPath DerivedData \
              | tee xcodebuild_build-for-testing.log \
              | xcpretty
      - store_artifacts:
          path: xcodebuild_build-for-testing.log
      - run:
-          name: Testing against Combine on iOS 13.4 with xcodebuild
+          name: Testing against Combine on iOS 14.1 with xcodebuild
          command: |
            set -o pipefail \
              && xcodebuild test-without-building \
                   -scheme OpenCombine-Package \
-                   -destination "platform=iOS Simulator,name=iPhone 11,OS=13.4" \
+                   -destination "platform=iOS Simulator,name=iPhone 11,OS=14.1" \
                   -derivedDataPath DerivedData \
              | tee xcodebuild_test-without-building.log \
              | xcpretty --report junit -o build/reports/results.xml
@@ -161,65 +212,33 @@ jobs:
          command: |
            bash <(curl -s https://codecov.io/bash) -D DerivedData

+  "Execute tests on Ubuntu 18.04 (Swift 5.0)":
+    docker:
+      - image: swift:5.0-bionic
+    environment:
+      SWIFT_VERSION: "5.0"
+    <<: *ubuntu_tests_steps
+
+  "Execute tests on Ubuntu 18.04 (Swift 5.1)":
+    docker:
+      - image: swift:5.1-bionic
+    environment:
+      SWIFT_VERSION: "5.1"
+    <<: *ubuntu_tests_steps
+
  "Execute tests on Ubuntu 18.04 (Swift 5.2)":
    docker:
      - image: swift:5.2-bionic
    environment:
      SWIFT_VERSION: "5.2"
-    steps:
-      - checkout
-      - run:
-          name: Installing dependencies
-          command: |
-            apt update -y
-            apt upgrade -y
-            apt install -y curl
-      - run:
-          name: Building and running tests in debug mode with coverage
-          command: | # We need to run the test command twice because of https://bugs.swift.org/browse/SR-10783
-            make test-debug \
-              SWIFT_TEST_FLAGS="--enable-test-discovery \
-                                --enable-index-store \
-                                --enable-code-coverage \
-                                --build-path .build-test-debug" \
-              > /dev/null 2>&1 \
-              || true
-            make test-debug \
-              SWIFT_BUILD_FLAGS="-Xswiftc -warnings-as-errors" \
-              SWIFT_TEST_FLAGS="--enable-test-discovery \
-                                --enable-index-store \
-                                --enable-code-coverage \
-                                --build-path .build-test-debug"
-            llvm-cov show \
-              -instr-profile=.build-test-debug/debug/codecov/default.profdata \
-              .build-test-debug/debug/OpenCombinePackageTests.xctest \
-              > coverage.txt
-      - run:
-          name: Building and running tests in debug mode with TSan
-          command: | # We need to run the test command twice because of https://bugs.swift.org/browse/SR-10783
-            make test-debug-sanitize-thread \
-              SWIFT_BUILD_FLAGS="-Xswiftc -warnings-as-errors" \
-              SWIFT_TEST_FLAGS="--enable-test-discovery \
-                                --enable-index-store \
-                                --build-path .build-test-debug-sanitize-thread" \
-              > /dev/null 2>&1 \
-              || true
-            make test-debug-sanitize-thread \
-              SWIFT_TEST_FLAGS="--enable-test-discovery \
-                                --enable-index-store \
-                                --build-path .build-test-debug-sanitize-thread" \
-      - run:
-          name: Building and running tests in release mode
-          command: |
-            make test-release \
-              SWIFT_BUILD_FLAGS="-Xswiftc -warnings-as-errors" \
-              SWIFT_TEST_FLAGS="--enable-test-discovery \
-                                --enable-index-store \
-                                --build-path .build-test-release"
-      - run:
-          name: Uploading code coverage
-          command: |
-            bash <(curl -s https://codecov.io/bash)
+    <<: *ubuntu_tests_steps
+
+  "Execute tests on Ubuntu 18.04 (Swift 5.3)":
+    docker:
+      - image: swift:5.3-bionic
+    environment:
+      SWIFT_VERSION: "5.3"
+    <<: *ubuntu_tests_steps

  "Run SwiftLint and Danger":
    macos:
@@ -257,15 +276,19 @@ workflows:
  "OpenCombine: execute tests on macOS":
    jobs:
      - "Execute tests on macOS 10.15.0 (Xcode 11.3.0, Swift 5.1.3)"
+      - "Execute tests on macOS 10.15.0 (Xcode 12.1.0, Swift 5.3.0)"
  "OpenCombine: execute compatibility tests":
    jobs:
-      - "Execute compatibility tests on iOS 13.4 (Xcode 11.4.0, Swift 5.2.0)"
+      - "Execute compatibility tests on iOS 14.1 (Xcode 12.1.0, Swift 5.3.0)"
  "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 5.0)"
+      - "Execute tests on Ubuntu 18.04 (Swift 5.1)"
      - "Execute tests on Ubuntu 18.04 (Swift 5.2)"
+      - "Execute tests on Ubuntu 18.04 (Swift 5.3)"
  "OpenCombine: run SwiftLint and Danger":
    jobs:
      - "Run SwiftLint and Danger"
@@ -3,6 +3,7 @@
 /Packages
 /*.xcodeproj
 /.swiftpm
+Tests/LinuxMain.swift

 # Created by https://www.gitignore.io/api/Xcode
 # Edit at https://www.gitignore.io/?templates=Xcode
@@ -93,7 +93,7 @@ GEM
    http-cookie (1.0.3)
      domain_name (~> 0.5)
    httpclient (2.8.3)
-    json (2.2.0)
+    json (2.3.1)
    jwt (2.1.0)
    memoist (0.16.1)
    mime-types (3.3)
@@ -1,6 +1,6 @@
 Pod::Spec.new do |spec|
  spec.name          = "OpenCombine"
-  spec.version       = "0.9.0"
+  spec.version       = "0.11.0"
  spec.summary       = "Open source implementation of Apple's Combine framework for processing values over time."

  spec.description   = <<-DESC
@@ -1,6 +1,6 @@
 Pod::Spec.new do |spec|
  spec.name          = "OpenCombineDispatch"
-  spec.version       = "0.9.0"
+  spec.version       = "0.11.0"
  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", '>= 0.8'
+  spec.dependency     "OpenCombine", '>= 0.10.2'
 end
@@ -1,6 +1,6 @@
 Pod::Spec.new do |spec|
    spec.name          = "OpenCombineFoundation"
-    spec.version       = "0.9.0"
+    spec.version       = "0.11.0"
    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", '>= 0.8'
+    spec.dependency     "OpenCombine", '>= 0.10.2'
  end
@@ -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)
@@ -10,8 +10,6 @@ Open-source implementation of Apple's [Combine](https://developer.apple.com/docu

 The main goal of this project is to provide a compatible, reliable and efficient implementation which can be used on Apple's operating systems before macOS 10.15 and iOS 13, as well as Linux and Windows.

-The project is in early development.
-
 ### Installation
 `OpenCombine` contains three public targets: `OpenCombine`, `OpenCombineFoundation` and `OpenCombineDispatch` (the fourth one, `COpenCombineHelpers`, is considered private. Don't import it in your projects).

@@ -23,7 +21,7 @@ To add `OpenCombine` to your [SPM](https://swift.org/package-manager/) package,

 ```swift
 dependencies: [
-    .package(url: "https://github.com/broadwaylamb/OpenCombine.git", from: "0.9.0")
+    .package(url: "https://github.com/OpenCombine/OpenCombine.git", from: "0.11.0")
 ],
 targets: [
    .target(name: "MyAwesomePackage", dependencies: ["OpenCombine",
@@ -35,7 +33,7 @@ targets: [
 ###### 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" 
@@ -46,18 +44,18 @@ 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', '~> 0.9'
-pod 'OpenCombineDispatch', '~> 0.9'
-pod 'OpenCombineFoundation', '~> 0.9'
+pod 'OpenCombine', '~> 0.11.0'
+pod 'OpenCombineDispatch', '~> 0.11.0'
+pod 'OpenCombineFoundation', '~> 0.11.0'
 ```

 ### 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.
+You can refer to [this repo](https://github.com/OpenCombine/combine-interfaces) to observe Apple's Combine API and documentation changes between different Xcode (beta) versions.

 You can run compatibility tests against Apple's Combine. In order to do that you will need either macOS 10.14 with iOS 13 simulator installed (since the only way we can get Apple's Combine on macOS 10.14 is using the simulator), or macOS 10.15 (Apple's Combine is bundled with the OS). Execute the following command from the root of the package:

@@ -67,7 +65,7 @@ $ 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

@@ -76,7 +74,7 @@ Or enable the `-DOPENCOMBINE_COMPATIBILITY_TEST` compiler flag in Xcode's build
 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/broadwaylamb/OpenCombine/releases) page, click the **Draft a new release** button.
+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.
@@ -164,13 +164,12 @@ extension Publisher {
 extension Publishers {

    /// A strategy for collecting received elements.
-    ///
-    /// - byTime: Collect and periodically publish items.
-    /// - byTimeOrCount: Collect and publish items, either periodically or when a buffer reaches its maximum size.
    public enum TimeGroupingStrategy<Context> where Context : Scheduler {

+        /// A grouping that collects and periodically publishes items.
        case byTime(Context, Context.SchedulerTimeType.Stride)

+        /// A grouping that collects and publishes items periodically or when a buffer reaches a maximum size.
        case byTimeOrCount(Context, Context.SchedulerTimeType.Stride, Int)
    }

@@ -208,15 +207,40 @@ extension Publishers {

 extension Publisher {

-    /// Collects elements by a given strategy, and emits a single array of the collection.
+    /// Collects elements by a given time-grouping strategy, and emits a single array of
+    /// the collection.
+    ///
+    /// Use `collect(_:options:)` to emit arrays of elements on a schedule specified by
+    /// a `Scheduler` and `Stride` that you provide. At the end of each scheduled
+    /// interval, the publisher sends an array that contains the items it collected.
+    /// If the upstream publisher finishes before filling the buffer, the publisher sends
+    /// an array that contains items it received. This may be fewer than the number of
+    /// elements specified in the requested `Stride`.
+    ///
+    /// If the upstream publisher fails with an error, this publisher forwards the error
+    /// to the downstream receiver instead of sending its output.
+    ///
+    /// The example above collects timestamps generated on a one-second `Timer` in groups
+    /// (`Stride`) of five.
+    ///
+    ///     let sub = Timer.publish(every: 1, on: .main, in: .default)
+    ///         .autoconnect()
+    ///         .collect(.byTime(RunLoop.main, .seconds(5)))
+    ///         .sink { print("\($0)", terminator: "\n\n") }
+    ///
+    ///     // Prints: "[2020-01-24 00:54:46 +0000, 2020-01-24 00:54:47 +0000,
+    ///     //          2020-01-24 00:54:48 +0000, 2020-01-24 00:54:49 +0000,
+    ///     //          2020-01-24 00:54:50 +0000]"
+    ///
+    /// > Note: When this publisher receives a request for `.max(n)` elements, it requests
+    /// `.max(count * n)` from the upstream publisher.
    ///
-    /// If the upstream publisher finishes before filling the buffer, this publisher sends an array of all the items it has received. This may be fewer than `count` elements.
-    /// If the upstream publisher fails with an error, this publisher forwards the error to the downstream receiver instead of sending its output.
-    /// Note: When this publisher receives a request for `.max(n)` elements, it requests `.max(count * n)` from the upstream publisher.
    /// - Parameters:
-    ///   - strategy: The strategy with which to collect and publish elements.
-    ///   - options: `Scheduler` options to use for the strategy.
-    /// - Returns: A publisher that collects elements by a given strategy, and emits a single array of the collection.
+    ///   - strategy: The timing group strategy used by the operator to collect and
+    ///     publish elements.
+    ///   - options: ``Scheduler`` options to use for the strategy.
+    /// - Returns: A publisher that collects elements by a given strategy, and emits
+    ///   a single array of the collection.
    public func collect<S>(_ strategy: Publishers.TimeGroupingStrategy<S>, options: S.SchedulerOptions? = nil) -> Publishers.CollectByTime<Self, S> where S : Scheduler
 }

@@ -750,113 +774,48 @@ extension Publishers {

 extension Publisher {

-    /// Publishes either the most-recent or first element published by the upstream publisher in the specified time interval.
+    /// Publishes either the most-recent or first element published by the upstream
+    /// publisher in the specified time interval.
+    ///
+    /// Use `throttle(for:scheduler:latest:`` to selectively republish elements from
+    /// an upstream publisher during an interval you specify. Other elements received from
+    /// the upstream in the throttling interval aren’t republished.
+    ///
+    /// In the example below, a `Timer.TimerPublisher` produces elements on 3-second
+    /// intervals; the `throttle(for:scheduler:latest:)` operator delivers the first
+    /// event, then republishes only the latest event in the following ten second
+    /// intervals:
+    ///
+    ///     cancellable = Timer.publish(every: 3.0, on: .main, in: .default)
+    ///         .autoconnect()
+    ///         .print("\(Date().description)")
+    ///         .throttle(for: 10.0, scheduler: RunLoop.main, latest: true)
+    ///         .sink(
+    ///             receiveCompletion: { print ("Completion: \($0).") },
+    ///             receiveValue: { print("Received Timestamp \($0).") }
+    ///          )
+    ///
+    ///     // Prints:
+    ///     //    Publish at: 2020-03-19 18:26:54 +0000: receive value: (2020-03-19 18:26:57 +0000)
+    ///     //    Received Timestamp 2020-03-19 18:26:57 +0000.
+    ///     //    Publish at: 2020-03-19 18:26:54 +0000: receive value: (2020-03-19 18:27:00 +0000)
+    ///     //    Publish at: 2020-03-19 18:26:54 +0000: receive value: (2020-03-19 18:27:03 +0000)
+    ///     //    Publish at: 2020-03-19 18:26:54 +0000: receive value: (2020-03-19 18:27:06 +0000)
+    ///     //    Publish at: 2020-03-19 18:26:54 +0000: receive value: (2020-03-19 18:27:09 +0000)
+    ///     //    Received Timestamp 2020-03-19 18:27:09 +0000.
    ///
    /// - Parameters:
-    ///   - interval: The interval at which to find and emit the most recent element, expressed in the time system of the scheduler.
+    ///   - interval: The interval at which to find and emit either the most recent or
+    ///     the first element, expressed in the time system of the scheduler.
    ///   - scheduler: The scheduler on which to publish elements.
-    ///   - latest: A Boolean value that indicates whether to publish the most recent element. If `false`, the publisher emits the first element received during the interval.
-    /// - Returns: A publisher that emits either the most-recent or first element received during the specified interval.
+    ///   - latest: A Boolean value that indicates whether to publish the most recent
+    ///     element. If `false`, the publisher emits the first element received during
+    ///     the interval.
+    /// - Returns: A publisher that emits either the most-recent or first element received
+    ///   during the specified interval.
    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 publisher created by applying the zip function to two upstream publishers.
@@ -37,73 +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
-  }
-}
@@ -10,7 +10,8 @@
 /// Subscriber implementations can use this type to provide a “cancellation token” that
 /// makes it possible for a caller to cancel a publisher, but not to use the
 /// `Subscription` object to request items.
-/// An AnyCancellable instance automatically calls `cancel()` when deinitialized.
+///
+/// An `AnyCancellable` instance automatically calls `cancel()` when deinitialized.
 public final class AnyCancellable: Cancellable, Hashable {

    private var _cancel: (() -> Void)?
@@ -46,18 +47,18 @@ public final class AnyCancellable: Cancellable, Hashable {

 extension AnyCancellable {

-    /// Stores this AnyCancellable in the specified collection.
-    /// Parameters:
-    ///    - collection: The collection to store this AnyCancellable.
+    /// Stores this type-erasing cancellable instance in the specified collection.
+    ///
+    /// - Parameter collection: The collection in which to store this `AnyCancellable`.
    public func store<Cancellables: RangeReplaceableCollection>(
        in collection: inout Cancellables
    ) where Cancellables.Element == AnyCancellable {
        collection.append(self)
    }

-    /// Stores this AnyCancellable in the specified set.
-    /// Parameters:
-    ///    - set: The set to store this AnyCancellable.
+    /// Stores this type-erasing cancellable instance in the specified collection.
+    ///
+    /// - Parameter collection: The collection in which to store this `AnyCancellable`.
    public func store(in set: inout Set<AnyCancellable>) {
        set.insert(self)
    }
@@ -9,8 +9,43 @@ extension Publisher {

    /// Wraps this publisher with a type eraser.
    ///
-    /// Use `eraseToAnyPublisher()` to expose an instance of `AnyPublisher` to
+    /// Use `eraseToAnyPublisher()` to expose an instance of `AnyPublishe`` to
    /// the downstream subscriber, rather than this publisher’s actual type.
+    /// This form of _type erasure_ preserves abstraction across API boundaries, such as
+    /// different modules.
+    /// When you expose your publishers as the `AnyPublisher` type, you can change
+    /// the underlying implementation over time without affecting existing clients.
+    ///
+    /// The following example shows two types that each have a `publisher` property.
+    /// `TypeWithSubject` exposes this property as its actual type, `PassthroughSubject`,
+    /// while `TypeWithErasedSubject` uses `eraseToAnyPublisher()` to expose it as
+    /// an `AnyPublisher`. As seen in the output, a caller from another module can access
+    /// `TypeWithSubject.publisher` as its native type. This means you can’t change your
+    /// publisher to a different type without breaking the caller. By comparison,
+    /// `TypeWithErasedSubject.publisher` appears to callers as an `AnyPublisher`, so you
+    /// can change the underlying publisher type at will.
+    ///
+    ///     public class TypeWithSubject {
+    ///         public let publisher: some Publisher = PassthroughSubject<Int,Never>()
+    ///     }
+    ///     public class TypeWithErasedSubject {
+    ///         public let publisher: some Publisher = PassthroughSubject<Int,Never>()
+    ///             .eraseToAnyPublisher()
+    ///     }
+    ///
+    ///     // In another module:
+    ///     let nonErased = TypeWithSubject()
+    ///     if let subject = nonErased.publisher as? PassthroughSubject<Int,Never> {
+    ///         print("Successfully cast nonErased.publisher.")
+    ///     }
+    ///     let erased = TypeWithErasedSubject()
+    ///     if let subject = erased.publisher as? PassthroughSubject<Int,Never> {
+    ///         print("Successfully cast erased.publisher.")
+    ///     }
+    ///
+    ///     // Prints "Successfully cast nonErased.publisher."
+    ///
+    /// - Returns: An ``AnyPublisher`` wrapping this publisher.
    @inlinable
    public func eraseToAnyPublisher() -> AnyPublisher<Output, Failure> {
        return .init(self)
@@ -20,7 +55,13 @@ extension Publisher {
 /// A type-erasing publisher.
 ///
 /// Use `AnyPublisher` to wrap a publisher whose type has details you don’t want to expose
-/// to subscribers or other publishers.
+/// across API boundaries, such as different modules. Wrapping a `Subject` with
+/// `AnyPublisher` also prevents callers from accessing its `send(_:)` method. When you
+/// use type erasure this way, you can change the underlying publisher implementation over
+/// time without affecting existing clients.
+///
+/// You can use OpenCombine’s `eraseToAnyPublisher()` operator to wrap a publisher with
+/// `AnyPublisher`.
 public struct AnyPublisher<Output, Failure: Error>
  : CustomStringConvertible,
    CustomPlaygroundDisplayConvertible
@@ -30,13 +71,17 @@ public struct AnyPublisher<Output, Failure: Error>

    /// Creates a type-erasing publisher to wrap the provided publisher.
    ///
-    /// - Parameters:
-    ///   - publisher: A publisher to wrap with a type-eraser.
+    /// - Parameter publisher: A publisher to wrap with a type-eraser.
    @inlinable
    public init<PublisherType: Publisher>(_ publisher: PublisherType)
        where Output == PublisherType.Output, Failure == PublisherType.Failure
    {
-        box = PublisherBox(base: publisher)
+        // If this has already been boxed, avoid boxing again
+        if let erased = publisher as? AnyPublisher<Output, Failure> {
+            box = erased.box
+        } else {
+            box = PublisherBox(base: publisher)
+        }
    }

    public var description: String {
@@ -61,7 +106,7 @@ extension AnyPublisher: Publisher {
    public func receive<Downstream: Subscriber>(subscriber: Downstream)
        where Output == Downstream.Input, Failure == Downstream.Failure
    {
-        box.subscribe(subscriber)
+        box.receive(subscriber: subscriber)
    }
 }

@@ -98,6 +143,6 @@ internal final class PublisherBox<PublisherType: Publisher>
    override internal func receive<Downstream: Subscriber>(subscriber: Downstream)
        where Failure == Downstream.Failure, Output == Downstream.Input
    {
-        base.subscribe(subscriber)
+        base.receive(subscriber: subscriber)
    }
 }
@@ -7,9 +7,10 @@

 /// A type-erasing subscriber.
 ///
-/// Use an `AnySubscriber` to wrap an existing subscriber whose details you don’t want
-/// to expose. You can also use `AnySubscriber` to create a custom subscriber by providing
-/// closures for `Subscriber`’s methods, rather than implementing `Subscriber` directly.
+/// Use an `AnySubscriber` to wrap an existing subscriber whose details you don’t want to
+/// expose. You can also use `AnySubscriber` to create a custom subscriber by providing
+/// closures for the methods defined in `Subscriber`, rather than implementing
+/// `Subscriber` directly.
 public struct AnySubscriber<Input, Failure: Error>: Subscriber,
                                                    CustomStringConvertible,
                                                    CustomReflectable,
@@ -44,6 +45,11 @@ public struct AnySubscriber<Input, Failure: Error>: Subscriber,
    public init<Subscriber: OpenCombine.Subscriber>(_ subscriber: Subscriber)
        where Input == Subscriber.Input, Failure == Subscriber.Failure
    {
+        if let erased = subscriber as? AnySubscriber<Input, Failure> {
+            self = erased
+            return
+        }
+
        combineIdentifier = subscriber.combineIdentifier

        box = AnySubscriberBox(subscriber)
@@ -62,8 +68,8 @@ public struct AnySubscriber<Input, Failure: Error>: Subscriber,

        if let playgroundDescription = subscriber as? CustomPlaygroundDisplayConvertible {
            playgroundDescriptionThunk = { playgroundDescription.playgroundDescription }
-        } else if let desccription = subscriber as? CustomStringConvertible {
-            playgroundDescriptionThunk = { desccription.description }
+        } else if let description = subscriber as? CustomStringConvertible {
+            playgroundDescriptionThunk = { description.description }
        } else {
            let fixedDescription = String(describing: type(of: subscriber))
            playgroundDescriptionThunk = { fixedDescription }
@@ -5,7 +5,7 @@
 //  Created by Sergej Jaskiewicz on 10.06.2019.
 //

-/// A protocol indicating that an activity or action may be canceled.
+/// A protocol indicating that an activity or action supports cancellation.
 ///
 /// Calling `cancel()` frees up any allocated resources. It also stops side effects such
 /// as timers, network access, or disk I/O.
@@ -17,18 +17,18 @@ public protocol Cancellable {

 extension Cancellable {

-    /// Stores this Cancellable in the specified collection.
-    /// Parameters:
-    ///    - collection: The collection to store this Cancellable.
+    /// Stores this cancellable instance in the specified collection.
+    ///
+    /// - Parameter collection: The collection in which to store this `Cancellable`.
    public func store<Cancellables: RangeReplaceableCollection>(
            in collection: inout Cancellables
    ) where Cancellables.Element == AnyCancellable {
        AnyCancellable(self).store(in: &collection)
    }

-    /// Stores this Cancellable in the specified set.
-    /// Parameters:
-    ///    - set: The set to store this Cancellable.
+    /// Stores this cancellable instance in the specified set.
+    ///
+    /// - Parameter set: The set in which to store this `Cancellable`.
    public func store(in set: inout Set<AnyCancellable>) {
        AnyCancellable(self).store(in: &set)
    }
@@ -5,17 +5,25 @@
 //  Created by Sergej Jaskiewicz on 11.06.2019.
 //

+/// A type that defines methods for decoding.
 public protocol TopLevelDecoder {

+    /// The type this decoder accepts.
    associatedtype Input

+    /// Decodes an instance of the indicated type.
    func decode<DecodablyType: Decodable>(_ type: DecodablyType.Type,
                                          from: Input) throws -> DecodablyType
 }

+/// A type that defines methods for encoding.
 public protocol TopLevelEncoder {

+    /// The type this encoder produces.
    associatedtype Output

+    /// Encodes an instance of the indicated type.
+    ///
+    /// - Parameter value: The instance to encode.
    func encode<EncodableType: Encodable>(_ value: EncodableType) throws -> Output
 }
@@ -9,19 +9,28 @@
 import COpenCombineHelpers
 #endif

+/// A unique identifier for identifying publisher streams.
+///
+/// To conform to `CustomCombineIdentifierConvertible` in a
+/// `Subscription` or `Subject` that you implement as a structure, create an instance of
+/// `CombineIdentifier` as follows:
+///
+///     let combineIdentifier = CombineIdentifier()
 public struct CombineIdentifier: Hashable, CustomStringConvertible {

-    private let value: UInt64
+    private let rawValue: UInt64

+    /// Creates a unique Combine identifier.
    public init() {
-        value = __nextCombineIdentifier()
+        rawValue = __nextCombineIdentifier()
    }

+    /// Creates a Combine identifier, using the bit pattern of the provided object.
    public init(_ obj: AnyObject) {
-        value = UInt64(UInt(bitPattern: ObjectIdentifier(obj)))
+        rawValue = UInt64(UInt(bitPattern: ObjectIdentifier(obj)))
    }

    public var description: String {
-        return "0x\(String(value, radix: 16))"
+        return "0x\(String(rawValue, radix: 16))"
    }
 }
@@ -7,13 +7,18 @@

 /// A publisher that provides an explicit means of connecting and canceling publication.
 ///
+/// Use a `ConnectablePublisher` when you need to perform additional configuration or
+/// setup prior to producing any elements.
+///
+/// This publisher doesn’t produce any elements until you call its `connect()` method.
+///
 /// Use `makeConnectable()` to create a `ConnectablePublisher` from any publisher whose
 /// failure type is `Never`.
 public protocol ConnectablePublisher: Publisher {

-    /// Connects to the publisher and returns a `Cancellable` instance with which
-    /// to cancel publishing.
+    /// Connects to the publisher, allowing it to produce elements, and returns
+    /// an instance with which to cancel publishing.
    ///
-    /// - Returns: A `Cancellable` instance that can be used to cancel publishing.
+    /// - Returns: A `Cancellable` instance that you use to cancel publishing.
    func connect() -> Cancellable
 }
@@ -7,28 +7,37 @@

 /// A subject that wraps a single value and publishes a new element whenever the value
 /// changes.
+///
+/// Unlike `PassthroughSubject`, `CurrentValueSubject` maintains a buffer of the most
+/// recently published element.
+///
+/// Calling `send(_:)` on a `CurrentValueSubject` also updates the current value, making
+/// it equivalent to updating the `value` directly.
 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 +45,213 @@ 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" }
-}
@@ -5,8 +5,18 @@
 //  Created by Sergej Jaskiewicz on 10.06.2019.
 //

+/// A protocol for uniquely identifying publisher streams.
+///
+/// If you create a custom `Subscription` or `Subscriber` type, implement this protocol
+/// so that development tools can uniquely identify publisher chains in your app.
+/// If your type is a class, OpenCombine provides an implementation of `combineIdentifier`
+/// for you.
+/// If your type is a structure, set up the identifier as follows:
+///
+///     let combineIdentifier = CombineIdentifier()
 public protocol CustomCombineIdentifierConvertible {

+    /// A unique identifier for identifying publisher streams.
    var combineIdentifier: CombineIdentifier { get }
 }

@@ -5,114 +5,208 @@
 //  Created by Max Desiatov on 24/11/2019.
 //

-/// A publisher that eventually produces one value and then finishes or fails.
-public final class Future<Output, Failure>: Publisher where Failure: Error {
+/// A publisher that eventually produces a single value and then finishes or fails.
+public final class Future<Output, Failure: Error>: Publisher {

+    /// A type that represents a closure to invoke in the future, when an element or error
+    /// is available.
+    ///
+    /// The promise closure receives one parameter: a `Result` that contains either
+    /// a single element published by a `Future`, or an error.
    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>?

+    /// Creates a publisher that invokes a promise closure when the publisher emits
+    /// an element.
+    ///
+    /// - Parameter attemptToFulfill: A `Promise` that the publisher invokes when
+    ///   the publisher emits an element or terminates with an error.
    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" }
-}
@@ -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))
+    }
+}
@@ -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
+    }
+}
@@ -0,0 +1,177 @@
+//
+//  DebugHook.swift
+//  
+//
+//  Created by Sergej Jaskiewicz on 27.09.2020.
+//
+
+internal final class DebugHook {
+
+    private struct Handler: Hashable {
+        let handler: _Introspection
+
+        static func == (lhs: Handler, rhs: Handler) -> Bool {
+            return lhs.handler === rhs.handler
+        }
+
+        func hash(into hasher: inout Hasher) {
+            hasher.combine(ObjectIdentifier(handler))
+        }
+    }
+
+    internal static func getGlobalHook() -> DebugHook? {
+        globalLock.lock()
+        defer { globalLock.unlock() }
+        return globalHook
+    }
+
+    internal static func enable(_ handler: _Introspection) {
+        let hook: DebugHook
+        DebugHook.globalLock.lock()
+        defer { DebugHook.globalLock.unlock() }
+        if let _hook = DebugHook.globalHook {
+            hook = _hook
+        } else {
+            hook = DebugHook()
+            DebugHook.globalHook = hook
+        }
+        hook.lock.lock()
+        defer { hook.lock.unlock() }
+        hook.handlers.insert(Handler(handler: handler))
+    }
+
+    internal static func disable(_ handler: _Introspection) {
+        DebugHook.globalLock.lock()
+        defer { DebugHook.globalLock.unlock() }
+        guard let hook = DebugHook.globalHook else { return }
+        hook.lock.lock()
+        hook.handlers.remove(Handler(handler: handler))
+        let noMoreHandlers = hook.handlers.isEmpty
+        hook.lock.unlock()
+        if noMoreHandlers {
+            DebugHook.globalHook = nil
+        }
+    }
+
+    internal static func handlerIsEnabled(_ handler: _Introspection) -> Bool {
+        DebugHook.globalLock.lock()
+        defer { DebugHook.globalLock.unlock() }
+        guard let hook = DebugHook.globalHook else { return false }
+        hook.lock.lock()
+        defer { hook.lock.unlock() }
+        return hook.handlers.contains(Handler(handler: handler))
+    }
+
+    private static var globalHook: DebugHook?
+
+    private static let globalLock = UnfairLock.allocate()
+
+    private let lock = UnfairLock.allocate()
+
+    private var handlers = Set<Handler>()
+
+    internal var debugHandlers: [_Introspection] {
+        lock.lock()
+        defer { lock.unlock() }
+        return handlers.map { $0.handler }
+    }
+
+    private init() {}
+
+    deinit {
+        lock.deallocate()
+    }
+
+    internal func willReceive<Upstream: Publisher, Downstream: Subscriber>(
+        publisher: Upstream,
+        subscriber: Downstream
+    ) where Upstream.Failure == Downstream.Failure, Upstream.Output == Downstream.Input {
+        for debugHandler in debugHandlers {
+            debugHandler.willReceive(publisher: publisher, subscriber: subscriber)
+        }
+    }
+
+    internal func didReceive<Upstream: Publisher, Downstream: Subscriber>(
+        publisher: Upstream,
+        subscriber: Downstream
+    ) where Upstream.Failure == Downstream.Failure, Upstream.Output == Downstream.Input {
+        for debugHandler in debugHandlers {
+            debugHandler.didReceive(publisher: publisher, subscriber: subscriber)
+        }
+    }
+
+    internal func willReceive<Downstream: Subscriber>(subscriber: Downstream,
+                                                      subscription: Subscription) {
+        for debugHandler in debugHandlers {
+            debugHandler.willReceive(subscriber: subscriber, subscription: subscription)
+        }
+    }
+
+    internal func didReceive<Downstream: Subscriber>(subscriber: Downstream,
+                                                     subscription: Subscription) {
+        for debugHandler in debugHandlers {
+            debugHandler.didReceive(subscriber: subscriber, subscription: subscription)
+        }
+    }
+
+    internal func willReceive<Downstream: Subscriber>(subscriber: Downstream,
+                                                      input: Downstream.Input) {
+        for debugHandler in debugHandlers {
+            debugHandler.willReceive(subscriber: subscriber, input: input)
+        }
+    }
+
+    internal func didReceive<Downstream: Subscriber>(
+        subscriber: Downstream,
+        input: Downstream.Input,
+        resultingDemand: Subscribers.Demand
+    ) {
+        for debugHandler in debugHandlers {
+            debugHandler.didReceive(subscriber: subscriber,
+                                    input: input,
+                                    resultingDemand: resultingDemand)
+        }
+    }
+
+    internal func willReceive<Downstream: Subscriber>(
+        subscriber: Downstream,
+        completion: Subscribers.Completion<Downstream.Failure>
+    ) {
+        for debugHandler in debugHandlers {
+            debugHandler.willReceive(subscriber: subscriber, completion: completion)
+        }
+    }
+
+    internal func didReceive<Downstream: Subscriber>(
+        subscriber: Downstream,
+        completion: Subscribers.Completion<Downstream.Failure>
+    ) {
+        for debugHandler in debugHandlers {
+            debugHandler.didReceive(subscriber: subscriber, completion: completion)
+        }
+    }
+
+    internal func willRequest(subscription: Subscription, demand: Subscribers.Demand) {
+        for debugHandler in debugHandlers {
+            debugHandler.willRequest(subscription: subscription, demand)
+        }
+    }
+
+    internal func didRequest(subscription: Subscription, demand: Subscribers.Demand) {
+        for debugHandler in debugHandlers {
+            debugHandler.didRequest(subscription: subscription, demand)
+        }
+    }
+
+    internal func willCancel(subscription: Subscription) {
+        for debugHandler in debugHandlers {
+            debugHandler.willCancel(subscription: subscription)
+        }
+    }
+
+    internal func didCancel(subscription: Subscription) {
+        for debugHandler in debugHandlers {
+            debugHandler.didCancel(subscription: subscription)
+        }
+    }
+}
@@ -9,7 +9,7 @@
 ///
 /// Filter-like operators send an instance of their `Inner` class that is subclass
 /// of this class to the upstream publisher (as subscriber) and
-/// to the downstream subcriber (as subscription).
+/// to the downstream subscriber (as subscription).
 ///
 /// Filter-like operators include `Publishers.Filter`,
 /// `Publishers.RemoveDuplicates`, `Publishers.PrefixWhile` and more.
@@ -24,8 +24,6 @@ internal class FilterProducer<Downstream: Subscriber,
      CustomReflectable
    where Downstream.Input == Output
 {
-    // NOTE: This class has been audited for thread safety
-
    // MARK: - State

    private enum State {
@@ -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()
-    }
-}
@@ -0,0 +1,212 @@
+//
+//  PublishedSubject.swift
+//  
+//
+//  Created by Sergej Jaskiewicz on 29.10.2020.
+//
+
+internal final class PublishedSubject<Output>: Subject {
+
+    internal typealias Failure = Never
+
+    private let lock = UnfairLock.allocate()
+
+    private var downstreams = ConduitList<Output, Failure>.empty
+
+    private var currentValue: Output
+
+    private var upstreamSubscriptions: [Subscription] = []
+
+    private var hasAnyDownstreamDemand = false
+
+    private var changePublisher: ObservableObjectPublisher?
+
+    internal var value: Output {
+        get {
+            lock.lock()
+            defer { lock.unlock() }
+            return currentValue
+        }
+        set {
+            send(newValue)
+        }
+    }
+
+    internal var objectWillChange: ObservableObjectPublisher? {
+        get {
+            lock.lock()
+            defer { lock.unlock() }
+            return changePublisher
+        }
+        set {
+            lock.lock()
+            defer { lock.unlock() }
+            changePublisher = newValue
+        }
+    }
+
+    internal init(_ value: Output) {
+        self.currentValue = value
+    }
+
+    deinit {
+        for subscription in upstreamSubscriptions {
+            subscription.cancel()
+        }
+        lock.deallocate()
+    }
+
+    internal func send(subscription: Subscription) {
+        lock.lock()
+        upstreamSubscriptions.append(subscription)
+        lock.unlock()
+        subscription.request(.unlimited)
+    }
+
+    internal func receive<Downstream: Subscriber>(subscriber: Downstream)
+        where Downstream.Input == Output, Downstream.Failure == Never
+    {
+        lock.lock()
+        let conduit = Conduit(parent: self, downstream: subscriber)
+        downstreams.insert(conduit)
+        lock.unlock()
+        subscriber.receive(subscription: conduit)
+    }
+
+    internal func send(_ input: Output) {
+        lock.lock()
+        let downstreams = self.downstreams
+        let changePublisher = self.changePublisher
+        lock.unlock()
+        changePublisher?.send()
+        downstreams.forEach { conduit in
+            conduit.offer(input)
+        }
+        lock.lock()
+        currentValue = input
+        lock.unlock()
+    }
+
+    internal func send(completion: Subscribers.Completion<Never>) {
+        fatalError("unreachable")
+    }
+
+    private func disassociate(_ conduit: ConduitBase<Output, Failure>) {
+        lock.lock()
+        downstreams.remove(conduit)
+        lock.unlock()
+    }
+}
+
+extension PublishedSubject {
+
+    private final class Conduit<Downstream: Subscriber>
+        : ConduitBase<Output, Failure>,
+          CustomStringConvertible,
+          CustomReflectable,
+          CustomPlaygroundDisplayConvertible
+        where Downstream.Input == Output, Downstream.Failure == Never
+    {
+
+        fileprivate var parent: PublishedSubject?
+
+        fileprivate var downstream: Downstream?
+
+        fileprivate var demand = Subscribers.Demand.none
+
+        private var lock = UnfairLock.allocate()
+
+        private var downstreamLock = UnfairRecursiveLock.allocate()
+
+        private var deliveredCurrentValue = false
+
+        fileprivate init(parent: PublishedSubject,
+                         downstream: Downstream) {
+            self.parent = parent
+            self.downstream = downstream
+        }
+
+        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()
+        }
+
+        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 "PublishedSubject" }
+
+        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 }
+    }
+}
@@ -0,0 +1,32 @@
+//
+//  PublishedSubscriber.swift
+//  
+//
+//  Created by Sergej Jaskiewicz on 29.10.2020.
+//
+
+internal struct PublishedSubscriber<Value>: Subscriber {
+
+    internal typealias Input = Value
+
+    internal typealias Failure = Never
+
+    internal let combineIdentifier = CombineIdentifier()
+
+    private weak var subject: PublishedSubject<Value>?
+
+    internal init(_ subject: PublishedSubject<Value>) {
+        self.subject = subject
+    }
+
+    internal func receive(subscription: Subscription) {
+        subject?.send(subscription: subscription)
+    }
+
+    internal func receive(_ input: Value) -> Subscribers.Demand {
+        subject?.send(input)
+        return .none
+    }
+
+    internal func receive(completion: Subscribers.Completion<Never>) {}
+}
@@ -9,7 +9,7 @@
 ///
 /// Reduce-like operators send an instance of their `Inner` class that is subclass
 /// of this class to the upstream publisher (as subscriber) and
-/// to the downstream subcriber (as subsription).
+/// to the downstream subscriber (as subscription).
 ///
 /// Reduce-like operators include `Publishers.Reduce`, `Publishers.TryReduce`,
 /// `Publishers.Count`, `Publishers.FirstWhere`, `Publishers.AllSatisfy` and more.
@@ -5,7 +5,6 @@
 //  Created by Sergej Jaskiewicz on 16/09/2019.
 //

-// NOTE: This class has been audited for thread safety.
 internal final class SubjectSubscriber<Downstream: Subject>
    : Subscriber,
      CustomStringConvertible,
@@ -14,7 +13,7 @@ internal final class SubjectSubscriber<Downstream: Subject>
      Subscription
 {
    private let lock = UnfairLock.allocate()
-    private var downstreamSubject: Downstream?
+    private weak var downstreamSubject: Downstream?
    private var upstreamSubscription: Subscription?

    private var isCancelled: Bool { return downstreamSubject == nil }
@@ -0,0 +1,69 @@
+//
+//  SubscriberTap.swift
+//  
+//
+//  Created by Sergej Jaskiewicz on 27.09.2020.
+//
+
+internal protocol SubscriberTapMarker {
+    var inner: Any { mutating get }
+}
+
+internal struct SubscriberTap<Subscriber: OpenCombine.Subscriber>
+: OpenCombine.Subscriber,
+  CustomStringConvertible,
+  SubscriberTapMarker
+{
+    internal typealias Input = Subscriber.Input
+
+    internal typealias Failure = Subscriber.Failure
+
+    private let subscriber: Subscriber
+
+    internal lazy var inner: Any = AnySubscriber(self.subscriber)
+
+    internal init(subscriber: Subscriber) {
+        self.subscriber = subscriber
+    }
+
+    internal var combineIdentifier: CombineIdentifier {
+        return subscriber.combineIdentifier
+    }
+
+    internal func receive(subscription: Subscription) {
+        let hook = DebugHook.getGlobalHook()
+        if let subscriptionTap = subscription as? SubscriptionTap {
+            hook?.willReceive(subscriber: subscriber,
+                              subscription: subscriptionTap.subscription)
+            subscriber.receive(subscription: subscriptionTap)
+            hook?.didReceive(subscriber: subscriber,
+                             subscription: subscriptionTap.subscription)
+        } else {
+            hook?.willReceive(subscriber: subscriber, subscription: subscription)
+            subscriber
+                .receive(subscription: SubscriptionTap(subscription: subscription))
+            hook?.didReceive(subscriber: subscriber, subscription: subscription)
+        }
+    }
+
+    internal func receive(_ input: Input) -> Subscribers.Demand {
+        let hook = DebugHook.getGlobalHook()
+        hook?.willReceive(subscriber: subscriber, input: input)
+        let newDemand = subscriber.receive(input)
+        hook?.didReceive(subscriber: subscriber,
+                         input: input,
+                         resultingDemand: newDemand)
+        return newDemand
+    }
+
+    internal func receive(completion: Subscribers.Completion<Subscriber.Failure>) {
+        let hook = DebugHook.getGlobalHook()
+        hook?.willReceive(subscriber: subscriber, completion: completion)
+        subscriber.receive(completion: completion)
+        hook?.didReceive(subscriber: subscriber, completion: completion)
+    }
+
+    internal var description: String {
+        return String(describing: subscriber)
+    }
+}
@@ -8,5 +8,26 @@
 internal enum SubscriptionStatus {
    case awaitingSubscription
    case subscribed(Subscription)
+    case pendingTerminal(Subscription)
    case terminal
 }
+
+extension SubscriptionStatus {
+    internal var isAwaitingSubscription: Bool {
+        switch self {
+        case .awaitingSubscription:
+            return true
+        default:
+            return false
+        }
+    }
+
+    internal var subscription: Subscription? {
+        switch self {
+        case .awaitingSubscription, .terminal:
+            return nil
+        case let .subscribed(subscription), let .pendingTerminal(subscription):
+            return subscription
+        }
+    }
+}
@@ -0,0 +1,33 @@
+//
+//  SubscriptionTap.swift
+//  
+//
+//  Created by Sergej Jaskiewicz on 27.09.2020.
+//
+
+internal struct SubscriptionTap: Subscription, CustomStringConvertible {
+
+    internal let subscription: Subscription
+
+    internal var combineIdentifier: CombineIdentifier {
+        return subscription.combineIdentifier
+    }
+
+    internal func request(_ demand: Subscribers.Demand) {
+        let hook = DebugHook.getGlobalHook()
+        hook?.willRequest(subscription: subscription, demand: demand)
+        subscription.request(demand)
+        hook?.didRequest(subscription: subscription, demand: demand)
+    }
+
+    internal func cancel() {
+        let hook = DebugHook.getGlobalHook()
+        hook?.willCancel(subscription: subscription)
+        subscription.cancel()
+        hook?.didCancel(subscription: subscription)
+    }
+
+    internal var description: String {
+        return String(describing: subscription)
+    }
+}
@@ -8,8 +8,8 @@
 /// A scheduler for performing synchronous actions.
 ///
 /// You can only use this scheduler for immediate actions. If you attempt to schedule
-/// actions after a specific date, this scheduler ignores the date and performs
-/// them immediately.
+/// actions after a specific date, this scheduler ignores the date and performs them
+/// immediately.
 public struct ImmediateScheduler: Scheduler {

    /// The time type used by the immediate scheduler.
@@ -41,29 +41,42 @@ public struct ImmediateScheduler: Scheduler {
                              Codable,
                              SchedulerTimeIntervalConvertible {

+            /// The type used when evaluating floating-point literals.
            public typealias FloatLiteralType = Double

+            /// The type used when evaluating integer literals.
            public typealias IntegerLiteralType = Int

+            /// The type used for expressing the stride’s magnitude.
            public typealias Magnitude = Int

+            /// The value of this time interval in seconds.
            public var magnitude: Int

+            /// Creates an immediate scheduler time interval from the given time interval.
            @inlinable
            public init(_ value: Int) {
                magnitude = value
            }

+            /// Creates an immediate scheduler time interval from an integer seconds
+            /// value.
            @inlinable
            public init(integerLiteral value: Int) {
                self.init(value)
            }

+            /// Creates an immediate scheduler time interval from a floating-point seconds
+            /// value.
            @inlinable
            public init(floatLiteral value: Double) {
                self.init(Int(value))
            }

+            /// Creates an immediate scheduler time interval from a binary integer type.
+            ///
+            /// If `exactly` can’t convert to an `Int`, the resulting time interval is
+            /// `nil`.
            @inlinable
            public init?<BinaryIntegerType: BinaryInteger>(
                exactly source: BinaryIntegerType
@@ -119,6 +132,7 @@ public struct ImmediateScheduler: Scheduler {
        }
    }

+    /// A type that defines options accepted by the immediate scheduler.
    public typealias SchedulerOptions = Never

    /// The shared instance of the immediate scheduler.
@@ -127,15 +141,21 @@ public struct ImmediateScheduler: Scheduler {
    /// the shared instance.
    public static let shared = ImmediateScheduler()

+    /// Performs the action at the next possible opportunity.
    @inlinable
    public func schedule(options: SchedulerOptions?, _ action: @escaping () -> Void) {
        action()
    }

+    /// The immediate scheduler’s definition of the current moment in time.
    public var now: SchedulerTimeType { return SchedulerTimeType() }

+    /// The minimum tolerance allowed by the immediate scheduler.
    public var minimumTolerance: SchedulerTimeType.Stride { return 0 }

+    /// Performs the action at some time after the specified date.
+    ///
+    /// The immediate scheduler ignores `date` and performs the action immediately.
    public func schedule(after date: SchedulerTimeType,
                         tolerance: SchedulerTimeType.Stride,
                         options: SchedulerOptions?,
@@ -145,6 +165,8 @@ public struct ImmediateScheduler: Scheduler {

    /// Performs the action at some time after the specified date, at the specified
    /// frequency, optionally taking into account tolerance if possible.
+    ///
+    /// The immediate scheduler ignores `date` and performs the action immediately.
    public func schedule(after date: SchedulerTimeType,
                         interval: SchedulerTimeType.Stride,
                         tolerance: SchedulerTimeType.Stride,
@@ -7,8 +7,8 @@

 /// A type of object with a publisher that emits before the object has changed.
 ///
-/// By default an `ObservableObject` will synthesize an `objectWillChange`
-/// publisher that emits before any of its `@Published` properties changes:
+/// By default an `ObservableObject` synthesizes an `objectWillChange` publisher that
+/// emits the changed value before any of its `@Published` properties changes.
 ///
 ///     class Contact : ObservableObject {
 ///         @Published var name: String
@@ -25,11 +25,13 @@
 ///     }
 ///
 ///     let john = Contact(name: "John Appleseed", age: 24)
-///     john.objectWillChange.sink { _ in print("will change") }
-///     print(john.haveBirthday)
-///     // Prints "will change"
+///     cancellable = john.objectWillChange
+///         .sink { _ in
+///             print("\(john.age) will change")
+///         }
+///     print(john.haveBirthday())
+///     // Prints "24 will change"
 ///     // Prints "25"
-///
 public protocol ObservableObject: AnyObject {

    /// The type of publisher that emits before the object has changed.
@@ -60,7 +62,7 @@ extension ObservableObject where ObjectWillChangePublisher == ObservableObjectPu
    // swiftlint:enable let_var_whitespace
 }

-/// The default publisher of an `ObservableObject`.
+/// A publisher that publishes changes from observable objects.
 public final class ObservableObjectPublisher: Publisher {

    public typealias Output = Void
@@ -74,6 +76,7 @@ public final class ObservableObjectPublisher: Publisher {
    // TODO: Combine needs this for some reason
    private var identifier: ObjectIdentifier?

+    /// Creates an observable object publisher instance.
    public init() {}

    deinit {
@@ -5,18 +5,24 @@
 //  Created by Sergej Jaskiewicz on 11.06.2019.
 //

-/// A subject that passes along values and completion.
+/// A subject that broadcasts elements to downstream subscribers.
 ///
-/// 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  {
+/// As a concrete implementation of `Subject`, the `PassthroughSubject` provides
+/// a convenient way to adapt existing imperative code to the Combine model.
+///
+/// Unlike `CurrentValueSubject`, a `PassthroughSubject` doesn’t have an initial value or
+/// a buffer of the most recently-published element.
+/// A `PassthroughSubject` drops values if there are no subscribers, or its current demand
+/// is zero.
+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 +31,197 @@ 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" }
-}
@@ -6,29 +6,83 @@
 //

 #if swift(>=5.1)
-/// Adds a `Publisher` to a property.
+
+extension Publisher where Failure == Never {
+
+    /// Republishes elements received from a publisher, by assigning them to a property
+    /// marked as a publisher.
+    ///
+    /// Use this operator when you want to receive elements from a publisher and republish
+    /// them through a property marked with the `@Published` attribute. The `assign(to:)`
+    /// operator manages the life cycle of the subscription, canceling the subscription
+    /// automatically when the `Published` instance deinitializes. Because of this,
+    /// the `assign(to:)` operator doesn't return an `AnyCancellable` that you're
+    /// responsible for like `assign(to:on:)` does.
+    ///
+    /// The example below shows a model class that receives elements from an internal
+    /// `Timer.TimerPublisher`, and assigns them to a `@Published` property called
+    /// `lastUpdated`:
+    ///
+    ///     class MyModel: ObservableObject {
+    ///             @Published var lastUpdated: Date = Date()
+    ///             init() {
+    ///                  Timer.publish(every: 1.0, on: .main, in: .common)
+    ///                      .autoconnect()
+    ///                      .assign(to: $lastUpdated)
+    ///             }
+    ///         }
+    ///
+    /// If you instead implemented `MyModel` with `assign(to: lastUpdated, on: self)`,
+    /// storing the returned `AnyCancellable` instance could cause a reference cycle,
+    /// because the `Subscribers.Assign` subscriber would hold a strong reference
+    /// to `self`. Using `assign(to:)` solves this problem.
+    ///
+    /// - Parameter published: A property marked with the `@Published` attribute, which
+    ///   receives and republishes all elements received from the upstream publisher.
+    public func assign(to published: inout Published<Output>.Publisher) {
+        subscribe(PublishedSubscriber(published.subject))
+    }
+}
+
+/// A type that publishes a property marked with an attribute.
 ///
-/// Properties annotated with `@Published` contain both the stored value
-/// and a publisher which sends any new values after the property value
-/// has been sent. New subscribers will receive the current value
-/// of the property first.
-/// Note that the `@Published` property is class-constrained.
-/// Use it with properties of classes, not with non-class types like structures.
+/// Publishing a property with the `@Published` attribute creates a publisher of this
+/// type. You access the publisher with the `$` operator, as shown here:
+///
+///     class Weather {
+///         @Published var temperature: Double
+///         init(temperature: Double) {
+///             self.temperature = temperature
+///         }
+///     }
+///
+///     let weather = Weather(temperature: 20)
+///     cancellable = weather.$temperature
+///         .sink() {
+///             print ("Temperature now: \($0)")
+///         }
+///     weather.temperature = 25
+///
+///     // Prints:
+///     // Temperature now: 20.0
+///     // Temperature now: 25.0
+///
+/// When the property changes, publishing occurs in the property's `willSet` block,
+/// meaning subscribers receive the new value before it's actually set on the property.
+/// In the above example, the second time the sink executes its closure, it receives
+/// the parameter value `25`. However, if the closure evaluated `weather.temperature`,
+/// the value returned would be `20`.
+///
+/// > Important: The `@Published` attribute is class constrained. Use it with properties
+/// of classes, not with non-class types like structures.
+///
+/// ### See Also
+///
+/// - `Publisher.assign(to:)`
@available(swift, introduced: 5.1)
@propertyWrapper
 public struct Published<Value> {

-    @inlinable // trivially forwarding
-    public init(initialValue: Value) {
-        self.init(wrappedValue: initialValue)
-    }
-
-    /// Initialize the storage of the `Published` property as well as the corresponding
-    /// `Publisher`.
-    public init(wrappedValue: Value) {
-        value = wrappedValue
-    }
-
    /// A publisher for properties marked with the `@Published` attribute.
    public struct Publisher: OpenCombine.Publisher {

@@ -36,35 +90,85 @@ public struct Published<Value> {

        public typealias Failure = Never

+        fileprivate let subject: PublishedSubject<Value>
+
        public func receive<Downstream: Subscriber>(subscriber: Downstream)
            where Downstream.Input == Value, Downstream.Failure == Never
        {
            subject.subscribe(subscriber)
        }

-        fileprivate let subject: OpenCombine.CurrentValueSubject<Value, Never>
-
        fileprivate init(_ output: Output) {
            subject = .init(output)
        }
    }

-    private var value: Value
+    private enum Storage {
+        case value(Value)
+        case publisher(Publisher)
+    }

-    private var publisher: Publisher?
+    private var storage: Storage

-    internal var objectWillChange: ObservableObjectPublisher?
+    internal var objectWillChange: ObservableObjectPublisher? {
+        get {
+            switch storage {
+            case .value:
+                return nil
+            case .publisher(let publisher):
+                return publisher.subject.objectWillChange
+            }
+        }
+        set {
+            projectedValue.subject.objectWillChange = newValue
+        }
+    }

-    /// The property that can be accessed with the `$` syntax and allows access to
-    /// the `Publisher`
+    /// Creates the published instance with an initial wrapped value.
+    ///
+    /// Don't use this initializer directly. Instead, create a property with
+    /// the `@Published` attribute, as shown here:
+    ///
+    ///     @Published var lastUpdated: Date = Date()
+    ///
+    /// - Parameter wrappedValue: The publisher's initial value.
+    public init(initialValue: Value) {
+        self.init(wrappedValue: initialValue)
+    }
+
+    /// Creates the published instance with an initial value.
+    ///
+    /// Don't use this initializer directly. Instead, create a property with
+    /// the `@Published` attribute, as shown here:
+    ///
+    ///     @Published var lastUpdated: Date = Date()
+    ///
+    /// - Parameter initialValue: The publisher's initial value.
+    public init(wrappedValue: Value) {
+        storage = .value(wrappedValue)
+    }
+
+    /// The property for which this instance exposes a publisher.
+    ///
+    /// The `projectedValue` is the property accessed with the `$` operator.
    public var projectedValue: Publisher {
        mutating get {
-            if let publisher = publisher {
+            switch storage {
+            case .value(let value):
+                let publisher = Publisher(value)
+                storage = .publisher(publisher)
+                return publisher
+            case .publisher(let publisher):
                return publisher
            }
-            let publisher = Publisher(value)
-            self.publisher = publisher
-            return publisher
+        }
+        set { // swiftlint:disable:this unused_setter_value
+            switch storage {
+            case .value(let value):
+                storage = .publisher(Publisher(value))
+            case .publisher:
+                break
+            }
        }
    }

@@ -84,12 +188,20 @@ public struct Published<Value> {
        storage storageKeyPath: ReferenceWritableKeyPath<EnclosingSelf, Published<Value>>
    ) -> Value {
        get {
-            return object[keyPath: storageKeyPath].value
+            switch object[keyPath: storageKeyPath].storage {
+            case .value(let value):
+                return value
+            case .publisher(let publisher):
+                return publisher.subject.value
+            }
        }
        set {
-            object[keyPath: storageKeyPath].objectWillChange?.send()
-            object[keyPath: storageKeyPath].publisher?.subject.send(newValue)
-            object[keyPath: storageKeyPath].value = newValue
+            switch object[keyPath: storageKeyPath].storage {
+            case .value:
+                object[keyPath: storageKeyPath].storage = .publisher(Publisher(newValue))
+            case .publisher(let publisher):
+                publisher.subject.value = newValue
+            }
        }
        // TODO: Benchmark and explore a possibility to use _modify
    }
@@ -7,18 +7,40 @@

 /// Declares that a type can transmit a sequence of values over time.
 ///
-/// There are four kinds of messages:
-///     subscription - A connection between `Publisher` and `Subscriber`.
-///     value - An element in the sequence.
-///     error - The sequence ended with an error (`.failure(e)`).
-///     complete - The sequence ended successfully (`.finished`).
+/// A publisher delivers elements to one or more `Subscriber` instances.
+/// The subscriber’s `Input` and `Failure` associated types must match the `Output` and
+/// `Failure` types declared by the publisher.
+/// The publisher implements the `receive(subscriber:)`method to accept a subscriber.
 ///
-/// Both `.failure` and `.finished` are terminal messages.
+/// After this, the publisher can call the following methods on the subscriber:
+/// - `receive(subscription:)`: Acknowledges the subscribe request and returns
+///   a `Subscription` instance. The subscriber uses the subscription to demand elements
+///   from the publisher and can use it to cancel publishing.
+/// - `receive(_:)`: Delivers one element from the publisher to the subscriber.
+/// - `receive(completion:)`: Informs the subscriber that publishing has ended,
+///   either normally or with an error.
 ///
-/// You can summarize these possibilities with a regular expression:
-///   value*(error|finished)?
+/// Every `Publisher` must adhere to this contract for downstream subscribers to function
+/// correctly.
 ///
-/// Every `Publisher` must adhere to this contract.
+/// Extensions on `Publisher` define a wide variety of _operators_ that you compose to
+/// create sophisticated event-processing chains.
+/// Each operator returns a type that implements the `Publisher` protocol
+/// Most of these types exist as extensions on the `Publishers` enumeration.
+/// For example, the `map(_:)` operator returns an instance of `Publishers.Map`.
+///
+/// # Creating Your Own Publishers
+///
+/// Rather than implementing the `Publisher` protocol yourself, you can create your own
+/// publisher by using one of several types provided by the OpenCombine framework:
+///
+/// - Use a concrete subclass of `Subject`, such as `PassthroughSubject`, to publish
+///   values on-demand by calling its `send(_:)` method.
+/// - Use a `CurrentValueSubject` to publish whenever you update the subject’s underlying
+///   value.
+/// - Add the `@Published` annotation to a property of one of your own types. In doing so,
+///   the property gains a publisher that emits an event whenever the property’s value
+///   changes. See the `Published` type for an example of this approach.
 public protocol Publisher {

    /// The kind of values published by this publisher.
@@ -29,13 +51,15 @@ public protocol Publisher {
    /// Use `Never` if this `Publisher` does not publish errors.
    associatedtype Failure: Error

-    /// This function is called to attach the specified `Subscriber` to this `Publisher`
-    /// by `subscribe(_:)`
+    /// Attaches the specified subscriber to this publisher.
    ///
-    /// - SeeAlso: `subscribe(_:)`
-    /// - Parameters:
-    ///     - subscriber: The subscriber to attach to this `Publisher`.
-    ///                   once attached it can begin to receive values.
+    /// Always call this function instead of `receive(subscriber:)`.
+    /// Adopters of `Publisher` must implement `receive(subscriber:)`. The implementation
+    /// of `subscribe(_:)` provided by `Publisher` calls through to
+    /// `receive(subscriber:)`.
+    ///
+    /// - Parameter subscriber: The subscriber to attach to this publisher. After
+    ///   attaching, the subscriber can start to receive values.
    func receive<Subscriber: OpenCombine.Subscriber>(subscriber: Subscriber)
        where Failure == Subscriber.Failure, Output == Subscriber.Input
 }
@@ -55,9 +79,27 @@ extension Publisher {
    public func subscribe<Subscriber: OpenCombine.Subscriber>(_ subscriber: Subscriber)
        where Failure == Subscriber.Failure, Output == Subscriber.Input
    {
-        receive(subscriber: subscriber)
+        if let hook = DebugHook.getGlobalHook() {
+            if var marker = subscriber as? SubscriberTapMarker {
+                let anySubscriber = marker.inner
+                    as! AnySubscriber<Subscriber.Input, Subscriber.Failure>
+                hook.willReceive(publisher: self, subscriber: anySubscriber)
+                receive(subscriber: subscriber)
+                hook.didReceive(publisher: self, subscriber: anySubscriber)
+            } else {
+                let tap = SubscriberTap(subscriber: subscriber)
+                hook.willReceive(publisher: self, subscriber: subscriber)
+                receive(subscriber: tap)
+                hook.didReceive(publisher: self, subscriber: subscriber)
+            }
+        } else {
+            receive(subscriber: subscriber)
+        }
    }

+    /// Attaches the specified subject to this publisher.
+    ///
+    /// - Parameter subject: The subject to attach to this publisher.
    public func subscribe<Subject: OpenCombine.Subject>(
        _ subject: Subject
    ) -> AnyCancellable
@@ -17,7 +17,7 @@ public struct Deferred<DeferredPublisher: Publisher>: Publisher {
    /// Use `Never` if this `Publisher` does not publish errors.
    public typealias Failure = DeferredPublisher.Failure

-    /// The closure to execute when it receives a subscription.
+    /// The closure to execute when this deferred publisher receives a subscription.
    ///
    /// The publisher returned by this closure immediately
    /// receives the incoming subscription.
@@ -24,6 +24,7 @@ public struct Empty<Output, Failure: Error>: Publisher, Equatable {
    ///
    /// Use this initializer to connect the empty publisher to subscribers or other
    /// publishers that have specific output and failure types.
+    /// 
    /// - Parameters:
    ///   - completeImmediately: A Boolean value that indicates whether the publisher
    ///     should immediately finish.
@@ -20,6 +20,7 @@ public struct Fail<Output, Failure: Error>: Publisher {
    ///
    /// Use this initializer to create a `Fail` publisher that can work with
    /// subscribers or publishers that expect a given output type.
+    /// 
    /// - Parameters:
    ///   - outputType: The output type exposed by this publisher.
    ///   - failure: The failure to send when terminating the publisher.
@@ -15,27 +15,30 @@ 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
-    ///     }
-    /// }
+    /// Use `catch()` to replace an error from an upstream publisher with a new publisher.
+    ///
+    /// In the example below, the `catch()` operator handles the `SimpleError` thrown by
+    /// the upstream publisher by replacing the error with a `Just` publisher. This
+    /// continues the stream by publishing a single value and completing normally.
+    ///
+    ///     struct SimpleError: Error {}
+    ///     let numbers = [5, 4, 3, 2, 1, 0, 9, 8, 7, 6]
+    ///     cancellable = numbers.publisher
+    ///         .tryLast(where: {
+    ///             guard $0 != 0 else { throw SimpleError() }
+    ///             return true
+    ///         })
+    ///         .catch { error in
+    ///             Just(-1)
+    ///         }
+    ///         .sink { print("\($0)") }
+    ///         // Prints: -1
    ///
-    /// 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`.
    ///
+    /// - SeeAlso: `replaceError`
    /// - 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
@@ -49,13 +52,48 @@ extension Publisher {
    }

    /// Handles errors from an upstream publisher by either replacing it with another
-    /// publisher or `throw`ing  a new error.
+    /// publisher or throwing 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.
+    /// Use `tryCatch(_:)` to decide how to handle from an upstream publisher by either
+    /// replacing the publisher with a new publisher, or throwing a new error.
+    ///
+    /// In the example below, an array publisher emits values that a `tryMap(_:)` operator
+    /// evaluates to ensure the values are greater than zero. If the values aren’t greater
+    /// than zero, the operator throws an error to the downstream subscriber to let it
+    /// know there was a problem. The subscriber, `tryCatch(_:)`, replaces the error with
+    /// a new publisher using ``Just`` to publish a final value before the stream ends
+    /// normally.
+    ///
+    ///     enum SimpleError: Error { case error }
+    ///     var numbers = [5, 4, 3, 2, 1, -1, 7, 8, 9, 10]
+    ///
+    ///     cancellable = numbers.publisher
+    ///         .tryMap { v in
+    ///             if v > 0 {
+    ///                 return v
+    ///             } else {
+    ///                 throw SimpleError.error
+    ///             }
+    ///         }
+    ///         .tryCatch { error in
+    ///             Just(0) // Send a final value before completing normally.
+    ///                     // Alternatively, throw a new error to terminate the stream.
+    ///     }
+    ///       .sink(receiveCompletion: { print ("Completion: \($0).") },
+    ///             receiveValue: { print ("Received \($0).") })
+    ///     //    Received 5.
+    ///     //    Received 4.
+    ///     //    Received 3.
+    ///     //    Received 2.
+    ///     //    Received 1.
+    ///     //    Received 0.
+    ///     //    Completion: finished.
+    ///
+    /// - Parameter handler: A throwing closure that accepts the upstream failure as
+    ///   input. This closure can either replace the upstream publisher with a new one,
+    ///   or throw a new error to the downstream subscriber.
    /// - Returns: A publisher that handles errors from an upstream publisher by replacing
-    ///   the failed publisher with another publisher.
+    ///   the failed publisher with another publisher, or an error.
    public func tryCatch<NewPublisher: Publisher>(
        _ handler: @escaping (Failure) throws -> NewPublisher
    ) -> Publishers.TryCatch<Self, NewPublisher>
@@ -105,8 +143,12 @@ extension Publishers {
        }
    }

-    /// A publisher that handles errors from an upstream publisher by replacing the failed
-    /// publisher with another publisher or optionally producing a new error.
+    /// A publisher that handles errors from an upstream publisher by replacing
+    /// the failed publisher with another publisher or producing a new error.
+    ///
+    /// Because this publisher’s handler can throw an error, `Publishers.TryCatch` defines
+    /// its `Failure` type as `Error`. This is different from `Publishers.Catch`, which
+    /// gets its failure type from the replacement publisher.
    public struct TryCatch<Upstream: Publisher, NewPublisher: Publisher>: Publisher
        where Upstream.Output == NewPublisher.Output
    {
@@ -114,10 +156,21 @@ extension Publishers {

        public typealias Failure = Error

+        /// The publisher that this publisher receives elements from.
        public let upstream: Upstream

+        /// A closure that accepts the upstream failure as input and either returns
+        /// a publisher to replace the upstream publisher or throws an error.
        public let handler: (Upstream.Failure) throws -> NewPublisher

+        /// Creates a publisher that handles errors from an upstream publisher by
+        /// replacing the failed publisher with another publisher or by throwing an error.
+        ///
+        /// - Parameters:
+        ///   - upstream: The publisher that this publisher receives elements from.
+        ///   - handler: A closure that accepts the upstream failure as input and either
+        ///     returns a publisher to replace the upstream publisher. If this closure
+        ///     throws an error, the publisher terminates with the thrown error.
        public init(upstream: Upstream,
                    handler: @escaping (Upstream.Failure) throws -> NewPublisher) {
            self.upstream = upstream
@@ -13,16 +13,100 @@

 extension Publisher {

-    /// Encodes the output from upstream using a specified `TopLevelEncoder`.
-    /// For example, use `JSONEncoder`.
+    /// Encodes the output from upstream using a specified encoder.
+    ///
+    /// Use `encode(encoder:)` with a `JSONDecoder` (or a `PropertyListDecoder` for
+    /// property lists) to encode an `Encodable` struct into `Data` that could be used to
+    /// make a JSON string (or written to disk as a binary plist in the case of property
+    /// lists).
+    ///
+    /// In this example, a `PassthroughSubject` publishes an `Article`.
+    /// The `encode(encoder:)` operator encodes the properties of the `Article` struct
+    /// into a new JSON string according to the `Codable` protocol adopted by `Article`.
+    /// The operator publishes the resulting JSON string to the downstream subscriber.
+    /// If the encoding operation fails, which can happen in the case of complex
+    /// properties that can’t be directly transformed into JSON, the stream terminates
+    /// and the error is passed to the downstream subscriber.
+    ///
+    ///     struct Article: Codable {
+    ///         let title: String
+    ///         let author: String
+    ///         let pubDate: Date
+    ///     }
+    ///
+    ///     let dataProvider = PassthroughSubject<Article, Never>()
+    ///     let cancellable = dataProvider
+    ///         .encode(encoder: JSONEncoder())
+    ///         .sink(receiveCompletion: { print ("Completion: \($0)") },
+    ///               receiveValue: {  data in
+    ///                 guard let stringRepresentation =
+    ///                     String(data: data, encoding: .utf8) else { return }
+    ///                 print("""
+    ///                       Data received \(data) string representation: \
+    ///                       \(stringRepresentation)
+    ///                       """)
+    ///         })
+    ///
+    ///     dataProvider.send(Article(title: "My First Article",
+    ///                               author: "Gita Kumar",
+    ///                               pubDate: Date()))
+    ///
+    ///     // Prints: "Data received 86 bytes string representation:
+    ///     // {"title":"My First Article","author":"Gita Kumar"
+    ///     // "pubDate":606211803.279603}"
+    ///
+    /// - Parameter encoder: An encoder that implements the `TopLevelEncoder` protocol.
+    /// - Returns: A publisher that encodes received elements using a specified encoder,
+    ///   and publishes the resulting data.
    public func encode<Coder: TopLevelEncoder>(
        encoder: Coder
    ) -> Publishers.Encode<Self, Coder> {
        return .init(upstream: self, encoder: encoder)
    }

-    /// Decodes the output from upstream using a specified `TopLevelDecoder`.
-    /// For example, use `JSONDecoder`.
+    /// Decodes the output from the upstream using a specified decoder.
+    ///
+    /// Use `decode(type:decoder:)` with a `JSONDecoder` (or a `PropertyListDecoder` for
+    /// property lists) to decode data received from a `URLSession.DataTaskPublisher` or
+    /// other data source using the `Decodable` protocol.
+    ///
+    /// In this example, a `PassthroughSubject` publishes a JSON string. The JSON decoder
+    /// parses the string, converting its fields according to the `Decodable` protocol
+    /// implemented by `Article`, and successfully populating a new `Article`.
+    /// The `Publishers.Decode` publisher then publishes the `Article` to the downstream.
+    /// If a decoding operation fails, which happens in the case of missing or malformed
+    /// data in the source JSON string, the stream terminates and passes the error to
+    /// the downstream subscriber.
+    ///
+    ///     struct Article: Codable {
+    ///         let title: String
+    ///         let author: String
+    ///         let pubDate: Date
+    ///     }
+    ///
+    ///     let dataProvider = PassthroughSubject<Data, Never>()
+    ///     cancellable = dataProvider
+    ///         .decode(type: Article.self, decoder: JSONDecoder())
+    ///         .sink(receiveCompletion: { print ("Completion: \($0)")},
+    ///               receiveValue: { print ("value: \($0)") })
+    ///
+    ///     dataProvider.send(Data("""
+    ///                            {\"pubDate\":1574273638.575666, \
+    ///                            \"title\" : \"My First Article\", \
+    ///                            \"author\" : \"Gita Kumar\" }
+    ///                            """.utf8))
+    ///
+    ///     // Prints:
+    ///     // ".sink() data received Article(title: "My First Article",
+    ///     //                                author: "Gita Kumar",
+    ///     //                                pubDate: 2050-11-20 18:13:58 +0000)"
+    ///
+    /// - Parameters:
+    ///   - type: The encoded data to decode into a struct that conforms to
+    ///     the `Decodable` protocol.
+    ///   - decoder:  A decoder that implements the `TopLevelDecoder` protocol.
+    /// - Returns: A publisher that decodes a given type using a specified decoder and
+    ///   publishes the result.
    public func decode<Item: Decodable, Coder: TopLevelDecoder>(
        type: Item.Type,
        decoder: Coder
@@ -88,9 +172,6 @@ extension Publishers.Encode {
          CustomPlaygroundDisplayConvertible
        where Downstream.Input == Output, Downstream.Failure == Error
    {
-        // NOTE: This class has been audited for thread safety.
-        // Combine doesn't use any locking here.
-
        typealias Input = Upstream.Output

        typealias Failure = Upstream.Failure
@@ -99,6 +180,8 @@ extension Publishers.Encode {

        private let encode: (Upstream.Output) throws -> Output

+        private let lock = UnfairLock.allocate()
+
        private var finished = false

        private var subscription: Subscription?
@@ -111,44 +194,72 @@ extension Publishers.Encode {
            self.encode = encode
        }

+        deinit {
+            lock.deallocate()
+        }
+
        func receive(subscription: Subscription) {
+            lock.lock()
            if finished || self.subscription != nil {
+                lock.unlock()
                subscription.cancel()
                return
            }
            self.subscription = subscription
+            lock.unlock()
            downstream.receive(subscription: self)
        }

        func receive(_ input: Input) -> Subscribers.Demand {
-            if finished { return .none }
+            lock.lock()
+            if finished {
+                lock.unlock()
+                return .none
+            }
+            lock.unlock()
            do {
                return try downstream.receive(encode(input))
            } catch {
+                lock.lock()
                finished = true
+                let subscription = self.subscription
+                self.subscription = nil
+                lock.unlock()
                subscription?.cancel()
-                subscription = nil
                downstream.receive(completion: .failure(error))
                return .none
            }
        }

        func receive(completion: Subscribers.Completion<Failure>) {
-            if finished { return }
+            lock.lock()
+            if finished {
+                lock.unlock()
+                return
+            }
            finished = true
            subscription = nil
+            lock.unlock()
            downstream.receive(completion: completion.eraseError())
        }

        func request(_ demand: Subscribers.Demand) {
+            lock.lock()
+            let subscription = self.subscription
+            lock.unlock()
            subscription?.request(demand)
        }

        func cancel() {
-            guard let subscription = self.subscription, !finished else { return }
-            subscription.cancel()
+            lock.lock()
+            guard !finished, let subscription = self.subscription else {
+                lock.unlock()
+                return
+            }
            self.subscription = nil
            finished = true
+            lock.unlock()
+            subscription.cancel()
        }

        var description: String { return "Encode" }
@@ -175,9 +286,6 @@ extension Publishers.Decode {
          CustomPlaygroundDisplayConvertible
        where Downstream.Input == Output, Downstream.Failure == Error
    {
-        // NOTE: This class has been audited for thread safety.
-        // Combine doesn't use any locking here.
-
        typealias Input = Upstream.Output

        typealias Failure = Upstream.Failure
@@ -186,6 +294,8 @@ extension Publishers.Decode {

        private let decode: (Upstream.Output) throws -> Output

+        private let lock = UnfairLock.allocate()
+
        private var finished = false

        private var subscription: Subscription?
@@ -198,44 +308,72 @@ extension Publishers.Decode {
            self.decode = decode
        }

+        deinit {
+            lock.deallocate()
+        }
+
        func receive(subscription: Subscription) {
+            lock.lock()
            if finished || self.subscription != nil {
+                lock.unlock()
                subscription.cancel()
                return
            }
            self.subscription = subscription
+            lock.unlock()
            downstream.receive(subscription: self)
        }

        func receive(_ input: Input) -> Subscribers.Demand {
-            if finished { return .none }
+            lock.lock()
+            if finished {
+                lock.unlock()
+                return .none
+            }
+            lock.unlock()
            do {
                return try downstream.receive(decode(input))
            } catch {
+                lock.lock()
                finished = true
+                let subscription = self.subscription
+                self.subscription = nil
+                lock.unlock()
                subscription?.cancel()
-                subscription = nil
                downstream.receive(completion: .failure(error))
                return .none
            }
        }

        func receive(completion: Subscribers.Completion<Failure>) {
-            if finished { return }
+            lock.lock()
+            if finished {
+                lock.unlock()
+                return
+            }
            finished = true
            subscription = nil
+            lock.unlock()
            downstream.receive(completion: completion.eraseError())
        }

        func request(_ demand: Subscribers.Demand) {
+            lock.lock()
+            let subscription = self.subscription
+            lock.unlock()
            subscription?.request(demand)
        }

        func cancel() {
-            guard let subscription = self.subscription, !finished else { return }
-            subscription.cancel()
+            lock.lock()
+            guard !finished, let subscription = self.subscription else {
+                lock.unlock()
+                return
+            }
            self.subscription = nil
            finished = true
+            lock.unlock()
+            subscription.cancel()
        }

        var description: String { return "Decode" }
@@ -12,11 +12,29 @@
 //

 extension Publisher {
-
-    /// Returns a publisher that publishes the values of a keyt path as a tuple.
+    /// Publishes the value of the key path.
+    ///
+    /// In the following example, the `map(_:)` operator uses the Swift
+    /// key path syntax to access the `die` member
+    /// of the `DiceRoll` structure published by the `Just` publisher.
+    ///
+    /// The downstream sink subscriber receives only
+    /// the value of this `Int`,
+    /// not the entire `DiceRoll`.
+    ///
+    ///     struct DiceRoll {
+    ///         let die: Int
+    ///     }
+    ///
+    ///     cancellable = Just(DiceRoll(die: Int.random(in: 1...6)))
+    ///         .map(\.die)
+    ///         .sink {
+    ///             print ("Rolled: \($0)")
+    ///         }
+    ///     // Prints "Rolled: 6 (or some other random value).
    ///
    /// - Parameters:
-    ///    - keyPath: The key path of a property on `Output`
+    ///    - keyPath: The key path of a property on `Output`.
    /// - Returns: A publisher that publishes the value of the key path.
    public func map<Result>(
        _ keyPath: KeyPath<Output, Result>
@@ -26,12 +44,35 @@ extension Publisher {
            keyPath: keyPath
        )
    }
-
-    /// Returns a publisher that publishes the values of two key paths as a tuple.
+    /// Publishes the values of two key paths as a tuple.
+    ///
+    /// In the following example, the `map(_:_:)` operator uses the Swift
+    /// key path syntax to access the `die1` and `die2` members
+    /// of the `DiceRoll` structure published by the `Just` publisher.
+    ///
+    /// The downstream sink subscriber receives only
+    /// these two values (as an `(Int, Int)` tuple),
+    /// not the entire `DiceRoll`.
+    ///
+    ///     struct DiceRoll {
+    ///         let die1: Int
+    ///         let die2: Int
+    ///     }
+    ///
+    ///     cancellable = Just(DiceRoll(die1: Int.random(in: 1...6),
+    ///                                 die2: Int.random(in: 1...6)))
+    ///         .map(\.die1, \.die2)
+    ///         .sink { values in
+    ///             print("""
+    ///             Rolled: \(values.0), \(values.1) \
+    ///             (total \(values.0 + values.1))
+    ///             """)
+    ///         }
+    ///     // Prints "Rolled: 5, 3 (total: 8)" (or other random values).
    ///
    /// - Parameters:
-    ///    - keyPath0: The key path of a property on `Output`
-    ///    - keyPath1: The key path of another property on `Output`
+    ///    - keyPath0: The key path of a property on `Output`.
+    ///    - keyPath1: The key path of another property on `Output`.
    /// - Returns: A publisher that publishes the values of two key paths as a tuple.
    public func map<Result0, Result1>(
        _ keyPath0: KeyPath<Output, Result0>,
@@ -43,13 +84,38 @@ extension Publisher {
            keyPath1: keyPath1
        )
    }
-
-    /// Returns a publisher that publishes the values of three key paths as a tuple.
+    /// Publishes the values of three key paths as a tuple.
+    ///
+    /// In the following example, the `map(_:_:_:)` operator uses the Swift
+    /// key path syntax to access the `die1`, `die2`, and `die3` members
+    /// of the `DiceRoll` structure published by the `Just` publisher.
+    ///
+    /// The downstream sink subscriber receives only
+    /// these three values (as an `(Int, Int, Int)` tuple),
+    /// not the entire `DiceRoll`.
+    ///
+    ///     struct DiceRoll {
+    ///         let die1: Int
+    ///         let die2: Int
+    ///         let die3: Int
+    ///     }
+    ///
+    ///     cancellable = Just(DiceRoll(die1: Int.random(in: 1...6),
+    ///                                 die2: Int.random(in: 1...6),
+    ///                                 die3: Int.random(in: 1...6)))
+    ///         .map(\.die1, \.die2, \.die3)
+    ///         .sink { values in
+    ///             print("""
+    ///             Rolled: \(values.0), \(values.1), \(values.2) \
+    ///             (total \(values.0 + values.1 + values.2))
+    ///             """)
+    ///         }
+    ///     // Prints "Rolled: 2, 4, 3 (total: 9)" (or other random values).
    ///
    /// - Parameters:
-    ///    - keyPath0: The key path of a property on `Output`
-    ///    - keyPath1: The key path of another property on `Output`
-    ///    - keyPath2: The key path of a third property on `Output`
+    ///    - keyPath0: The key path of a property on `Output`.
+    ///    - keyPath1: The key path of a second property on `Output`.
+    ///    - keyPath2: The key path of a third property on `Output`.
    /// - Returns: A publisher that publishes the values of three key paths as a tuple.
    public func map<Result0, Result1, Result2>(
        _ keyPath0: KeyPath<Output, Result0>,
@@ -7,10 +7,11 @@

 /// A publisher that emits an output to each subscriber just once, and then finishes.
 ///
-/// You can use a `Just` publisher to start a chain of publishers. A `Just` publisher
-/// is also useful when replacing a value with `Catch`.
+/// You can use a `Just` publisher to start a chain of publishers. A `Just` publisher is
+/// also useful when replacing a value with `Publishers.Catch`.
 ///
-/// In contrast with `Publishers.Once`, a `Just` publisher cannot fail with an error.
+/// In contrast with `Result.Publisher`, a `Just` publisher can’t fail with an error.
+/// And unlike `Optional.Publisher`, a `Just` publisher always produces a value.
 public struct Just<Output>: Publisher {

    public typealias Failure = Never
@@ -28,28 +28,45 @@ extension Optional {
            self.optional = optional
        }

-        /// A publisher that publishes an optional value to each subscriber
-        /// exactly once, if the optional has a value.
+        public var publisher: Publisher {
+            return Publisher(optional)
+        }
+
+        /// The type of a Combine publisher that publishes the value of a Swift optional
+        /// instance to each subscriber exactly once, if the instance has any value at
+        /// all.
        ///
-        /// In contrast with `Just`, an `Optional` publisher may send
-        /// no value before completion.
+        /// In contrast with the `Just` publisher, which always produces a single value,
+        /// this publisher might not send any values and instead finish normally,
+        /// if `output` is `nil`.
        public struct Publisher: OpenCombine.Publisher {

+            /// The kind of value published by this publisher.
+            ///
+            /// This publisher produces the type wrapped by the optional.
            public typealias Output = Wrapped

+            /// The kind of error this publisher might publish.
+            ///
+            /// The optional publisher never produces errors.
            public typealias Failure = Never

-            /// The result to deliver to each subscriber.
+            /// The output to deliver to each subscriber.
            public let output: Wrapped?

-            /// Creates a publisher to emit the optional value of a successful result,
-            /// or fail with an error.
+            /// Creates a publisher to emit the value of the optional, or to finish
+            /// immediately if the optional doesn't have a value.
            ///
-            /// - Parameter result: The result to deliver to each subscriber.
+            /// - Parameter output: The result to deliver to each subscriber.
            public init(_ output: Output?) {
                self.output = output
            }

+            /// Implements the Publisher protocol by accepting the subscriber and
+            /// immediately publishing the optional’s value if it has one, or finishing
+            /// normally if it doesn’t.
+            ///
+            /// - Parameter subscriber: The subscriber to add.
            public func receive<Downstream: Subscriber>(subscriber: Downstream)
                where Output == Downstream.Input, Failure == Downstream.Failure
            {
@@ -64,13 +81,23 @@ extension Optional {
        }
    }

+    public var ocombine: OCombine {
+        return .init(self)
+    }
+
 #if !canImport(Combine)
-    /// A publisher that publishes an optional value to each subscriber
-    /// exactly once, if the optional has a value.
+    /// The type of a Combine publisher that publishes the value of a Swift optional
+    /// instance to each subscriber exactly once, if the instance has any value at
+    /// all.
    ///
-    /// In contrast with `Just`, an `Optional` publisher may send
-    /// no value before completion.
+    /// In contrast with the `Just` publisher, which always produces a single value,
+    /// this publisher might not send any values and instead finish normally,
+    /// if `output` is `nil`.
    public typealias Publisher = OCombine.Publisher
+
+    public var publisher: Publisher {
+        return Publisher(self)
+    }
 #endif
 }

@@ -235,7 +262,7 @@ extension Optional.OCombine.Publisher {
        in range: RangeExpression
    ) -> Optional<Output>.OCombine.Publisher where RangeExpression.Bound == Int {
        let range = range.relative(to: 0 ..< Int.max)
-        precondition(range.lowerBound >= 0, "lowerBould must not be negative")
+        precondition(range.lowerBound >= 0, "lowerBound must not be negative")

        // I don't know why, but Combine has this precondition
        precondition(range.upperBound < .max - 1)
@@ -10,13 +10,28 @@ extension Publisher {
    /// Publishes a single Boolean value that indicates whether all received elements pass
    /// a given predicate.
    ///
-    /// When this publisher receives an element, it runs the predicate against
-    /// the element. If the predicate returns `false`, the publisher produces a `false`
-    /// value and finishes. If the upstream publisher finishes normally, this publisher
-    /// produces a `true` value and finishes.
-    /// As a `reduce`-style operator, this publisher produces at most one value.
-    /// Backpressure note: Upon receiving any request greater than zero, this publisher
-    /// requests unlimited elements from the upstream publisher.
+    /// Use the `allSatisfy(_:)` operator to determine if all elements in a stream satisfy
+    /// a criteria you provide. When this publisher receives an element, it runs
+    /// the predicate against the element. If the predicate returns `false`, the publisher
+    /// produces a `false` value and finishes. If the upstream publisher finishes
+    /// normally, this publisher produces a `true` value and finishes.
+    ///
+    /// In the example below, the `allSatisfy(_:)` operator tests if each an integer array
+    /// publisher’s elements fall into the `targetRange`:
+    ///
+    ///     let targetRange = (-1...100)
+    ///     let numbers = [-1, 0, 10, 5]
+    ///     numbers.publisher
+    ///         .allSatisfy { targetRange.contains($0) }
+    ///         .sink { print("\($0)") }
+    ///
+    ///     // Prints: "true"
+    ///
+    /// With operators similar to `reduce(_:_:)`, this publisher produces at most one
+    /// value.
+    ///
+    ///  > Note: Upon receiving any request greater than zero, this publisher requests
+    ///  unlimited elements from the upstream publisher.
    ///
    /// - Parameter predicate: A closure that evaluates each received element.
    ///   Return `true` to continue, or `false` to cancel the upstream and complete.
@@ -31,21 +46,45 @@ extension Publisher {
    /// Publishes a single Boolean value that indicates whether all received elements pass
    /// a given error-throwing predicate.
    ///
-    /// When this publisher receives an element, it runs the predicate against
-    /// the element. If the predicate returns `false`, the publisher produces a `false`
-    /// value and finishes. If the upstream publisher finishes normally, this publisher
-    /// produces a `true` value and finishes. If the predicate throws an error,
-    /// the publisher fails, passing the error to its downstream.
-    /// As a `reduce`-style operator, this publisher produces at most one value.
-    /// Backpressure note: Upon receiving any request greater than zero, this publisher
-    /// requests unlimited elements from the upstream publisher.
+    /// Use the `tryAllSatisfy(_:)` operator to determine if all elements in a stream
+    /// satisfy a criteria in an error-throwing predicate you provide. When this publisher
+    /// receives an element, it runs the predicate against the element. If the predicate
+    /// returns `false`, the publisher produces a `false` value and finishes.
+    /// If the upstream publisher finishes normally, this publisher produces a `true`
+    /// value and finishes. If the predicate throws an error, the publisher fails and
+    /// passes the error to its downstream subscriber.
    ///
-    /// - Parameter predicate:  A closure that evaluates each received element.
-    ///   Return `true` to continue, or `false` to cancel the upstream and complete.
-    ///   The closure may throw, in which case the publisher cancels the upstream
-    ///   publisher and fails with the thrown error.
-    /// - Returns: A publisher that publishes a Boolean value that indicates whether
-    ///   all received elements pass a given predicate.
+    /// In the example below, an error-throwing predicate tests if each of an integer
+    /// array publisher’s elements fall into the `targetRange`; the predicate throws
+    /// an error if an element is zero and terminates the stream.
+    ///
+    ///     let targetRange = (-1...100)
+    ///     let numbers = [-1, 10, 5, 0]
+    ///
+    ///     numbers.publisher
+    ///         .tryAllSatisfy { anInt in
+    ///             guard anInt != 0 else { throw RangeError() }
+    ///             return targetRange.contains(anInt)
+    ///         }
+    ///         .sink(
+    ///             receiveCompletion: { print ("completion: \($0)") },
+    ///             receiveValue: { print ("value: \($0)") }
+    ///         )
+    ///
+    ///     // Prints: "completion: failure(RangeError())"
+    ///
+    /// With operators similar to `reduce(_:_:)`, this publisher produces at most one
+    /// value.
+    ///
+    ///  > Note: Upon receiving any request greater than zero, this publisher requests
+    ///  unlimited elements from the upstream publisher.
+    ///
+    /// - Parameter predicate: A closure that evaluates each received element. Return
+    ///   `true` to continue, or `false` to cancel the upstream and complete. The closure
+    ///   may throw an error, in which case the publisher cancels the upstream publisher
+    ///   and fails with the thrown error.
+    /// - Returns: A publisher that publishes a Boolean value that indicates whether all
+    ///   received elements pass a given predicate.
    public func tryAllSatisfy(
        _ predicate: @escaping (Output) throws -> Bool
    ) -> Publishers.TryAllSatisfy<Self> {
@@ -10,8 +10,34 @@ extension Publisher {
    /// Raises a fatal error when its upstream publisher fails, and otherwise republishes
    /// all received input.
    ///
-    /// Use this function for internal sanity checks that are active during testing but
-    /// do not impact performance of shipping code.
+    /// Use `assertNoFailure()` for internal sanity checks that are active during testing.
+    /// However, it is important to note that, like its Swift counterpart
+    /// `fatalError(_:)`, the `assertNoFailure()` operator asserts a fatal exception when
+    /// triggered in both development/testing _and_ shipping versions of code.
+    ///
+    /// In the example below, a `CurrentValueSubject` publishes the initial and second
+    /// values successfully. The third value, containing a `genericSubjectError`, causes
+    /// the `assertNoFailure()` operator to assert a fatal exception stopping the process:
+    ///
+    ///     public enum SubjectError: Error {
+    ///         case genericSubjectError
+    ///     }
+    ///
+    ///     let subject = CurrentValueSubject<String, Error>("initial value")
+    ///     subject
+    ///         .assertNoFailure()
+    ///         .sink(receiveCompletion: { print ("completion: \($0)") },
+    ///               receiveValue: { print ("value: \($0).") }
+    ///         )
+    ///
+    ///     subject.send("second value")
+    ///     subject.send(completion: .failure(SubjectError.genericSubjectError))
+    ///
+    ///     // Prints:
+    ///     //  value: initial value.
+    ///     //  value: second value.
+    ///     // The process then terminates in the debugger as the assertNoFailure
+    ///     // operator catches the genericSubjectError.
    ///
    /// - Parameters:
    ///   - prefix: A string used at the beginning of the fatal error message.
@@ -11,13 +11,19 @@ extension ConnectablePublisher {
    /// publisher.
    ///
    /// Use `autoconnect()` to simplify working with `ConnectablePublisher` instances,
-    /// such as those created with `makeConnectable()`.
+    /// such as `TimerPublisher` in `OpenCombineFoundation`.
    ///
-    ///     let autoconnectedPublisher = somePublisher
-    ///         .makeConnectable()
+    /// In the following example, the `Timer.publish()` operator creates
+    /// a `TimerPublisher`, which is a `ConnectablePublisher`. As a result, subscribers
+    /// don’t receive any values until after a call to `connect()`.
+    /// For convenience when working with a single subscriber, the `.autoconnect()`
+    /// operator performs the `connect()` call when attached to by the subscriber.
+    ///
+    ///     cancellable = Timer.publish(every: 1, on: .main, in: .default)
    ///         .autoconnect()
-    ///         .subscribe(someSubscriber)
-    ///
+    ///         .sink { date in
+    ///             print ("Date now: \(date)")
+    ///         }
    /// - Returns: A publisher which automatically connects to its upstream connectable
    ///   publisher.
    public func autoconnect() -> Publishers.Autoconnect<Self> {
@@ -27,12 +33,12 @@ extension ConnectablePublisher {

 extension Publishers {

-    /// A publisher that automatically connects and disconnects from this connectable
-    /// publisher.
+    /// A publisher that automatically connects to an upstream connectable publisher.
+    ///
+    /// This publisher calls `connect()` on the upstream `ConnectablePublisher` when first
+    /// attached to by a subscriber.
    public class Autoconnect<Upstream: ConnectablePublisher>: Publisher {

-        // NOTE: This class has been audited for thread safety
-
        public typealias Output = Upstream.Output

        public typealias Failure = Upstream.Failure
@@ -14,9 +14,29 @@ extension Publisher {
    /// Raises a debugger signal when a provided closure needs to stop the process in
    /// the debugger.
    ///
-    /// When any of the provided closures returns `true`, this publisher raises
-    /// the `SIGTRAP` signal to stop the process in the debugger.
-    /// Otherwise, this publisher passes through values and completions as-is.
+    /// Use `breakpoint(receiveSubscription:receiveOutput:receiveCompletion:)` to examine
+    /// one or more stages of the subscribe/publish/completion process and stop in
+    /// the debugger, based on conditions you specify. When any of the provided closures
+    /// returns `true`, this operator raises the `SIGTRAP` signal to stop the process
+    /// in the debugger. Otherwise, this publisher passes through values and completions
+    /// as-is.
+    ///
+    /// In the example below, a `PassthroughSubject` publishes strings to a breakpoint
+    /// republisher. When the breakpoint receives the string “`DEBUGGER`”, it returns
+    /// `true`, which stops the app in the debugger.
+    ///
+    ///     let publisher = PassthroughSubject<String?, Never>()
+    ///     cancellable = publisher
+    ///         .breakpoint(
+    ///             receiveOutput: { value in return value == "DEBUGGER" }
+    ///         )
+    ///         .sink { print("\(String(describing: $0))" , terminator: " ") }
+    ///
+    ///     publisher.send("DEBUGGER")
+    ///
+    ///     // Prints: "error: Execution was interrupted, reason: signal SIGTRAP."
+    ///     // Depending on your specific environment, the console messages may
+    ///     // also include stack trace information, which is not shown here.
    ///
    /// - Parameters:
    ///   - receiveSubscription: A closure that executes when when the publisher receives
@@ -44,19 +64,46 @@ extension Publisher {
    /// Raises a debugger signal upon receiving a failure.
    ///
    /// When the upstream publisher fails with an error, this publisher raises
-    /// the `SIGTRAP` signal, which stops the process in the debugger.
-    /// Otherwise, this publisher passes through values and completions as-is.
+    /// the `SIGTRAP` signal, which stops the process in the debugger. Otherwise, this
+    /// publisher passes through values and completions as-is.
+    ///
+    /// In this example a `PassthroughSubject` publishes strings, but its downstream
+    /// `Publisher/tryMap(_:)` operator throws an error. This sends the error downstream
+    /// as a `Subscribers.Completion.failure(_:)`. The `breakpointOnError()`
+    /// operator receives this completion and stops the app in the debugger.
+    ///
+    ///      struct CustomError : Error {}
+    ///      let publisher = PassthroughSubject<String?, Error>()
+    ///      cancellable = publisher
+    ///          .tryMap { stringValue in
+    ///              throw CustomError()
+    ///          }
+    ///          .breakpointOnError()
+    ///          .sink(
+    ///              receiveCompletion: { completion in
+    ///                  print("Completion: \(String(describing: completion))")
+    ///              },
+    ///              receiveValue: { aValue in
+    ///                  print("Result: \(String(describing: aValue))")
+    ///              }
+    ///          )
+    ///
+    ///      publisher.send("TEST DATA")
+    ///
+    ///      // Prints: "error: Execution was interrupted, reason: signal SIGTRAP."
+    ///      // Depending on your specific environment, the console messages may
+    ///      // also include stack trace information, which is not shown here.
    ///
    /// - Returns: A publisher that raises a debugger signal upon receiving a failure.
    public func breakpointOnError() -> Publishers.Breakpoint<Self> {
-        return breakpoint { completion in
+        return breakpoint(receiveCompletion: { completion in
            switch completion {
            case .finished:
                return false
            case .failure:
                return true
            }
-        }
+        })
    }
 }

@@ -172,10 +219,7 @@ extension Publishers.Breakpoint {
        var description: String { return "Breakpoint" }

        var customMirror: Mirror {
-            let children = CollectionOfOne<Mirror.Child>(
-                ("upstream", breakpoint.upstream)
-            )
-            return Mirror(self, children: children)
+            return Mirror(self, children: EmptyCollection())
        }

        var playgroundDescription: Any { return description }
@@ -8,9 +8,20 @@
 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.
+    ///
+    /// Use `buffer(size:prefetch:whenFull:)` to collect a specific number of elements
+    /// from an upstream publisher before republishing them to the downstream subscriber
+    /// according to the `Publishers.BufferingStrategy` and `Publishers.PrefetchStrategy`
+    /// strategy you specify.
+    ///
+    /// If the publisher completes before reaching the `size` threshold, it buffers
+    /// the elements and publishes them downstream prior to completion.
+    ///
+    /// - Parameters:
+    ///   - size: The maximum number of elements to store.
+    ///   - prefetch: The strategy to initially populate the buffer.
+    ///   - whenFull: The action to take when the buffer becomes full.
+    /// - Returns: A publisher that buffers elements received from an upstream publisher.
    public func buffer(
        size: Int,
        prefetch: Publishers.PrefetchStrategy,
@@ -26,37 +37,33 @@ extension Publisher {
 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.
+        ///
+        /// This strategy starts by making a demand equal to the buffer’s size from
+        /// the upstream when the subscriber first connects. Afterwards, it continues
+        /// to demand elements from the upstream to try to keep the buffer full.
        case keepFull

-        /// A strategy that avoids prefetching and instead performs requests
-        /// on demand.
+        /// A strategy that avoids prefetching and instead performs requests on demand.
+        ///
+        /// This strategy just forwards the downstream’s requests to the upstream
+        /// publisher.
        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.
+    /// A strategy that handles exhaustion of a buffer’s capacity.
    public enum BufferingStrategy<Failure: Error> {

-        /// When full, discard the newly-received element without buffering it.
+        /// When the buffer is full, discard the newly received element.
        case dropNewest

-        /// When full, remove the least recently-received element from the buffer.
+        /// When the buffer is full, discard the oldest element in the buffer.
        case dropOldest

-        /// When full, execute the closure to provide a custom error.
+        /// When the buffer is full, execute the closure to provide a custom error.
        case customError(() -> Failure)
    }

@@ -98,7 +105,11 @@ extension Publishers {
        public func receive<Downstream: Subscriber>(subscriber: Downstream)
            where Downstream.Input == Output, Downstream.Failure == Failure
        {
-            upstream.subscribe(Inner(downstream: subscriber, buffer: self))
+            let inner = Inner(downstream: subscriber,
+                              size: size,
+                              prefetch: prefetch,
+                              whenFull: whenFull)
+            upstream.subscribe(inner)
        }
    }
 }
@@ -120,17 +131,19 @@ extension Publishers.Buffer {

        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 let size: Int
+
+        private let prefetch: Publishers.PrefetchStrategy // keepFull is 0x0
+
+        private let whenFull: Publishers.BufferingStrategy<Failure>
+
+        private let downstream: Downstream
+
+        private var state = SubscriptionStatus.awaitingSubscription

        private var downstreamDemand = Subscribers.Demand.none

@@ -142,8 +155,14 @@ extension Publishers.Buffer {

        private var terminal: Subscribers.Completion<Failure>?

-        init(downstream: Downstream, buffer: Publishers.Buffer<Upstream>) {
-            state = .ready(buffer, downstream)
+        init(downstream: Downstream,
+             size: Int,
+             prefetch: Publishers.PrefetchStrategy,
+             whenFull: Publishers.BufferingStrategy<Failure>) {
+            self.size = size
+            self.prefetch = prefetch
+            self.whenFull = whenFull
+            self.downstream = downstream
        }

        deinit {
@@ -152,18 +171,18 @@ extension Publishers.Buffer {

        func receive(subscription: Subscription) {
            lock.lock()
-            guard case let .ready(buffer, downstream) = state else {
+            guard case .awaitingSubscription = state else {
                lock.unlock()
                subscription.cancel()
                return
            }
-            state = .subscribed(buffer, downstream, subscription)
+            state = .subscribed(subscription)
            lock.unlock()

            let upstreamDemand: Subscribers.Demand
-            switch buffer.prefetch {
+            switch prefetch {
            case .keepFull:
-                upstreamDemand = .max(buffer.size)
+                upstreamDemand = .max(size)
            case .byRequest:
                upstreamDemand = .unlimited
            }
@@ -173,14 +192,14 @@ extension Publishers.Buffer {

        func receive(_ input: Input) -> Subscribers.Demand {
            lock.lock()
-            guard case let .subscribed(buffer, _, subscription) = state else {
+            guard case let .subscribed(subscription) = state else {
                lock.unlock()
                return .none
            }
            switch terminal {
            case nil, .finished?:
-                if values.count >= buffer.size {
-                    switch buffer.whenFull {
+                if values.count >= size {
+                    switch whenFull {
                    case .dropNewest:
                        lock.unlock()
                        return drain()
@@ -216,7 +235,7 @@ extension Publishers.Buffer {

        func request(_ demand: Subscribers.Demand) {
            lock.lock()
-            guard case let .subscribed(_, _, subscription) = state else {
+            guard case let .subscribed(subscription) = state else {
                lock.unlock()
                return
            }
@@ -227,13 +246,16 @@ extension Publishers.Buffer {
                return
            }

-            // Request the number of items just enough to fill the buffer.
-            subscription.request(drain() + demand)
+            let more = drain()
+            if more != .none {
+                // Request the number of items just enough to fill the buffer.
+                subscription.request(more)
+            }
        }

        func cancel() {
            lock.lock()
-            guard case let .subscribed(_, _, subscription) = state else {
+            guard case let .subscribed(subscription) = state else {
                lock.unlock()
                return
            }
@@ -247,7 +269,7 @@ extension Publishers.Buffer {
            var upstreamDemand = Subscribers.Demand.none
            lock.lock()
            while true {
-                guard case let .subscribed(buffer, downstream, _) = state else {
+                guard case .subscribed = state else {
                    lock.unlock()
                    return upstreamDemand
                }
@@ -296,7 +318,7 @@ extension Publishers.Buffer {
                    additionalUpstreamDemand += 1
                }

-                if buffer.prefetch == .keepFull {
+                if prefetch == .keepFull {
                    upstreamDemand += additionalUpstreamDemand
                }

@@ -13,27 +13,30 @@ 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
-    ///     }
-    /// }
+    /// Use `catch()` to replace an error from an upstream publisher with a new publisher.
+    ///
+    /// In the example below, the `catch()` operator handles the `SimpleError` thrown by
+    /// the upstream publisher by replacing the error with a `Just` publisher. This
+    /// continues the stream by publishing a single value and completing normally.
+    ///
+    ///     struct SimpleError: Error {}
+    ///     let numbers = [5, 4, 3, 2, 1, 0, 9, 8, 7, 6]
+    ///     cancellable = numbers.publisher
+    ///         .tryLast(where: {
+    ///             guard $0 != 0 else { throw SimpleError() }
+    ///             return true
+    ///         })
+    ///         .catch { error in
+    ///             Just(-1)
+    ///         }
+    ///         .sink { print("\($0)") }
+    ///         // Prints: -1
    ///
-    /// 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`.
    ///
+    /// - SeeAlso: `replaceError`
    /// - 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
@@ -47,13 +50,48 @@ extension Publisher {
    }

    /// Handles errors from an upstream publisher by either replacing it with another
-    /// publisher or `throw`ing  a new error.
+    /// publisher or throwing 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.
+    /// Use `tryCatch(_:)` to decide how to handle from an upstream publisher by either
+    /// replacing the publisher with a new publisher, or throwing a new error.
+    ///
+    /// In the example below, an array publisher emits values that a `tryMap(_:)` operator
+    /// evaluates to ensure the values are greater than zero. If the values aren’t greater
+    /// than zero, the operator throws an error to the downstream subscriber to let it
+    /// know there was a problem. The subscriber, `tryCatch(_:)`, replaces the error with
+    /// a new publisher using ``Just`` to publish a final value before the stream ends
+    /// normally.
+    ///
+    ///     enum SimpleError: Error { case error }
+    ///     var numbers = [5, 4, 3, 2, 1, -1, 7, 8, 9, 10]
+    ///
+    ///     cancellable = numbers.publisher
+    ///         .tryMap { v in
+    ///             if v > 0 {
+    ///                 return v
+    ///             } else {
+    ///                 throw SimpleError.error
+    ///             }
+    ///         }
+    ///         .tryCatch { error in
+    ///             Just(0) // Send a final value before completing normally.
+    ///                     // Alternatively, throw a new error to terminate the stream.
+    ///     }
+    ///       .sink(receiveCompletion: { print ("Completion: \($0).") },
+    ///             receiveValue: { print ("Received \($0).") })
+    ///     //    Received 5.
+    ///     //    Received 4.
+    ///     //    Received 3.
+    ///     //    Received 2.
+    ///     //    Received 1.
+    ///     //    Received 0.
+    ///     //    Completion: finished.
+    ///
+    /// - Parameter handler: A throwing closure that accepts the upstream failure as
+    ///   input. This closure can either replace the upstream publisher with a new one,
+    ///   or throw a new error to the downstream subscriber.
    /// - Returns: A publisher that handles errors from an upstream publisher by replacing
-    ///   the failed publisher with another publisher.
+    ///   the failed publisher with another publisher, or an error.
    public func tryCatch<NewPublisher: Publisher>(
        _ handler: @escaping (Failure) throws -> NewPublisher
    ) -> Publishers.TryCatch<Self, NewPublisher>
@@ -103,8 +141,12 @@ extension Publishers {
        }
    }

-    /// A publisher that handles errors from an upstream publisher by replacing the failed
-    /// publisher with another publisher or optionally producing a new error.
+    /// A publisher that handles errors from an upstream publisher by replacing
+    /// the failed publisher with another publisher or producing a new error.
+    ///
+    /// Because this publisher’s handler can throw an error, `Publishers.TryCatch` defines
+    /// its `Failure` type as `Error`. This is different from `Publishers.Catch`, which
+    /// gets its failure type from the replacement publisher.
    public struct TryCatch<Upstream: Publisher, NewPublisher: Publisher>: Publisher
        where Upstream.Output == NewPublisher.Output
    {
@@ -112,10 +154,21 @@ extension Publishers {

        public typealias Failure = Error

+        /// The publisher that this publisher receives elements from.
        public let upstream: Upstream

+        /// A closure that accepts the upstream failure as input and either returns
+        /// a publisher to replace the upstream publisher or throws an error.
        public let handler: (Upstream.Failure) throws -> NewPublisher

+        /// Creates a publisher that handles errors from an upstream publisher by
+        /// replacing the failed publisher with another publisher or by throwing an error.
+        ///
+        /// - Parameters:
+        ///   - upstream: The publisher that this publisher receives elements from.
+        ///   - handler: A closure that accepts the upstream failure as input and either
+        ///     returns a publisher to replace the upstream publisher. If this closure
+        ///     throws an error, the publisher terminates with the thrown error.
        public init(upstream: Upstream,
                    handler: @escaping (Upstream.Failure) throws -> NewPublisher) {
            self.upstream = upstream
@@ -10,13 +10,30 @@ extension Publisher {
    /// Collects all received elements, and emits a single array of the collection when
    /// the upstream publisher finishes.
    ///
+    /// Use `collect()` to gather elements into an array that the operator emits after
+    /// the upstream publisher finishes.
+    ///
    /// If the upstream publisher fails with an error, this publisher forwards the error
    /// to the downstream receiver instead of sending its output.
+    ///
    /// This publisher requests an unlimited number of elements from the upstream
-    /// publisher. It only sends the collected array to its downstream after a request
-    /// whose demand is greater than 0 items.
-    /// Note: This publisher uses an unbounded amount of memory to store the received
-    /// values.
+    /// publisher and uses an unbounded amount of memory to store the received values.
+    /// The publisher may exert memory pressure on the system for very large sets of
+    /// elements.
+    ///
+    /// The `collect()` operator only sends the collected array to its downstream receiver
+    /// after a request whose demand is greater than 0 items. Otherwise, `collect()` waits
+    /// until it receives a non-zero request.
+    ///
+    /// In the example below, an Integer range is a publisher that emits an array of
+    /// integers:
+    ///
+    ///     let numbers = (0...10)
+    ///     cancellable = numbers.publisher
+    ///         .collect()
+    ///         .sink { print("\($0)") }
+    ///
+    ///     // Prints: "[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]"
    ///
    /// - Returns: A publisher that collects all received items and returns them as
    ///   an array upon completion.
@@ -10,12 +10,25 @@ extension Publisher {
    /// Collects up to the specified number of elements, and then emits a single array of
    /// the collection.
    ///
-    /// If the upstream publisher finishes before filling the buffer, this publisher sends
-    /// an array of all the items it has received. This may be fewer than `count`
-    /// elements.
+    /// Use `collect(_:)` to emit arrays of at most `count` elements from an upstream
+    /// publisher. If the upstream publisher finishes before collecting the specified
+    /// number of elements, the publisher sends an array of only the items it received
+    /// This may be fewer than `count` elements.
+    ///
    /// If the upstream publisher fails with an error, this publisher forwards the error
    /// to the downstream receiver instead of sending its output.
-    /// Note: When this publisher receives a request for `.max(n)` elements, it requests
+    ///
+    /// In the example below, the `collect(_:)` operator emits one partial and two full
+    /// arrays based on the requested collection size of `5`:
+    ///
+    ///     let numbers = (0...10)
+    ///     cancellable = numbers.publisher
+    ///         .collect(5)
+    ///         .sink { print("\($0), terminator: " "") }
+    ///
+    ///     // Prints "[0, 1, 2, 3, 4] [5, 6, 7, 8, 9] [10] "
+    ///
+    /// > Note: When this publisher receives a request for `.max(n)` elements, it requests
    /// `.max(count * n)` from the upstream publisher.
    ///
    /// - Parameter count: The maximum number of received elements to buffer before
@@ -7,29 +7,79 @@

 extension Publisher {

-    /// Calls a closure with each received element and publishes any returned
-    /// optional that has a value.
+    /// Calls a closure with each received element and publishes any returned optional
+    /// that has a value.
    ///
-    /// - Parameter transform: A closure that receives a value and returns
-    ///   an optional value.
-    /// - Returns: A publisher that republishes all non-`nil` results of calling
-    ///   the transform closure.
+    /// OpenCombine’s `compactMap(_:)` operator performs a function similar to that of
+    /// `compactMap(_:)` in the Swift standard library: the `compactMap(_:)` operator in
+    /// OpenCombine removes `nil` elements in a publisher’s stream and republishes
+    /// non-`nil` elements to the downstream subscriber.
+    ///
+    /// The example below uses a range of numbers as the source for a collection based
+    /// publisher. The `compactMap(_:)` operator consumes each element from the `numbers`
+    /// publisher attempting to access the dictionary using the element as the key.
+    /// If the example’s dictionary returns a `nil`, due to a non-existent key,
+    /// `compactMap(_:)` filters out the `nil` (missing) elements.
+    ///
+    ///     let numbers = (0...5)
+    ///     let romanNumeralDict: [Int : String] =
+    ///         [1: "I", 2: "II", 3: "III", 5: "V"]
+    ///
+    ///     cancellable = numbers.publisher
+    ///         .compactMap { romanNumeralDict[$0] }
+    ///         .sink { print("\($0)", terminator: " ") }
+    ///
+    ///     // Prints: "I II III V"
+    ///
+    /// - Parameter transform: A closure that receives a value and returns an optional
+    ///   value.
+    /// - Returns: Any non-`nil` optional results of the calling the supplied closure.
    public func compactMap<ElementOfResult>(
        _ transform: @escaping (Output) -> ElementOfResult?
    ) -> Publishers.CompactMap<Self, ElementOfResult> {
        return .init(upstream: self, transform: transform)
    }

-    /// Calls an error-throwing closure with each received element and publishes
-    /// any returned optional that has a value.
+    /// Calls an error-throwing closure with each received element and publishes any
+    /// returned optional that has a value.
    ///
-    /// If the closure throws an error, the publisher cancels the upstream and sends
-    /// the thrown error to the downstream receiver as a `Failure`.
+    /// Use `tryCompactMap(_:)` to remove `nil` elements from a publisher’s stream based
+    /// on an error-throwing closure you provide. If the closure throws an error,
+    /// the publisher cancels the upstream publisher and sends the thrown error to
+    /// the downstream subscriber as a `Publisher.Failure`.
    ///
-    /// - Parameter transform: an error-throwing closure that receives a value
-    ///   and returns an optional value.
-    /// - Returns: A publisher that republishes all non-`nil` results of calling
-    ///   the `transform` closure.
+    /// The following example uses an array of numbers as the source for
+    /// a collection-based publisher. A `tryCompactMap(_:)` operator consumes each integer
+    /// from the publisher and uses a dictionary to transform the numbers from its Arabic
+    /// to Roman numerals, as an optional `String`.
+    ///
+    /// If the closure called by `tryCompactMap(_:)` fails to look up a Roman numeral,
+    /// it returns the optional String `(unknown)`.
+    ///
+    /// If the closure called by `tryCompactMap(_:)` determines the input is `0`, it
+    /// throws an error. The `tryCompactMap(_:)` operator catches this error and stops
+    /// publishing, sending a `Subscribers.Completion.failure(_:)` that wraps the error.
+    ///
+    ///     struct ParseError: Error {}
+    ///     func romanNumeral(from: Int) throws -> String? {
+    ///         let romanNumeralDict: [Int : String] =
+    ///             [1: "I", 2: "II", 3: "III", 4: "IV", 5: "V"]
+    ///         guard from != 0 else { throw ParseError() }
+    ///         return romanNumeralDict[from]
+    ///     }
+    ///     let numbers = [6, 5, 4, 3, 2, 1, 0]
+    ///     cancellable = numbers.publisher
+    ///         .tryCompactMap { try romanNumeral(from: $0) }
+    ///         .sink(
+    ///               receiveCompletion: { print ("\($0)") },
+    ///               receiveValue: { print ("\($0)", terminator: " ") }
+    ///          )
+    ///
+    ///     // Prints: "(Unknown) V IV III II I failure(ParseError())"
+    ///
+    /// - Parameter transform: An error-throwing closure that receives a value and returns
+    ///   an optional value.
+    /// - Returns: Any non-`nil` optional results of calling the supplied closure.
    public func tryCompactMap<ElementOfResult>(
        _ transform: @escaping (Output) throws -> ElementOfResult?
    ) -> Publishers.TryCompactMap<Self, ElementOfResult> {
@@ -122,23 +172,48 @@ extension Publishers {
 }

 extension Publishers.CompactMap {
-    private final class Inner<Downstream: Subscriber>
-        : FilterProducer<Downstream,
-                         Upstream.Output,
-                         Output,
-                         Upstream.Failure,
-                         (Upstream.Output) -> Output?>
-        where Downstream.Failure == Upstream.Failure, Downstream.Input == Output
+    private struct Inner<Downstream: Subscriber>
+        : Subscriber,
+          CustomStringConvertible,
+          CustomReflectable,
+          CustomPlaygroundDisplayConvertible
+        where Upstream.Failure == Downstream.Failure
    {
-        // NOTE: This class has been audited for thread safety
+        typealias Input = Upstream.Output
+        typealias Failure = Upstream.Failure

-        override func receive(
-            newValue: Upstream.Output
-        ) -> PartialCompletion<Output?, Downstream.Failure> {
-            return .continue(filter(newValue))
+        private let downstream: Downstream
+        private let filter: (Input) -> Downstream.Input?
+
+        let combineIdentifier = CombineIdentifier()
+
+        init(downstream: Downstream, filter: @escaping (Input) -> Downstream.Input?) {
+            self.downstream = downstream
+            self.filter = filter
        }

-        override var description: String { return "CompactMap" }
+        func receive(subscription: Subscription) {
+            downstream.receive(subscription: subscription)
+        }
+
+        func receive(_ input: Input) -> Subscribers.Demand {
+            if let output = filter(input) {
+                return downstream.receive(output)
+            }
+            return .max(1)
+        }
+
+        func receive(completion: Subscribers.Completion<Failure>) {
+            downstream.receive(completion: completion)
+        }
+
+        var description: String { return "CompactMap" }
+
+        var customMirror: Mirror {
+            return Mirror(self, children: EmptyCollection())
+        }
+
+        var playgroundDescription: Any { return description }
    }
 }

@@ -151,8 +226,6 @@ extension Publishers.TryCompactMap {
                         (Upstream.Output) throws -> Output?>
        where Downstream.Failure == Error, Downstream.Input == Output
    {
-        // NOTE: This class has been audited for thread safety
-
        override func receive(
            newValue: Upstream.Output
        ) -> PartialCompletion<Output?, Error> {
@@ -10,6 +10,20 @@ extension Publisher where Output: Comparable {
    /// Publishes the minimum value received from the upstream publisher, after it
    /// finishes.
    ///
+    /// Use `min()` to find the minimum value in a stream of elements from
+    /// an upstream publisher.
+    ///
+    /// In the example below, the `min()` operator emits a value when the publisher
+    /// finishes, that value is the minimum of the values received from upstream, which
+    /// is `-1`.
+    ///
+    ///     let numbers = [-1, 0, 10, 5]
+    ///     numbers.publisher
+    ///         .min()
+    ///         .sink { print("\($0)") }
+    ///
+    ///     // Prints: "-1"
+    ///
    /// After this publisher receives a request for more than 0 items, it requests
    /// unlimited items from its upstream publisher.
    ///
@@ -22,6 +36,20 @@ extension Publisher where Output: Comparable {
    /// Publishes the maximum value received from the upstream publisher, after it
    /// finishes.
    ///
+    /// Use `max()` to determine the maximum value in the stream of elements from
+    /// an upstream publisher.
+    ///
+    /// In the example below, the `max()` operator emits a value when the publisher
+    /// finishes, that value is the maximum of the values received from upstream, which
+    /// is `10`.
+    ///
+    ///     let numbers = [0, 10, 5]
+    ///     cancellable = numbers.publisher
+    ///         .max()
+    ///         .sink { print("\($0)") }
+    ///
+    ///     // Prints: "10"
+    ///
    /// After this publisher receives a request for more than 0 items, it requests
    /// unlimited items from its upstream publisher.
    ///
@@ -37,11 +65,36 @@ extension Publisher {
    /// Publishes the minimum value received from the upstream publisher, after it
    /// finishes.
    ///
+    /// Use `min(by:)` to determine the minimum value in the stream of elements from
+    /// an upstream publisher using a comparison operation you specify.
+    ///
+    /// This operator is useful when the value received from the upstream publisher isn’t
+    /// `Comparable`.
+    ///
+    /// In the example below an array publishes enumeration elements representing playing
+    /// card ranks. The `min(by:)` operator compares the current and next elements using
+    /// the `rawValue` property of each enumeration value in the user supplied closure and
+    /// prints the minimum value found after publishing all of the elements.
+    ///
+    ///     enum Rank: Int {
+    ///         case ace = 1, two, three, four, five, six, seven, eight, nine,
+    ///         ten, jack, queen, king
+    ///     }
+    ///
+    ///     let cards: [Rank] = [.five, .queen, .ace, .eight, .king]
+    ///     cancellable = cards.publisher
+    ///         .min {
+    ///             return  $0.rawValue < $1.rawValue
+    ///         }
+    ///         .sink { print("\($0)") }
+    ///
+    ///     // Prints: "ace"
+    ///
    /// After this publisher receives a request for more than 0 items, it requests
    /// unlimited items from its upstream publisher.
    ///
    /// - Parameter areInIncreasingOrder: A closure that receives two elements and returns
-    ///   `true` if they are in increasing order.
+    ///   `true` if they’re in increasing order.
    /// - Returns: A publisher that publishes the minimum value received from the upstream
    ///   publisher, after the upstream publisher finishes.
    public func min(
@@ -50,15 +103,39 @@ extension Publisher {
        return max(by: { areInIncreasingOrder($1, $0) })
    }

-    /// Publishes the minimum value received from the upstream publisher, using the
-    /// provided error-throwing closure to order the items.
+    /// Publishes the minimum value received from the upstream publisher, using
+    /// the provided error-throwing closure to order the items.
+    ///
+    /// Use `tryMin(by:)` to determine the minimum value of elements received from
+    /// the upstream publisher using an error-throwing closure you specify.
+    ///
+    /// In the example below, an array publishes elements. The `tryMin(by:)` operator
+    /// executes the error-throwing closure that throws when the `first` element is an odd
+    /// number, terminating the publisher.
+    ///
+    ///     struct IllegalValueError: Error {}
+    ///
+    ///     let numbers: [Int]  = [0, 10, 6, 13, 22, 22]
+    ///     numbers.publisher
+    ///         .tryMin { first, second -> Bool in
+    ///             if (first % 2 != 0) {
+    ///                 throw IllegalValueError()
+    ///             }
+    ///             return first < second
+    ///         }
+    ///         .sink(
+    ///             receiveCompletion: { print ("completion: \($0)") },
+    ///             receiveValue: { print ("value: \($0)") }
+    ///         )
+    ///
+    ///     // Prints: "completion: failure(IllegalValueError())"
    ///
    /// After this publisher receives a request for more than 0 items, it requests
    /// unlimited items from its upstream publisher.
    ///
    /// - Parameter areInIncreasingOrder: A throwing closure that receives two elements
-    ///   and returns `true` if they are in increasing order. If this closure throws, the
-    ///   publisher terminates with a `Failure`.
+    ///   and returns `true` if they’re in increasing order. If this closure throws,
+    ///   the publisher terminates with a `Subscribers.Completion.failure(_:)`.
    /// - Returns: A publisher that publishes the minimum value received from the upstream
    ///   publisher, after the upstream publisher finishes.
    public func tryMin(
@@ -67,14 +144,36 @@ extension Publisher {
        return tryMax(by: { try areInIncreasingOrder($1, $0) })
    }

-    /// Publishes the maximum value received from the upstream publisher, using the
-    /// provided ordering closure.
+    /// Publishes the maximum value received from the upstream publisher, using
+    /// the provided ordering closure.
+    ///
+    /// Use `max(by:)` to determine the maximum value of elements received from
+    /// the upstream publisher based on an ordering closure you specify.
+    ///
+    /// In the example below, an array publishes enumeration elements representing playing
+    /// card ranks. The `max(by:)` operator compares the current and next elements using
+    /// the `rawValue` property of each enumeration value in the user supplied closure and
+    /// prints the maximum value found after publishing all of the elements.
+    ///
+    ///     enum Rank: Int {
+    ///         case ace = 1, two, three, four, five, six, seven, eight, nine,
+    ///         ten, jack, queen, king
+    ///     }
+    ///
+    ///     let cards: [Rank] = [.five, .queen, .ace, .eight, .jack]
+    ///     cancellable = cards.publisher
+    ///         .max {
+    ///             return  $0.rawValue > $1.rawValue
+    ///         }
+    ///         .sink { print("\($0)") }
+    ///
+    ///     // Prints: "queen"
    ///
    /// After this publisher receives a request for more than 0 items, it requests
    /// unlimited items from its upstream publisher.
    ///
    /// - Parameter areInIncreasingOrder: A closure that receives two elements and returns
-    ///   `true` if they are in increasing order.
+    ///   `true` if they’re in increasing order.
    /// - Returns: A publisher that publishes the maximum value received from the upstream
    ///   publisher, after the upstream publisher finishes.
    public func max(
@@ -83,16 +182,42 @@ extension Publisher {
        return .init(upstream: self, areInIncreasingOrder: areInIncreasingOrder)
    }

-    /// Publishes the maximum value received from the upstream publisher, using the
-    /// provided error-throwing closure to order the items.
+    /// Publishes the maximum value received from the upstream publisher, using
+    /// the provided error-throwing closure to order the items.
+    ///
+    /// Use `tryMax(by:)` to determine the maximum value of elements received from
+    /// the upstream publisher using an error-throwing closure you specify.
+    ///
+    /// In the example below, an array publishes elements. The `tryMax(by:)` operator
+    /// executes the error-throwing closure that throws when the `first` element is
+    /// an odd number, terminating the publisher.
+    ///
+    ///     struct IllegalValueError: Error {}
+    ///
+    ///     let numbers: [Int]  = [0, 10, 6, 13, 22, 22]
+    ///     cancellable = numbers.publisher
+    ///         .tryMax { first, second -> Bool in
+    ///             if (first % 2 != 0) {
+    ///                 throw IllegalValueError()
+    ///             }
+    ///             return first > second
+    ///         }
+    ///         .sink(
+    ///             receiveCompletion: { print ("completion: \($0)") },
+    ///             receiveValue: { print ("value: \($0)") }
+    ///         )
+    ///
+    ///     // Prints: completion: failure(IllegalValueError())
    ///
    /// After this publisher receives a request for more than 0 items, it requests
    /// unlimited items from its upstream publisher.
+    ///
    /// - Parameter areInIncreasingOrder: A throwing closure that receives two elements
-    /// and returns `true` if they are in increasing order. If this closure throws, the
-    /// publisher terminates with a `Failure`.
+    ///   and returns `true` if they’re in increasing order. If this closure throws,
+    ///   the publisher terminates with a ``Subscribers/Completion/failure(_:)``.
+    ///
    /// - Returns: A publisher that publishes the maximum value received from the upstream
-    /// publisher, after the upstream publisher finishes.
+    ///   publisher, after the upstream publisher finishes.
    public func tryMax(
        by areInIncreasingOrder:  @escaping (Output, Output) throws -> Bool
    ) -> Publishers.TryComparison<Self> {
@@ -7,7 +7,20 @@

 extension Publisher {

-    /// Prefixes a `Publisher`'s output with the specified sequence.
+    /// Prefixes a publisher’s output with the specified values.
+    ///
+    /// Use `prepend(_:)` when you need to prepend specific elements before the output
+    /// of a publisher.
+    ///
+    /// In the example below, the `prepend(_:)` operator publishes the provided elements
+    /// before republishing all elements from `dataElements`:
+    ///
+    ///     let dataElements = (0...10)
+    ///     cancellable = dataElements.publisher
+    ///         .prepend(0, 1, 255)
+    ///         .sink { print("\($0)", terminator: " ") }
+    ///
+    ///     // Prints: "0 1 255 0 1 2 3 4 5 6 7 8 9 10"
    ///
    /// - Parameter elements: The elements to publish before this publisher’s elements.
    /// - Returns: A publisher that prefixes the specified elements prior to this
@@ -18,7 +31,21 @@ extension Publisher {
        return prepend(elements)
    }

-    /// Prefixes a `Publisher`'s output with the specified sequence.
+    /// Prefixes a publisher’s output with the specified sequence.
+    ///
+    /// Use `prepend(_:)` to publish values from two publishers when you need to prepend
+    /// one publisher’s elements to another.
+    ///
+    /// In this example the `/prepend(_:)-v9sb` operator publishes the provided sequence
+    /// before republishing all elements from `dataElements`:
+    ///
+    ///     let prefixValues = [0, 1, 255]
+    ///     let dataElements = (0...10)
+    ///     cancellable = dataElements.publisher
+    ///         .prepend(prefixValues)
+    ///         .sink { print("\($0)", terminator: " ") }
+    ///
+    ///     // Prints: "0 1 255 0 1 2 3 4 5 6 7 8 9 10"
    ///
    /// - Parameter elements: A sequence of elements to publish before this publisher’s
    ///   elements.
@@ -32,10 +59,22 @@ extension Publisher {
        return prepend(.init(sequence: elements))
    }

-    /// Prefixes this publisher’s output with the elements emitted by the given publisher.
+    /// Prefixes the output of this publisher with the elements emitted by the given
+    /// publisher.
    ///
-    /// The resulting publisher doesn’t emit any elements until the prefixing publisher
-    /// finishes.
+    /// Use `prepend(_:)` to publish values from two publishers when you need to prepend
+    /// one publisher’s elements to another.
+    ///
+    /// In the example below, a publisher of `prefixValues` publishes its elements before
+    /// the `dataElements` publishes its elements:
+    ///
+    ///     let prefixValues = [0, 1, 255]
+    ///     let dataElements = (0...10)
+    ///     cancellable = dataElements.publisher
+    ///         .prepend(prefixValues.publisher)
+    ///         .sink { print("\($0)", terminator: " ") }
+    ///
+    ///     // Prints: "0 1 255 0 1 2 3 4 5 6 7 8 9 10"
    ///
    /// - Parameter publisher: The prefixing publisher.
    /// - Returns: A publisher that prefixes the prefixing publisher’s elements prior to
@@ -48,14 +87,51 @@ extension Publisher {
        return .init(prefix: publisher, suffix: self)
    }

-    /// Append a `Publisher`'s output with the specified sequence.
+    /// Appends a publisher’s output with the specified elements.
+    ///
+    /// Use `append(_:)` when you need to prepend specific elements after the output of
+    /// a publisher.
+    ///
+    /// In the example below, the `append(_:)` operator publishes the provided elements
+    /// after republishing all elements from `dataElements`:
+    ///
+    ///     let dataElements = (0...10)
+    ///     cancellable = dataElements.publisher
+    ///         .append(0, 1, 255)
+    ///         .sink { print("\($0)", terminator: " ") }
+    ///
+    ///     // Prints: "0 1 2 3 4 5 6 7 8 9 10 0 1 255"
+    ///
+    ///
+    /// - Parameter elements: Elements to publish after this publisher’s elements.
+    /// - Returns: A publisher that appends the specifiecd elements after this publisher’s
+    ///   elements.
    public func append(
        _ elements: Output...
    ) -> Publishers.Concatenate<Self, Publishers.Sequence<[Output], Failure>> {
        return append(elements)
    }

-    /// Appends a `Publisher`'s output with the specified sequence.
+    /// Appends a publisher’s output with the specified sequence.
+    ///
+    /// Use `append(_:)` to append a sequence to the end of
+    /// a publisher’s output.
+    ///
+    /// In the example below, the `append(_:)` publisher republishes all elements from
+    /// `groundTransport` until it finishes, then publishes the members of `airTransport`:
+    ///
+    ///     let groundTransport = ["car", "bus", "truck", "subway", "bicycle"]
+    ///     let airTransport = ["parasail", "jet", "helicopter", "rocket"]
+    ///     cancellable = groundTransport.publisher
+    ///         .append(airTransport)
+    ///         .sink { print("\($0)", terminator: " ") }
+    ///
+    ///     // Prints: "car bus truck subway bicycle parasail jet helicopter rocket"
+    ///
+    /// - Parameter elements: A sequence of elements to publish after this publisher’s
+    ///   elements.
+    /// - Returns: A publisher that appends the sequence of elements after this
+    ///   publisher’s elements.
    public func append<Elements: Sequence>(
        _ elements: Elements
    ) -> Publishers.Concatenate<Self, Publishers.Sequence<Elements, Failure>>
@@ -64,12 +140,26 @@ extension Publisher {
        return append(.init(sequence: elements))
    }

-    /// Appends this publisher’s output with the elements emitted by the given publisher.
+    /// Appends the output of this publisher with the elements emitted by the given
+    /// publisher.
    ///
-    /// This operator produces no elements until this publisher finishes. It then produces
-    /// this publisher’s elements, followed by the given publisher’s elements.
-    /// If this publisher fails with an error, the prefixing publisher does not publish
-    /// the provided publisher’s elements.
+    /// Use `append(_:)` to append the output of one publisher to another.
+    /// The `append(_:)` operator produces no elements until this publisher finishes.
+    /// It then produces this publisher’s elements, followed by the given publisher’s
+    /// elements. If this publisher fails with an error, the given publishers elements
+    /// aren’t published.
+    ///
+    /// In the example below, the `append` publisher republishes all elements from
+    /// the `numbers` publisher until it finishes, then publishes all elements from
+    /// the `otherNumbers` publisher:
+    ///
+    ///     let numbers = (0...10)
+    ///     let otherNumbers = (25...35)
+    ///     cancellable = numbers.publisher
+    ///         .append(otherNumbers.publisher)
+    ///         .sink { print("\($0)", terminator: " ") }
+    ///
+    ///     // Prints: "0 1 2 3 4 5 6 7 8 9 10 25 26 27 28 29 30 31 32 33 34 35 "
    ///
    /// - Parameter publisher: The appending publisher.
    /// - Returns: A publisher that appends the appending publisher’s elements after this
@@ -85,7 +175,7 @@ extension Publisher {

 extension Publishers {

-    /// A publisher that emits all of one publisher’s elements before those from anothe
+    /// A publisher that emits all of one publisher’s elements before those from another
    /// publisher.
    public struct Concatenate<Prefix: Publisher, Suffix: Publisher>: Publisher
        where Prefix.Failure == Suffix.Failure, Prefix.Output == Suffix.Output
@@ -110,8 +200,7 @@ extension Publishers {
            where Suffix.Failure == Downstream.Failure, Suffix.Output == Downstream.Input
        {
            let inner = Inner(downstream: subscriber, suffix: suffix)
-            subscriber.receive(subscription: inner)
-            prefix.subscribe(inner)
+            prefix.subscribe(Inner<Downstream>.PrefixSubscriber(inner: inner))
        }
    }
 }
@@ -119,9 +208,8 @@ extension Publishers {
 extension Publishers.Concatenate: Equatable where Prefix: Equatable, Suffix: Equatable {}

 extension Publishers.Concatenate {
-    private final class Inner<Downstream: Subscriber>
-        : Subscriber,
-          Subscription,
+    fileprivate final class Inner<Downstream: Subscriber>
+        : Subscription,
          CustomStringConvertible,
          CustomReflectable,
          CustomPlaygroundDisplayConvertible
@@ -131,22 +219,26 @@ extension Publishers.Concatenate {

        typealias Failure = Suffix.Failure

+        fileprivate struct PrefixSubscriber {
+            let inner: Inner<Downstream>
+        }
+
+        fileprivate struct SuffixSubscriber {
+            let inner: Inner<Downstream>
+        }
+
        private let downstream: Downstream

+        private var prefixState = SubscriptionStatus.awaitingSubscription
+
+        private var suffixState = SubscriptionStatus.awaitingSubscription
+
        private let suffix: Suffix

-        private var prefixFinished = false
-
-        private var demand = Subscribers.Demand.none
-
-        private var upstream: Subscription?
-
-        private var expectedSubscriptions = 2
+        private var pending = Subscribers.Demand.none

        private let lock = UnfairLock.allocate()

-        private let downstreamLock = UnfairRecursiveLock.allocate()
-
        fileprivate init(downstream: Downstream, suffix: Suffix) {
            self.downstream = downstream
            self.suffix = suffix
@@ -154,65 +246,13 @@ extension Publishers.Concatenate {

        deinit {
            lock.deallocate()
-            downstreamLock.deallocate()
-        }
-
-        func receive(subscription: Subscription) {
-            lock.lock()
-            guard upstream == nil, expectedSubscriptions > 0 else {
-                lock.unlock()
-                subscription.cancel()
-                return
-            }
-            upstream = subscription
-            expectedSubscriptions -= 1
-            let demand = self.demand
-            lock.unlock()
-            if demand > 0 {
-                subscription.request(demand)
-            }
-        }
-
-        func receive(_ input: Input) -> Subscribers.Demand {
-            lock.lock()
-            demand -= 1
-            lock.unlock()
-            downstreamLock.lock()
-            let newDemand = downstream.receive(input)
-            downstreamLock.unlock()
-            lock.lock()
-            demand += newDemand
-            lock.unlock()
-            return newDemand
-        }
-
-        func receive(completion: Subscribers.Completion<Failure>) {
-            // Reading prefixFinished should be locked. Combine doesn't lock here.
-            if prefixFinished {
-                downstreamLock.lock()
-                downstream.receive(completion: completion)
-                downstreamLock.unlock()
-                return
-            }
-
-            guard case .finished = completion else {
-                downstreamLock.lock()
-                downstream.receive(completion: completion)
-                downstreamLock.unlock()
-                return
-            }
-
-            prefixFinished = true // Should be locked as well?
-            lock.lock()
-            upstream = nil
-            lock.unlock()
-            suffix.subscribe(self)
        }

        func request(_ demand: Subscribers.Demand) {
            lock.lock()
-            self.demand += demand
-            guard let subscription = upstream else {
+            pending += demand
+            guard let subscription = prefixState.subscription ?? suffixState.subscription
+            else {
                lock.unlock()
                return
            }
@@ -222,27 +262,203 @@ extension Publishers.Concatenate {

        func cancel() {
            lock.lock()
-            guard let subscription = upstream else {
-                lock.unlock()
-                return
-            }
-            upstream = nil
+            let upstreamSubscription =
+                prefixState.subscription ?? suffixState.subscription
+            prefixState = .terminal
+            suffixState = .terminal
            lock.unlock()
-            subscription.cancel()
+            upstreamSubscription?.cancel()
        }

        var description: String { return "Concatenate" }

        var customMirror: Mirror {
-            let children: [Mirror.Child] = [
-                ("downstream", downstream),
-                ("upstreamSubscription", upstream as Any),
-                ("suffix", suffix),
-                ("demand", demand)
-            ]
-            return Mirror(self, children: children)
+            return Mirror(self, children: EmptyCollection())
        }

        var playgroundDescription: Any { return description }
+
+        // MARK: - Private
+
+        private func prefixReceive(subscription: Subscription) {
+            lock.lock()
+            guard case .awaitingSubscription = prefixState else {
+                lock.unlock()
+                subscription.cancel()
+                return
+            }
+            prefixState = .subscribed(subscription)
+            lock.unlock()
+            downstream.receive(subscription: self)
+        }
+
+        private func prefixReceive(_ input: Input) -> Subscribers.Demand {
+            lock.lock()
+            guard case .subscribed = prefixState, pending != .none else {
+                lock.unlock()
+                return .none
+            }
+            pending -= 1
+            lock.unlock()
+            let newDemand = downstream.receive(input)
+            if newDemand == .none {
+                return .none
+            }
+            lock.lock()
+            pending += newDemand
+            lock.unlock()
+            return newDemand
+        }
+
+        private func prefixReceive(completion: Subscribers.Completion<Failure>) {
+            lock.lock()
+            guard case .subscribed = prefixState else {
+                lock.unlock()
+                return
+            }
+            prefixState = .terminal
+            lock.unlock()
+            switch completion {
+            case .finished:
+                suffix.subscribe(SuffixSubscriber(inner: self))
+            case .failure:
+                downstream.receive(completion: completion)
+            }
+        }
+
+        private func suffixReceive(subscription: Subscription) {
+            lock.lock()
+            guard case .awaitingSubscription = suffixState else {
+                lock.unlock()
+                subscription.cancel()
+                return
+            }
+            suffixState = .subscribed(subscription)
+            let pending = self.pending
+            lock.unlock()
+            if pending != .none {
+                subscription.request(pending)
+            }
+        }
+
+        private func suffixReceive(_ input: Input) -> Subscribers.Demand {
+            lock.lock()
+            guard case .subscribed = suffixState else {
+                lock.unlock()
+                return .none
+            }
+            lock.unlock()
+            return downstream.receive(input)
+        }
+
+        private func suffixReceive(completion: Subscribers.Completion<Failure>) {
+            lock.lock()
+            guard case .subscribed = suffixState else {
+                lock.unlock()
+                return
+            }
+            prefixState = .terminal
+            suffixState = .terminal
+            lock.unlock()
+            downstream.receive(completion: completion)
+        }
+    }
+}
+
+// MARK: - PrefixSubscriber conformances
+
+extension Publishers.Concatenate.Inner.PrefixSubscriber: Subscriber {
+
+    fileprivate typealias Input = Suffix.Output
+
+    fileprivate typealias Failure = Suffix.Failure
+
+    fileprivate var combineIdentifier: CombineIdentifier {
+        return inner.combineIdentifier
+    }
+
+    fileprivate func receive(subscription: Subscription) {
+        inner.prefixReceive(subscription: subscription)
+    }
+
+    fileprivate func receive(_ input: Input) -> Subscribers.Demand {
+        return inner.prefixReceive(input)
+    }
+
+    fileprivate func receive(completion: Subscribers.Completion<Failure>) {
+        inner.prefixReceive(completion: completion)
+    }
+}
+
+extension Publishers.Concatenate.Inner.PrefixSubscriber
+    : CustomStringConvertible
+{
+    fileprivate var description: String {
+        return inner.description
+    }
+}
+
+extension Publishers.Concatenate.Inner.PrefixSubscriber
+    : CustomReflectable
+{
+    fileprivate var customMirror: Mirror {
+        return inner.customMirror
+    }
+}
+
+extension Publishers.Concatenate.Inner.PrefixSubscriber
+    : CustomPlaygroundDisplayConvertible
+{
+    fileprivate var playgroundDescription: Any {
+        return inner.playgroundDescription
+    }
+}
+
+// MARK: - SuffixSubscriber conformances
+
+extension Publishers.Concatenate.Inner.SuffixSubscriber: Subscriber {
+
+    fileprivate typealias Input = Suffix.Output
+
+    fileprivate typealias Failure = Suffix.Failure
+
+    fileprivate var combineIdentifier: CombineIdentifier {
+        return inner.combineIdentifier
+    }
+
+    fileprivate func receive(subscription: Subscription) {
+        inner.suffixReceive(subscription: subscription)
+    }
+
+    fileprivate func receive(_ input: Input) -> Subscribers.Demand {
+        return inner.suffixReceive(input)
+    }
+
+    fileprivate func receive(completion: Subscribers.Completion<Failure>) {
+        inner.suffixReceive(completion: completion)
+    }
+}
+
+extension Publishers.Concatenate.Inner.SuffixSubscriber
+    : CustomStringConvertible
+{
+    fileprivate var description: String {
+        return inner.description
+    }
+}
+
+extension Publishers.Concatenate.Inner.SuffixSubscriber
+    : CustomReflectable
+{
+    fileprivate var customMirror: Mirror {
+        return inner.customMirror
+    }
+}
+
+extension Publishers.Concatenate.Inner.SuffixSubscriber
+    : CustomPlaygroundDisplayConvertible
+{
+    fileprivate var playgroundDescription: Any {
+        return inner.playgroundDescription
    }
 }
@@ -9,10 +9,22 @@ extension Publisher where Output: Equatable {

    /// Publishes a Boolean value upon receiving an element equal to the argument.
    ///
-    /// The contains publisher consumes all received elements until the upstream publisher
-    /// produces a matching element. At that point, it emits `true` and finishes normally.
-    /// If the upstream finishes normally without producing a matching element,
-    /// this publisher emits `false`, then finishes.
+    /// Use `contains(_:)` to find the first element in an upstream that’s equal to
+    /// the supplied argument. The `Publishers.Contains` publisher consumes all received
+    /// elements until the upstream publisher produces a matching element. Upon finding
+    /// the first match, it emits `true` and finishes normally. If the upstream finishes
+    /// normally without producing a matching element, this publisher emits `false` and
+    /// finishes.
+    ///
+    /// In the example below, the `contains(_:)` operator emits `true` the first time it
+    /// receives the value `5` from the `numbers.publisher`, and then finishes normally.
+    ///
+    ///     let numbers = [-1, 5, 10, 5]
+    ///     numbers.publisher
+    ///         .contains(5)
+    ///         .sink { print("\($0)") }
+    ///
+    ///     // Prints: "true"
    ///
    /// - Parameter output: An element to match against.
    /// - Returns: A publisher that emits the Boolean value `true` when the upstream
@@ -27,12 +39,27 @@ extension Publisher {
    /// Publishes a Boolean value upon receiving an element that satisfies the predicate
    /// closure.
    ///
-    /// This operator consumes elements produced from the upstream publisher until
-    /// the upstream publisher produces a matching element.
+    /// Use `contains(where:)` to find the first element in an upstream that satisfies
+    /// the closure you provide. This operator consumes elements produced from
+    /// the upstream publisher until the upstream publisher produces a matching element.
+    ///
+    /// This operator is useful when the upstream publisher produces elements that don’t
+    /// conform to `Equatable`.
+    ///
+    /// In the example below, the `contains(where:)` operator tests elements against
+    /// the supplied closure and emits `true` for the first elements that’s greater than
+    /// `4`, and then finishes normally.
+    ///
+    ///     let numbers = [-1, 0, 10, 5]
+    ///     numbers.publisher
+    ///         .contains {$0 > 4}
+    ///         .sink { print("\($0)") }
+    ///
+    ///     // Prints: "true"
    ///
    /// - Parameter predicate: A closure that takes an element as its parameter and
-    ///   returns a Boolean value indicating whether the element satisfies the closure’s
-    ///   comparison logic.
+    ///   returns a Boolean value that indicates whether the element satisfies
+    ///   the closure’s comparison logic.
    /// - Returns: A publisher that emits the Boolean value `true` when the upstream
    ///   publisher emits a matching value.
    public func contains(
@@ -41,16 +68,47 @@ extension Publisher {
        return .init(upstream: self, predicate: predicate)
    }

-    /// Publishes a Boolean value upon receiving an element that satisfies
-    /// the throwing predicate closure.
+    /// Publishes a Boolean value upon receiving an element that satisfies the throwing
+    /// predicate closure.
+    ///
+    /// Use `tryContains(where:)` to find the first element in an upstream that satisfies
+    /// the error-throwing closure you provide.
    ///
    /// This operator consumes elements produced from the upstream publisher until
-    /// the upstream publisher produces a matching element. If the closure throws,
-    /// the stream fails with an error.
+    /// the upstream publisher either:
+    ///
+    /// - Produces a matching element, after which it emits `true` and the publisher
+    ///   finishes normally.
+    /// - Emits `false` if no matching element is found and the publisher finishes
+    ///   normally.
+    ///
+    /// If the predicate throws an error, the publisher fails, passing the error to its
+    /// downstream.
+    ///
+    /// In the example below, the `tryContains(where:)` operator tests values to find
+    /// an element less than `10`; when the closure finds an odd number, like `3`,
+    /// the publisher terminates with an `IllegalValueError`.
+    ///
+    ///     struct IllegalValueError: Error {}
+    ///
+    ///     let numbers = [3, 2, 10, 5, 0, 9]
+    ///     numbers.publisher
+    ///         .tryContains {
+    ///             if ($0 % 2 != 0) {
+    ///                 throw IllegalValueError()
+    ///             }
+    ///            return $0 < 10
+    ///         }
+    ///         .sink(
+    ///             receiveCompletion: { print ("completion: \($0)") },
+    ///             receiveValue: { print ("value: \($0)") }
+    ///         )
+    ///
+    ///     // Prints: "completion: failure(IllegalValueError())"
    ///
    /// - Parameter predicate: A closure that takes an element as its parameter and
-    ///   returns a Boolean value indicating whether the element satisfies the closure’s
-    ///   comparison logic.
+    ///   returns a Boolean value that indicates whether the element satisfies
+    ///   the closure’s comparison logic.
    /// - Returns: A publisher that emits the Boolean value `true` when the upstream
    ///   publisher emits a matching value.
    public func tryContains(
@@ -9,8 +9,18 @@ extension Publisher {

    /// Publishes the number of elements received from the upstream publisher.
    ///
+    /// Use `count(`` to determine the number of elements received from the upstream
+    /// publisher before it completes:
+    ///
+    ///     let numbers = (0...10)
+    ///     cancellable = numbers.publisher
+    ///         .count()
+    ///         .sink { print("\($0)") }
+    ///
+    ///     // Prints: "11"
+    ///
    /// - Returns: A publisher that consumes all elements until the upstream publisher
-    /// finishes, then emits a single value with the total number of elements received.
+    ///   finishes, then emits a single value with the total number of elements received.
    public func count() -> Publishers.Count<Self> {
        return Publishers.Count(upstream: self)
    }
@@ -0,0 +1,336 @@
+//
+//  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 the `debounce(for:scheduler:options:)` operator to control the number of
+    /// values and time between delivery of values from the upstream publisher. This
+    /// operator is useful to process bursty or high-volume event streams where you need
+    /// to reduce the number of values delivered to the downstream to a rate you specify.
+    ///
+    /// In this example, a `PassthroughSubject` publishes elements on a schedule defined
+    /// by the `bounces` array. The array is composed of tuples representing a value sent
+    /// by the `PassthroughSubject`, and a `TimeInterval` ranging from one-quarter second
+    /// up to 2 seconds that drives a delivery timer. As the queue builds, elements
+    /// arriving faster than one-half second `debounceInterval` are discarded, while
+    /// elements arriving at a rate slower than `debounceInterval` are passed through to
+    /// the `sink(receiveValue:)` operator.
+    ///
+    ///     let bounces:[(Int,TimeInterval)] = [
+    ///         (0, 0),
+    ///         (1, 0.25),  // 0.25s interval since last index
+    ///         (2, 1),     // 0.75s interval since last index
+    ///         (3, 1.25),  // 0.25s interval since last index
+    ///         (4, 1.5),   // 0.25s interval since last index
+    ///         (5, 2)      // 0.5s interval since last index
+    ///     ]
+    ///
+    ///     let subject = PassthroughSubject<Int, Never>()
+    ///     cancellable = subject
+    ///         .debounce(for: .seconds(0.5), scheduler: RunLoop.main)
+    ///         .sink { index in
+    ///             print ("Received index \(index)")
+    ///         }
+    ///
+    ///     for bounce in bounces {
+    ///         DispatchQueue.main.asyncAfter(deadline: .now() + bounce.1) {
+    ///             subject.send(bounce.0)
+    ///         }
+    ///     }
+    ///
+    ///     // Prints:
+    ///     //  Received index 1
+    ///     //  Received index 4
+    ///     //  Received index 5
+    ///
+    ///     //  Here is the event flow shown from the perspective of time, showing value
+    ///     //  delivery through the `debounce()` operator:
+    ///
+    ///     //  Time 0: Send index 0.
+    ///     //  Time 0.25: Send index 1. Index 0 was waiting and is discarded.
+    ///     //  Time 0.75: Debounce period ends, publish index 1.
+    ///     //  Time 1: Send index 2.
+    ///     //  Time 1.25: Send index 3. Index 2 was waiting and is discarded.
+    ///     //  Time 1.5: Send index 4. Index 3 was waiting and is discarded.
+    ///     //  Time 2: Debounce period ends, publish index 4. Also, send index 5.
+    ///     //  Time 2.5: Debounce period ends, publish index 5.
+    ///
+    /// - 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 enum CancellerState {
+            case pending
+            case active(Cancellable)
+
+            fileprivate func cancel() {
+                if case let .active(cancellable) = self {
+                    cancellable.cancel()
+                }
+            }
+        }
+
+        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 currentCancellers = [Generation : CancellerState]()
+
+        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()
+            guard case .subscribed = state else {
+                lock.unlock()
+                return .none
+            }
+            currentGeneration += 1
+            let generation = currentGeneration
+            currentValue = input
+            let due = scheduler.now.advanced(by: dueTime)
+            let previousCancellers = self.currentCancellers
+            currentCancellers.removeAll()
+            currentCancellers[generation] = .pending
+            lock.unlock()
+            let newCanceller = scheduler.schedule(after: due,
+                                                  interval: dueTime,
+                                                  tolerance: scheduler.minimumTolerance,
+                                                  options: options) {
+                self.due(generation: generation)
+            }
+            lock.lock()
+            currentCancellers[generation] = .active(newCanceller)
+            lock.unlock()
+            for canceller in previousCancellers.values {
+                canceller.cancel()
+            }
+            return .none
+        }
+
+        func receive(completion: Subscribers.Completion<Failure>) {
+            lock.lock()
+            guard case .subscribed = state else {
+                lock.unlock()
+                return
+            }
+            state = .terminal
+            let previousCancellers = currentCancellers
+            currentCancellers.removeAll()
+            lock.unlock()
+            for canceller in previousCancellers.values {
+                canceller.cancel()
+            }
+            scheduler.schedule {
+                self.downstreamLock.lock()
+                self.downstream.receive(completion: completion)
+                self.downstreamLock.unlock()
+            }
+        }
+
+        func request(_ demand: Subscribers.Demand) {
+            lock.lock()
+            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
+            let previousCancellers = currentCancellers
+            currentCancellers.removeAll()
+            lock.unlock()
+            for canceller in previousCancellers.values {
+                canceller.cancel()
+            }
+            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 = currentCancellers[generation]
+                lock.unlock()
+                canceller?.cancel()
+                return
+            }
+
+            guard let canceller = currentCancellers[generation] else {
+                lock.unlock()
+                return
+            }
+            currentCancellers[generation] = nil
+
+            let hasAnyDemand = downstreamDemand != .none
+            if hasAnyDemand {
+                downstreamDemand -= 1
+            }
+
+            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()
+        }
+    }
+}
@@ -7,8 +7,52 @@

 extension Publisher {

-    /// Delays delivery of all output to the downstream receiver by a specified amount
-    /// of time on a particular scheduler.
+    /// Delays delivery of all output to the downstream receiver by a specified amount of
+    /// time on a particular scheduler.
+    ///
+    /// Use `delay(for:tolerance:scheduler:options:)` when you need to delay the delivery
+    /// of elements to a downstream by a specified amount of time.
+    ///
+    /// In this example, a `Timer` publishes an event every second.
+    /// The `delay(for:tolerance:scheduler:options:)` operator holds the delivery of
+    /// the initial element for 3 seconds (±0.5 seconds), after which each element is
+    /// delivered to the downstream on the main run loop after the specified delay:
+    ///
+    ///     let df = DateFormatter()
+    ///     df.dateStyle = .none
+    ///     df.timeStyle = .long
+    ///     cancellable = Timer.publish(every: 1.0, on: .main, in: .default)
+    ///         .autoconnect()
+    ///         .handleEvents(receiveOutput: { date in
+    ///             print ("Sending Timestamp \'\(df.string(from: date))\' to delay()")
+    ///         })
+    ///         .delay(for: .seconds(3), scheduler: RunLoop.main, options: .none)
+    ///         .sink(
+    ///             receiveCompletion: { print ("completion: \($0)", terminator: "\n") },
+    ///             receiveValue: { value in
+    ///                 let now = Date()
+    ///                 print("""
+    ///                 At \(df.string(from: now)) received Timestamp \
+    ///                 \'\(df.string(from: value))\' \
+    ///                 sent: \(String(format: "%.2f", now.timeIntervalSince(value)))
+    ///                 secs ago
+    ///                 """)
+    ///             }
+    ///         )
+    ///
+    ///     // Prints:
+    ///     //    Sending Timestamp '5:02:33 PM PDT' to delay()
+    ///     //    Sending Timestamp '5:02:34 PM PDT' to delay()
+    ///     //    Sending Timestamp '5:02:35 PM PDT' to delay()
+    ///     //    Sending Timestamp '5:02:36 PM PDT' to delay()
+    ///     //    At 5:02:36 PM PDT received  Timestamp '5:02:33 PM PDT' sent: 3.00
+    ///     //    secs ago
+    ///     //    Sending Timestamp '5:02:37 PM PDT' to delay()
+    ///     //    At 5:02:37 PM PDT received  Timestamp '5:02:34 PM PDT' sent: 3.00
+    ///     //    secs ago
+    ///     //    Sending Timestamp '5:02:38 PM PDT' to delay()
+    ///     //    At 5:02:38 PM PDT received  Timestamp '5:02:35 PM PDT' sent: 3.00
+    ///     //    secs ago
    ///
    /// The delay affects the delivery of elements and completion, but not of the original
    /// subscription.
@@ -17,6 +61,7 @@ extension Publisher {
    ///   - interval: The amount of time to delay.
    ///   - tolerance: The allowed tolerance in firing delayed events.
    ///   - scheduler: The scheduler to deliver the delayed events.
+    ///   - options: Options relevant to the scheduler’s behavior.
    /// - Returns: A publisher that delays delivery of elements and completion to
    ///   the downstream receiver.
    public func delay<Context: Scheduler>(
@@ -74,7 +119,12 @@ extension Publishers {
            where Upstream.Failure == Downstream.Failure,
                  Upstream.Output == Downstream.Input
        {
-            upstream.subscribe(Inner(self, downstream: subscriber))
+            let inner = Inner(downstream: subscriber,
+                              interval: interval,
+                              tolerance: tolerance,
+                              scheduler: scheduler,
+                              options: options)
+            upstream.subscribe(inner)
        }
    }
 }
@@ -85,25 +135,28 @@ extension Publishers.Delay {
          Subscription
        where Downstream.Input == Upstream.Output, Downstream.Failure == Upstream.Failure
    {
-        // NOTE: This class has been audited for thread safety
-
        typealias Input = Upstream.Output
        typealias Failure = Upstream.Failure

-        fileprivate typealias Delay = Publishers.Delay<Upstream, Context>
-
-        private enum State {
-            case ready(Delay, Downstream)
-            case subscribed(Delay, Downstream, Subscription)
-            case terminal
-        }
-
        private let lock = UnfairLock.allocate()
-        private var state: State
+        private let downstream: Downstream
+        private let interval: Context.SchedulerTimeType.Stride
+        private let tolerance: Context.SchedulerTimeType.Stride
+        private let scheduler: Context
+        private let options: Context.SchedulerOptions?
+        private var state = SubscriptionStatus.awaitingSubscription
        private let downstreamLock = UnfairRecursiveLock.allocate()

-        fileprivate init(_ publisher: Delay, downstream: Downstream) {
-            state = .ready(publisher, downstream)
+        fileprivate init(downstream: Downstream,
+                         interval: Context.SchedulerTimeType.Stride,
+                         tolerance: Context.SchedulerTimeType.Stride,
+                         scheduler: Context,
+                         options: Context.SchedulerOptions?) {
+            self.downstream = downstream
+            self.interval = interval
+            self.tolerance = tolerance
+            self.scheduler = scheduler
+            self.options = options
        }

        deinit {
@@ -111,73 +164,82 @@ extension Publishers.Delay {
            downstreamLock.deallocate()
        }

-        private func schedule(_ delay: Delay, work: @escaping () -> Void) {
-            delay
-                .scheduler
-                .schedule(after: delay.scheduler.now.advanced(by: delay.interval),
-                          tolerance: delay.tolerance,
-                          options: delay.options,
+        private func schedule(_ work: @escaping () -> Void) {
+            scheduler
+                .schedule(after: scheduler.now.advanced(by: interval),
+                          tolerance: tolerance,
+                          options: options,
                          work)
        }

        func receive(subscription: Subscription) {
            lock.lock()
-            guard case let .ready(delay, downstream) = state else {
+            guard case .awaitingSubscription = state else {
                lock.unlock()
                subscription.cancel()
                return
            }
-            state = .subscribed(delay, downstream, subscription)
+            state = .subscribed(subscription)
            lock.unlock()
            downstreamLock.lock()
            downstream.receive(subscription: self)
            downstreamLock.unlock()
        }

-        func receive(_ input: Upstream.Output) -> Subscribers.Demand {
+        func receive(_ input: Input) -> Subscribers.Demand {
            lock.lock()
-            guard case let .subscribed(delay, downstream, _) = state else {
+            guard case .subscribed = state else {
                lock.unlock()
                return .none
            }
            lock.unlock()
-            schedule(delay) {
-                self.scheduledReceive(input, downstream: downstream)
+            schedule {
+                self.scheduledReceive(input)
            }
            return .none
        }

-        private func scheduledReceive(_ input: Upstream.Output, downstream: Downstream) {
-            downstreamLock.lock()
-            let newDemand = downstream.receive(input)
-            downstreamLock.unlock()
-            guard newDemand > 0 else {
-                return
-            }
+        private func scheduledReceive(_ input: Input) {
            lock.lock()
-            guard case let .subscribed(_, _, subscription) = state else {
+            guard state.subscription != nil else {
                lock.unlock()
                return
            }
            lock.unlock()
-            subscription.request(newDemand)
+            downstreamLock.lock()
+            let newDemand = downstream.receive(input)
+            downstreamLock.unlock()
+            if newDemand == .none { return }
+            lock.lock()
+            let subscription = state.subscription
+            lock.unlock()
+            subscription?.request(newDemand)
        }

        func receive(completion: Subscribers.Completion<Failure>) {
            lock.lock()
-            guard case let .subscribed(delay, downstream, _) = state else {
+            guard case let .subscribed(subscription) = state else {
+                lock.unlock()
+                return
+            }
+            state = .pendingTerminal(subscription)
+            lock.unlock()
+            schedule {
+                self.scheduledReceive(completion: completion)
+            }
+        }
+
+        private func scheduledReceive(completion: Subscribers.Completion<Failure>) {
+            lock.lock()
+            guard case .pendingTerminal = state else {
+                assertionFailure(
+                    "This branch should not be reachable! Please report a bug."
+                )
                lock.unlock()
                return
            }
            state = .terminal
            lock.unlock()
-            schedule(delay) {
-                self.scheduledReceive(completion: completion, downstream: downstream)
-            }
-        }
-
-        private func scheduledReceive(completion: Subscribers.Completion<Failure>,
-                                      downstream: Downstream) {
            downstreamLock.lock()
            downstream.receive(completion: completion)
            downstreamLock.unlock()
@@ -185,7 +247,7 @@ extension Publishers.Delay {

        func request(_ demand: Subscribers.Demand) {
            lock.lock()
-            guard case let .subscribed(_, _, subscription) = state else {
+            guard case let .subscribed(subscription) = state else {
                lock.unlock()
                return
            }
@@ -195,7 +257,7 @@ extension Publishers.Delay {

        func cancel() {
            lock.lock()
-            guard case let .subscribed(_, _, subscription) = state else {
+            guard case let .subscribed(subscription) = state else {
                lock.unlock()
                return
            }
@@ -8,8 +8,20 @@
 extension Publisher {
    /// Omits the specified number of elements before republishing subsequent elements.
    ///
-    /// - Parameter count: The number of elements to omit.
-    /// - Returns: A publisher that does not republish the first `count` elements.
+    /// Use `dropFirst(_:)` when you want to drop the first `n` elements from the upstream
+    /// publisher, and republish the remaining elements.
+    ///
+    /// The example below drops the first five elements from the stream:
+    ///
+    ///     let numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
+    ///     cancellable = numbers.publisher
+    ///         .dropFirst(5)
+    ///         .sink { print("\($0)", terminator: " ") }
+    ///
+    ///     // Prints: "6 7 8 9 10 "
+    ///
+    /// - Parameter count: The number of elements to omit. The default is `1`.
+    /// - Returns: A publisher that doesn’t republish the first `count` elements.
    public func dropFirst(_ count: Int = 1) -> Publishers.Drop<Self> {
        return .init(upstream: self, count: count)
    }
@@ -40,8 +52,8 @@ extension Publishers {
                  Upstream.Output == Downstream.Input
        {
            let inner = Inner(downstream: subscriber, count: count)
-            upstream.subscribe(inner)
            subscriber.receive(subscription: inner)
+            upstream.subscribe(inner)
        }
    }
 }
@@ -58,8 +70,6 @@ extension Publishers.Drop {
        where Upstream.Output == Downstream.Input,
              Upstream.Failure == Downstream.Failure
    {
-        // NOTE: This class has been audited for thread safety.
-
        typealias Input = Upstream.Output

        typealias Failure = Upstream.Failure
@@ -94,7 +104,7 @@ extension Publishers.Drop {
            precondition(count >= 0, "count must not be negative")
            let demandToRequestFromUpstream = pendingDemand + count
            lock.unlock()
-            if demandToRequestFromUpstream > 0 {
+            if demandToRequestFromUpstream != .none {
                subscription.request(demandToRequestFromUpstream)
            }
        }
@@ -109,8 +119,9 @@ extension Publishers.Drop {
        }

        func receive(completion: Subscribers.Completion<Upstream.Failure>) {
-            // Combine doesn't lock here!
+            lock.lock()
            subscription = nil
+            lock.unlock()
            downstream.receive(completion: completion)
        }

@@ -127,9 +138,11 @@ extension Publishers.Drop {
        }

        func cancel() {
-            // Combine doesn't lock here!
+            lock.lock()
+            let subscription = self.subscription
+            self.subscription = nil
+            lock.unlock()
            subscription?.cancel()
-            subscription = nil
        }

        var description: String { return "Drop" }
@@ -10,20 +10,39 @@ extension Publisher {
    /// Ignores elements from the upstream publisher until it receives an element from
    /// a second publisher.
    ///
-    /// This publisher requests a single value from the upstream publisher, and it ignores
-    /// (drops) all elements from that publisher until the upstream publisher produces
-    /// a value. After the `other` publisher produces an element, this publisher cancels
-    /// its subscription to the `other` publisher, and allows events from the `upstream`
-    /// publisher to pass through.
+    /// Use `drop(untilOutputFrom:)` to ignore elements from the upstream publisher until
+    /// another, second, publisher delivers its first element.
+    /// This publisher requests a single value from the second publisher, and it ignores
+    /// (drops) all elements from the upstream publisher until the second publisher
+    /// produces a value. After the second publisher produces an element,
+    /// `drop(untilOutputFrom:)` cancels its subscription to the second publisher, and
+    /// allows events from the upstream publisher to pass through.
+    ///
    /// After this publisher receives a subscription from the upstream publisher, it
    /// passes through backpressure requests from downstream to the upstream publisher.
    /// If the upstream publisher acts on those requests before the other publisher
    /// produces an item, this publisher drops the elements it receives from the upstream
    /// publisher.
    ///
+    /// In the example below, the `pub1` publisher defers publishing its elements until
+    /// the `pub2` publisher delivers its first element:
+    ///
+    ///     let upstream = PassthroughSubject<Int, Never>()
+    ///     let second = PassthroughSubject<String, Never>()
+    ///     cancellable = upstream
+    ///         .drop(untilOutputFrom: second)
+    ///         .sink { print("\($0)", terminator: " ") }
+    ///
+    ///     upstream.send(1)
+    ///     upstream.send(2)
+    ///     second.send("A")
+    ///     upstream.send(3)
+    ///     upstream.send(4)
+    ///     // Prints "3 4"
+    ///
    /// - Parameter publisher: A publisher to monitor for its first emitted element.
-    /// - Returns: A publisher that drops elements from the upstream publisher until the
-    ///   `other` publisher produces a value.
+    /// - Returns: A publisher that drops elements from the upstream publisher until
+    ///   the `other` publisher produces a value.
    public func drop<Other: Publisher>(
        untilOutputFrom publisher: Other
    ) -> Publishers.DropUntilOutput<Self, Other> where Failure == Other.Failure {
@@ -10,6 +10,21 @@ extension Publisher {
    /// Omits elements from the upstream publisher until a given closure returns false,
    /// before republishing all remaining elements.
    ///
+    /// Use `drop(while:)` to omit elements from an upstream publisher until the element
+    /// received meets a condition you specify.
+    ///
+    /// In the example below, the operator omits all elements in the stream until
+    /// the first element arrives that’s a positive integer, after which the operator
+    /// publishes all remaining elements:
+    ///
+    ///     let numbers = [-62, -1, 0, 10, 0, 22, 41, -1, 5]
+    ///     cancellable = numbers.publisher
+    ///         .drop { $0 <= 0 }
+    ///         .sink { print("\($0)") }
+    ///
+    ///     // Prints: "10 0 22 41 -1 5"
+    ///
+    ///
    /// - Parameter predicate: A closure that takes an element as a parameter and returns
    ///   a Boolean value indicating whether to drop the element from the publisher’s
    ///   output.
@@ -24,7 +39,33 @@ extension Publisher {
    /// Omits elements from the upstream publisher until an error-throwing closure returns
    /// false, before republishing all remaining elements.
    ///
-    /// If the predicate closure throws, the publisher fails with an error.
+    /// Use `Publisher/tryDrop(while:)` to omit elements from an upstream until
+    /// an error-throwing closure you provide returns false, after which the remaining
+    /// items in the stream are published. If the closure throws, no elements are emitted
+    /// and the publisher fails with an error.
+    ///
+    /// In the example below, elements are ignored until `-1` is encountered in the stream
+    /// and the closure returns `false`. The publisher then republishes the remaining
+    /// elements and finishes normally. Conversely, if the `guard` value in the closure
+    /// had been encountered, the closure would throw and the publisher would fail with
+    /// an error.
+    ///
+    ///     struct RangeError: Error {}
+    ///     var numbers = [1, 2, 3, 4, 5, 6, -1, 7, 8, 9, 10]
+    ///     let range: CountableClosedRange<Int> = (1...100)
+    ///     cancellable = numbers.publisher
+    ///         .tryDrop {
+    ///             guard $0 != 0 else { throw RangeError() }
+    ///             return range.contains($0)
+    ///         }
+    ///         .sink(
+    ///             receiveCompletion: { print ("completion: \($0)") },
+    ///             receiveValue: { print ("value: \($0)") }
+    ///         )
+    ///
+    ///     // Prints: "-1 7 8 9 10 completion: finished"
+    ///     // If instead numbers was [1, 2, 3, 4, 5, 6, 0, -1, 7, 8, 9, 10],
+    ///     // tryDrop(while:) would fail with a RangeError.
    ///
    /// - Parameter predicate: A closure that takes an element as a parameter and returns
    ///   a Boolean value indicating whether to drop the element from the publisher’s
@@ -11,16 +11,100 @@ instantiations = ['Encode', 'Decode']
 }%
 extension Publisher {

-    /// Encodes the output from upstream using a specified `TopLevelEncoder`.
-    /// For example, use `JSONEncoder`.
+    /// Encodes the output from upstream using a specified encoder.
+    ///
+    /// Use `encode(encoder:)` with a `JSONDecoder` (or a `PropertyListDecoder` for
+    /// property lists) to encode an `Encodable` struct into `Data` that could be used to
+    /// make a JSON string (or written to disk as a binary plist in the case of property
+    /// lists).
+    ///
+    /// In this example, a `PassthroughSubject` publishes an `Article`.
+    /// The `encode(encoder:)` operator encodes the properties of the `Article` struct
+    /// into a new JSON string according to the `Codable` protocol adopted by `Article`.
+    /// The operator publishes the resulting JSON string to the downstream subscriber.
+    /// If the encoding operation fails, which can happen in the case of complex
+    /// properties that can’t be directly transformed into JSON, the stream terminates
+    /// and the error is passed to the downstream subscriber.
+    ///
+    ///     struct Article: Codable {
+    ///         let title: String
+    ///         let author: String
+    ///         let pubDate: Date
+    ///     }
+    ///
+    ///     let dataProvider = PassthroughSubject<Article, Never>()
+    ///     let cancellable = dataProvider
+    ///         .encode(encoder: JSONEncoder())
+    ///         .sink(receiveCompletion: { print ("Completion: \($0)") },
+    ///               receiveValue: {  data in
+    ///                 guard let stringRepresentation =
+    ///                     String(data: data, encoding: .utf8) else { return }
+    ///                 print("""
+    ///                       Data received \(data) string representation: \
+    ///                       \(stringRepresentation)
+    ///                       """)
+    ///         })
+    ///
+    ///     dataProvider.send(Article(title: "My First Article",
+    ///                               author: "Gita Kumar",
+    ///                               pubDate: Date()))
+    ///
+    ///     // Prints: "Data received 86 bytes string representation:
+    ///     // {"title":"My First Article","author":"Gita Kumar"
+    ///     // "pubDate":606211803.279603}"
+    ///
+    /// - Parameter encoder: An encoder that implements the `TopLevelEncoder` protocol.
+    /// - Returns: A publisher that encodes received elements using a specified encoder,
+    ///   and publishes the resulting data.
    public func encode<Coder: TopLevelEncoder>(
        encoder: Coder
    ) -> Publishers.Encode<Self, Coder> {
        return .init(upstream: self, encoder: encoder)
    }

-    /// Decodes the output from upstream using a specified `TopLevelDecoder`.
-    /// For example, use `JSONDecoder`.
+    /// Decodes the output from the upstream using a specified decoder.
+    ///
+    /// Use `decode(type:decoder:)` with a `JSONDecoder` (or a `PropertyListDecoder` for
+    /// property lists) to decode data received from a `URLSession.DataTaskPublisher` or
+    /// other data source using the `Decodable` protocol.
+    ///
+    /// In this example, a `PassthroughSubject` publishes a JSON string. The JSON decoder
+    /// parses the string, converting its fields according to the `Decodable` protocol
+    /// implemented by `Article`, and successfully populating a new `Article`.
+    /// The `Publishers.Decode` publisher then publishes the `Article` to the downstream.
+    /// If a decoding operation fails, which happens in the case of missing or malformed
+    /// data in the source JSON string, the stream terminates and passes the error to
+    /// the downstream subscriber.
+    ///
+    ///     struct Article: Codable {
+    ///         let title: String
+    ///         let author: String
+    ///         let pubDate: Date
+    ///     }
+    ///
+    ///     let dataProvider = PassthroughSubject<Data, Never>()
+    ///     cancellable = dataProvider
+    ///         .decode(type: Article.self, decoder: JSONDecoder())
+    ///         .sink(receiveCompletion: { print ("Completion: \($0)")},
+    ///               receiveValue: { print ("value: \($0)") })
+    ///
+    ///     dataProvider.send(Data("""
+    ///                            {\"pubDate\":1574273638.575666, \
+    ///                            \"title\" : \"My First Article\", \
+    ///                            \"author\" : \"Gita Kumar\" }
+    ///                            """.utf8))
+    ///
+    ///     // Prints:
+    ///     // ".sink() data received Article(title: "My First Article",
+    ///     //                                author: "Gita Kumar",
+    ///     //                                pubDate: 2050-11-20 18:13:58 +0000)"
+    ///
+    /// - Parameters:
+    ///   - type: The encoded data to decode into a struct that conforms to
+    ///     the `Decodable` protocol.
+    ///   - decoder:  A decoder that implements the `TopLevelDecoder` protocol.
+    /// - Returns: A publisher that decodes a given type using a specified decoder and
+    ///   publishes the result.
    public func decode<Item: Decodable, Coder: TopLevelDecoder>(
        type: Item.Type,
        decoder: Coder
@@ -87,9 +171,6 @@ extension Publishers.${instantiation} {
          CustomPlaygroundDisplayConvertible
        where Downstream.Input == Output, Downstream.Failure == Error
    {
-        // NOTE: This class has been audited for thread safety.
-        // Combine doesn't use any locking here.
-
        typealias Input = Upstream.Output

        typealias Failure = Upstream.Failure
@@ -98,6 +179,8 @@ extension Publishers.${instantiation} {

        private let ${instantiation.lower()}: (Upstream.Output) throws -> Output

+        private let lock = UnfairLock.allocate()
+
        private var finished = false

        private var subscription: Subscription?
@@ -110,44 +193,72 @@ extension Publishers.${instantiation} {
            self.${instantiation.lower()} = ${instantiation.lower()}
        }

+        deinit {
+            lock.deallocate()
+        }
+
        func receive(subscription: Subscription) {
+            lock.lock()
            if finished || self.subscription != nil {
+                lock.unlock()
                subscription.cancel()
                return
            }
            self.subscription = subscription
+            lock.unlock()
            downstream.receive(subscription: self)
        }

        func receive(_ input: Input) -> Subscribers.Demand {
-            if finished { return .none }
+            lock.lock()
+            if finished {
+                lock.unlock()
+                return .none
+            }
+            lock.unlock()
            do {
                return try downstream.receive(${instantiation.lower()}(input))
            } catch {
+                lock.lock()
                finished = true
+                let subscription = self.subscription
+                self.subscription = nil
+                lock.unlock()
                subscription?.cancel()
-                subscription = nil
                downstream.receive(completion: .failure(error))
                return .none
            }
        }

        func receive(completion: Subscribers.Completion<Failure>) {
-            if finished { return }
+            lock.lock()
+            if finished {
+                lock.unlock()
+                return
+            }
            finished = true
            subscription = nil
+            lock.unlock()
            downstream.receive(completion: completion.eraseError())
        }

        func request(_ demand: Subscribers.Demand) {
+            lock.lock()
+            let subscription = self.subscription
+            lock.unlock()
            subscription?.request(demand)
        }

        func cancel() {
-            guard let subscription = self.subscription, !finished else { return }
-            subscription.cancel()
+            lock.lock()
+            guard !finished, let subscription = self.subscription else {
+                lock.unlock()
+                return
+            }
            self.subscription = nil
            finished = true
+            lock.unlock()
+            subscription.cancel()
        }

        var description: String { return "${instantiation}" }
@@ -9,6 +9,20 @@ extension Publisher {

    /// Republishes all elements that match a provided closure.
    ///
+    /// OpenCombine’s `filter(_:)` operator performs an operation similar to that of
+    /// `filter(_:)` in the Swift Standard Library: it uses a closure to test each element
+    /// to determine whether to republish the element to the downstream subscriber.
+    ///
+    /// The following example, uses a filter operation that receives an `Int` and only
+    /// republishes a value if it’s even.
+    ///
+    ///     let numbers: [Int] = [1, 2, 3, 4, 5]
+    ///     cancellable = numbers.publisher
+    ///         .filter { $0 % 2 == 0 }
+    ///         .sink { print("\($0)", terminator: " ") }
+    ///
+    ///     // Prints: "2 4"
+    ///
    /// - Parameter isIncluded: A closure that takes one element and returns
    ///   a Boolean value indicating whether to republish the element.
    /// - Returns: A publisher that republishes all elements that satisfy the closure.
@@ -20,11 +34,34 @@ extension Publisher {

    /// Republishes all elements that match a provided error-throwing closure.
    ///
+    /// Use `tryFilter(_:)` to filter elements evaluated in an error-throwing closure.
    /// If the `isIncluded` closure throws an error, the publisher fails with that error.
    ///
-    /// - Parameter isIncluded:  A closure that takes one element and returns a
-    ///   Boolean value indicating whether to republish the element.
-    /// - Returns:  A publisher that republishes all elements that satisfy the closure.
+    /// In the example below, `tryFilter(_:)` checks to see if the divisor provided by
+    /// the publisher is zero, and throws a `DivisionByZeroError` and then terminates
+    /// the publisher with the thrown error:
+    ///
+    ///     struct DivisionByZeroError: Error {}
+    ///
+    ///     let numbers: [Int] = [1, 2, 3, 4, 0, 5]
+    ///     cancellable = numbers.publisher
+    ///         .tryFilter {
+    ///             if $0 == 0 {
+    ///                 throw DivisionByZeroError()
+    ///             } else {
+    ///                 return $0 % 2 == 0
+    ///             }
+    ///         }
+    ///         .sink(
+    ///             receiveCompletion: { print ("\($0)") },
+    ///             receiveValue: { print ("\($0)", terminator: " ") }
+    ///          )
+    ///
+    ///     // Prints: "2 4 failure(DivisionByZeroError())".
+    ///
+    /// - Parameter isIncluded: A closure that takes one element and returns a Boolean
+    ///   value that indicated whether to republish the element or throws an error.
+    /// - Returns: A publisher that republishes all elements that satisfy the closure.
    public func tryFilter(
        _ isIncluded: @escaping (Output) throws -> Bool
    ) -> Publishers.TryFilter<Self> {
@@ -142,23 +179,48 @@ extension Publishers {
 }

 extension Publishers.Filter {
-    private final class Inner<Downstream: Subscriber>
-        : FilterProducer<Downstream,
-                         Upstream.Output,
-                         Upstream.Output,
-                         Upstream.Failure,
-                         (Upstream.Output) -> Bool>
+    private struct Inner<Downstream: Subscriber>
+        : Subscriber,
+          CustomStringConvertible,
+          CustomReflectable,
+          CustomPlaygroundDisplayConvertible
        where Upstream.Output == Downstream.Input, Upstream.Failure == Downstream.Failure
    {
-        // NOTE: This class has been audited for thread safety
+        typealias Input = Upstream.Output
+        typealias Failure = Upstream.Failure

-        override func receive(
-            newValue: Upstream.Output
-        ) -> PartialCompletion<Upstream.Output?, Downstream.Failure> {
-            return filter(newValue) ? .continue(newValue) : .continue(nil)
+        private let downstream: Downstream
+        private let filter: (Input) -> Bool
+
+        let combineIdentifier = CombineIdentifier()
+
+        init(downstream: Downstream, filter: @escaping (Input) -> Bool) {
+            self.downstream = downstream
+            self.filter = filter
        }

-        override var description: String { return "Filter" }
+        func receive(subscription: Subscription) {
+            downstream.receive(subscription: subscription)
+        }
+
+        func receive(_ input: Input) -> Subscribers.Demand {
+            if filter(input) {
+                return downstream.receive(input)
+            }
+            return .max(1)
+        }
+
+        func receive(completion: Subscribers.Completion<Failure>) {
+            downstream.receive(completion: completion)
+        }
+
+        var description: String { return "Filter" }
+
+        var customMirror: Mirror {
+            return Mirror(self, children: EmptyCollection())
+        }
+
+        var playgroundDescription: Any { return description }
    }
 }

@@ -171,8 +233,6 @@ extension Publishers.TryFilter {
                         (Upstream.Output) throws -> Bool>
        where Downstream.Input == Upstream.Output, Downstream.Failure == Error
    {
-        // NOTE: This class has been audited for thread safety
-
        override func receive(
            newValue: Upstream.Output
        ) -> PartialCompletion<Upstream.Output?, Error> {
@@ -9,38 +9,89 @@ extension Publisher {

    /// Publishes the first element of a stream, then finishes.
    ///
-    /// If this publisher doesn’t receive any elements, it finishes without publishing.
+    /// Use `first()` to publish just the first element from an upstream publisher, then
+    /// finish normally. The `first()` operator requests `Subscribers.Demand.unlimited`
+    /// from its upstream as soon as downstream requests at least one element.
+    /// If the upstream completes before `first()` receives any elements, it completes
+    /// without emitting any values.
+    ///
+    /// In this example, the `first()` publisher republishes the first element received
+    /// from the sequence publisher, `-10`, then finishes normally.
+    ///
+    ///     let numbers = (-10...10)
+    ///     cancellable = numbers.publisher
+    ///         .first()
+    ///         .sink { print("\($0)") }
+    ///
+    ///     // Print: "-10"
+    ///
    /// - Returns: A publisher that only publishes the first element of a stream.
    public func first() -> Publishers.First<Self> {
        return .init(upstream: self)
    }

-    /// Publishes the first element of a stream to
-    /// satisfy a predicate closure, then finishes.
+    /// Publishes the first element of a stream to satisfy a predicate closure, then
+    /// finishes normally.
    ///
-    /// The publisher ignores all elements after the first.
-    /// If this publisher doesn’t receive any elements,
-    /// it finishes without publishing.
-    /// - Parameter predicate: A closure that takes an element as a parameter and
-    ///   returns a Boolean value that indicates whether to publish the element.
-    /// - Returns: A publisher that only publishes the first element of a stream
-    ///   that satifies the predicate.
+    /// Use `first(where:)` to republish only the first element of a stream that satisfies
+    /// a closure you specify. The publisher ignores all elements after the first element
+    /// that satisfies the closure and finishes normally.
+    /// If this publisher doesn’t receive any elements, it finishes without publishing.
+    ///
+    /// In the example below, the provided closure causes the `Publishers.FirstWhere`
+    /// publisher to republish the first received element that’s greater than `0`,
+    /// then finishes normally.
+    ///
+    ///     let numbers = (-10...10)
+    ///     cancellable = numbers.publisher
+    ///         .first { $0 > 0 }
+    ///         .sink { print("\($0)") }
+    ///
+    ///     // Prints: "1"
+    ///
+
+    /// - Parameter predicate: A closure that takes an element as a parameter and returns
+    ///   a Boolean value that indicates whether to publish the element.
+    /// - Returns: A publisher that only publishes the first element of a stream that
+    ///   satisfies the predicate.
    public func first(
        where predicate: @escaping (Output) -> Bool
    ) -> Publishers.FirstWhere<Self> {
        return .init(upstream: self, predicate: predicate)
    }

-    /// Publishes the first element of a stream to satisfy a
-    /// throwing predicate closure, then finishes.
+    /// Publishes the first element of a stream to satisfy a throwing predicate closure,
+    /// then finishes normally.
    ///
-    /// The publisher ignores all elements after the first. If this publisher
-    /// doesn’t receive any elements, it finishes without publishing. If the
-    /// predicate closure throws, the publisher fails with an error.
-    /// - Parameter predicate: A closure that takes an element as a parameter and
-    ///   returns a Boolean value that indicates whether to publish the element.
-    /// - Returns: A publisher that only publishes the first element of a stream
-    ///   that satifies the predicate.
+    /// Use `tryFirst(where:)` when you need to republish only the first element of
+    /// a stream that satisfies an error-throwing closure you specify.
+    /// The publisher ignores all elements after the first. If this publisher doesn’t
+    /// receive any elements, it finishes without publishing. If the predicate closure
+    /// throws an error, the publisher fails.
+    ///
+    /// In the example below, a range publisher emits the first element in the range then
+    /// finishes normally:
+    ///
+    ///     let numberRange: ClosedRange<Int> = (-1...50)
+    ///     numberRange.publisher
+    ///         .tryFirst {
+    ///             guard $0 < 99 else {throw RangeError()}
+    ///             return true
+    ///         }
+    ///         .sink(
+    ///             receiveCompletion: { print ("completion: \($0)", terminator: " ") },
+    ///             receiveValue: { print ("\($0)", terminator: " ") }
+    ///          )
+    ///
+    ///     // Prints: "-1 completion: finished"
+    ///     // If instead the number range were ClosedRange<Int> = (100...200),
+    ///     // the tryFirst operator would terminate publishing with a RangeError.
+    ///
+
+    /// - Parameter predicate: A closure that takes an element as a parameter and returns
+    ///   a Boolean value that indicates whether to publish the element.
+    /// - Returns: A publisher that only publishes the first element of a stream that
+    /// satisfies the predicate.
    public func tryFirst(
        where predicate: @escaping (Output) throws -> Bool
    ) -> Publishers.TryFirstWhere<Self> {
@@ -5,16 +5,57 @@
 //

 extension Publisher {
-    /// Transforms all elements from an upstream publisher into a new or existing
-    /// publisher.
+
+    /// Transforms all elements from an upstream publisher into a new publisher up to
+    /// a maximum number of publishers you specify.
    ///
-    /// `flatMap` merges the output from all returned publishers into a single stream of
-    /// output.
+    /// OpenCombine‘s `flatMap(maxPublishers:_:)` operator performs a similar function
+    /// to the `flatMap(_:)` operator in the Swift standard library, but turns
+    /// the elements from one kind of publisher into a new publisher that is sent
+    /// to subscribers. Use `flatMap(maxPublishers:_:)` when you want to create a new
+    /// series of events for downstream subscribers based on the received value.
+    /// The closure creates the new `Publishe`` based on the received value.
+    /// The new `Publisher` can emit more than one event, and successful completion of
+    /// the new `Publisher` does not complete the overall stream.
+    /// Failure of the new `Publisher` will fail the overall stream.
+    ///
+    /// In the example below, a `PassthroughSubject` publishes `WeatherStation` elements.
+    /// The `flatMap(maxPublishers:_:)` receives each element, creates a `URL` from it,
+    /// and produces a new `URLSession.DataTaskPublisher`, which will publish the data
+    /// loaded from that `URL`.
+    ///
+    ///     public struct WeatherStation {
+    ///         public let stationID: String
+    ///     }
+    ///
+    ///     var weatherPublisher = PassthroughSubject<WeatherStation, URLError>()
+    ///
+    ///     cancellable = weatherPublisher
+    ///         .flatMap { station -> URLSession.DataTaskPublisher in
+    ///             let url = URL(string: """
+    ///             https://weatherapi.example.com/stations/\(station.stationID)\
+    ///             /observations/latest
+    ///             """)!
+    ///             return URLSession.shared.dataTaskPublisher(for: url)
+    ///         }
+    ///         .sink(
+    ///             receiveCompletion: { completion in
+    ///                 // Handle publisher completion (normal or error).
+    ///             },
+    ///             receiveValue: {
+    ///                 // Process the received data.
+    ///             }
+    ///          )
+    ///
+    ///     weatherPublisher.send(WeatherStation(stationID: "KSFO")) // San Francisco, CA
+    ///     weatherPublisher.send(WeatherStation(stationID: "EGLC")) // London, UK
+    ///     weatherPublisher.send(WeatherStation(stationID: "ZBBB")) // Beijing, CN
    ///
    /// - Parameters:
-    ///   - maxPublishers: The maximum number of publishers produced by this method.
-    ///   - transform: A closure that takes an element as a parameter and returns a
-    ///     publisher that produces elements of that type.
+    ///   - maxPublishers: Specifies the maximum number of concurrent publisher
+    ///     subscriptions, or `Subscribers.Demand.unlimited` if unspecified.
+    ///   - transform: A closure that takes an element as a parameter and returns
+    ///     a publisher that produces elements of that type.
    /// - Returns: A publisher that transforms elements from an upstream publisher into
    ///   a publisher of that element’s type.
    public func flatMap<Result, Child: Publisher>(
@@ -28,7 +69,72 @@ extension Publisher {
    }
 }

+extension Publisher where Failure == Never {
+
+    /// Transforms all elements from an upstream publisher into a new publisher up to
+    /// a maximum number of publishers you specify.
+    ///
+    /// - Parameters:
+    ///   - maxPublishers: Specifies the maximum number of concurrent publisher
+    ///     subscriptions, or `Subscribers.Demand.unlimited` if unspecified.
+    ///   - transform: A closure that takes an element as a parameter and returns
+    ///     a publisher that produces elements of that type.
+    /// - Returns: A publisher that transforms elements from an upstream publisher into
+    ///   a publisher of that element’s type.
+    public func flatMap<Child: Publisher>(
+        maxPublishers: Subscribers.Demand = .unlimited,
+        _ transform: @escaping (Output) -> Child
+    ) -> Publishers.FlatMap<Child, Publishers.SetFailureType<Self, Child.Failure>> {
+        return setFailureType(to: Child.Failure.self)
+            .flatMap(maxPublishers: maxPublishers, transform)
+    }
+
+    /// Transforms all elements from an upstream publisher into a new publisher up to
+    /// a maximum number of publishers you specify.
+    ///
+    /// - Parameters:
+    ///   - maxPublishers: Specifies the maximum number of concurrent publisher
+    ///     subscriptions, or `Subscribers.Demand.unlimited` if unspecified.
+    ///   - transform: A closure that takes an element as a parameter and returns
+    ///     a publisher that produces elements of that type.
+    /// - Returns: A publisher that transforms elements from an upstream publisher
+    ///   into a publisher of that element’s type.
+    public func flatMap<Child: Publisher>(
+        maxPublishers: Subscribers.Demand = .unlimited,
+        _ transform: @escaping (Output) -> Child
+    ) -> Publishers.FlatMap<Child, Self> where Child.Failure == Never {
+        return .init(upstream: self, maxPublishers: maxPublishers, transform: transform)
+    }
+}
+
+extension Publisher {
+
+    /// Transforms all elements from an upstream publisher into a new publisher up to
+    /// a maximum number of publishers you specify.
+    ///
+    /// - Parameters:
+    ///   - maxPublishers: Specifies the maximum number of concurrent publisher
+    ///     subscriptions, or `Subscribers.Demand.unlimited` if unspecified.
+    ///   - transform: A closure that takes an element as a parameter and returns
+    ///     a publisher that produces elements of that type.
+    /// - Returns: A publisher that transforms elements from an upstream publisher into
+    ///   a publisher of that element’s type.
+    public func flatMap<Child: Publisher>(
+        maxPublishers: Subscribers.Demand = .unlimited,
+        _ transform: @escaping (Output) -> Child
+    ) -> Publishers.FlatMap<Publishers.SetFailureType<Child, Failure>, Self>
+        where Child.Failure == Never
+    {
+        return flatMap(maxPublishers: maxPublishers) {
+            transform($0).setFailureType(to: Failure.self)
+        }
+    }
+}
+
 extension Publishers {
+
+    /// A publisher that transforms elements from an upstream publisher into a new
+    /// publisher.
    public struct FlatMap<Child: Publisher, Upstream: Publisher>: Publisher
        where Child.Failure == Upstream.Failure
    {
@@ -9,6 +9,35 @@ extension Publisher {

    /// Performs the specified closures when publisher events occur.
    ///
+    /// Use `handleEvents` when you want to examine elements as they progress through
+    /// the stages of the publisher’s lifecycle.
+    ///
+    /// In the example below, a publisher of integers shows the effect of printing
+    /// debugging information at each stage of the element-processing lifecycle:
+    ///
+    ///     let integers = (0...2)
+    ///     cancellable = integers.publisher
+    ///         .handleEvents(receiveSubscription: { subs in
+    ///             print("Subscription: \(subs.combineIdentifier)")
+    ///         }, receiveOutput: { anInt in
+    ///             print("in output handler, received \(anInt)")
+    ///         }, receiveCompletion: { _ in
+    ///             print("in completion handler")
+    ///         }, receiveCancel: {
+    ///             print("received cancel")
+    ///         }, receiveRequest: { (demand) in
+    ///             print("received demand: \(demand.description)")
+    ///         })
+    ///         .sink { _ in return }
+    ///
+    ///     // Prints:
+    ///     //   received demand: unlimited
+    ///     //   Subscription: 0x7f81284734c0
+    ///     //   in output handler, received 0
+    ///     //   in output handler, received 1
+    ///     //   in output handler, received 2
+    ///     //   in completion handler
+    ///
    /// - Parameters:
    ///   - receiveSubscription: A closure that executes when the publisher receives
    ///     the subscription from the upstream publisher. Defaults to `nil`.
@@ -91,7 +120,6 @@ extension Publishers {
                  Upstream.Output == Downstream.Input
        {
            let inner = Inner(self, downstream: subscriber)
-            subscriber.receive(subscription: inner)
            upstream.subscribe(inner)
        }
    }
@@ -110,13 +138,21 @@ extension Publishers.HandleEvents {
        typealias Failure = Upstream.Failure

        private var status = SubscriptionStatus.awaitingSubscription
-        private var pendingDemand = Subscribers.Demand.none
        private let lock = UnfairLock.allocate()
-        private var events: Publishers.HandleEvents<Upstream>?
+        public var receiveSubscription: ((Subscription) -> Void)?
+        public var receiveOutput: ((Upstream.Output) -> Void)?
+        public var receiveCompletion:
+            ((Subscribers.Completion<Upstream.Failure>) -> Void)?
+        public var receiveCancel: (() -> Void)?
+        public var receiveRequest: ((Subscribers.Demand) -> Void)?
        private let downstream: Downstream

        init(_ events: Publishers.HandleEvents<Upstream>, downstream: Downstream) {
-            self.events = events
+            self.receiveSubscription = events.receiveSubscription
+            self.receiveOutput = events.receiveOutput
+            self.receiveCompletion = events.receiveCompletion
+            self.receiveCancel = events.receiveCancel
+            self.receiveRequest = events.receiveRequest
            self.downstream = downstream
        }

@@ -125,61 +161,83 @@ extension Publishers.HandleEvents {
        }

        func receive(subscription: Subscription) {
-            events?.receiveSubscription?(subscription)
            lock.lock()
+            if let receiveSubscription = self.receiveSubscription {
+                lock.unlock()
+                receiveSubscription(subscription)
+                lock.lock()
+            }
            guard case .awaitingSubscription = status else {
                lock.unlock()
                subscription.cancel()
                return
            }
            status = .subscribed(subscription)
-            let pendingDemand = self.pendingDemand
-            self.pendingDemand = .none
            lock.unlock()
-            if pendingDemand > 0 {
-                subscription.request(pendingDemand)
-            }
+            downstream.receive(subscription: self)
        }

-        func receive(_ input: Upstream.Output) -> Subscribers.Demand {
-            events?.receiveOutput?(input)
+        func receive(_ input: Input) -> Subscribers.Demand {
+            lock.lock()
+            if let receiveOutput = self.receiveOutput {
+                lock.unlock()
+                receiveOutput(input)
+            } else {
+                lock.unlock()
+            }
            let newDemand = downstream.receive(input)
-            if newDemand > 0 {
-                events?.receiveRequest?(newDemand)
+            if newDemand == .none {
+                return newDemand
+            }
+            lock.lock()
+            if let receiveRequest = self.receiveRequest {
+                lock.unlock()
+                receiveRequest(newDemand)
+            } else {
+                lock.unlock()
            }
            return newDemand
        }

-        func receive(completion: Subscribers.Completion<Upstream.Failure>) {
-            events?.receiveCompletion?(completion)
+        func receive(completion: Subscribers.Completion<Failure>) {
            lock.lock()
-            events = nil
-            status = .terminal
+            if let receiveCompletion = self.receiveCompletion {
+                lock.unlock()
+                receiveCompletion(completion)
+                lock.lock()
+            }
+            lockedTerminate()
            lock.unlock()
            downstream.receive(completion: completion)
        }

        func request(_ demand: Subscribers.Demand) {
-            events?.receiveRequest?(demand)
            lock.lock()
-            if case let .subscribed(subscription) = status {
+            if let receiveRequest = self.receiveRequest {
                lock.unlock()
-                subscription.request(demand)
-                return
+                receiveRequest(demand)
+                lock.lock()
            }
-            pendingDemand += demand
-            lock.unlock()
-        }
-
-        func cancel() {
-            events?.receiveCancel?()
-            lock.lock()
            guard case let .subscribed(subscription) = status else {
                lock.unlock()
                return
            }
-            events = nil
-            status = .terminal
+            lock.unlock()
+            subscription.request(demand)
+        }
+
+        func cancel() {
+            lock.lock()
+            if let receiveCancel = self.receiveCancel {
+                lock.unlock()
+                receiveCancel()
+                lock.lock()
+            }
+            guard case let .subscribed(subscription) = status else {
+                lock.unlock()
+                return
+            }
+            lockedTerminate()
            lock.unlock()
            subscription.cancel()
        }
@@ -189,5 +247,14 @@ extension Publishers.HandleEvents {
        var customMirror: Mirror { return Mirror(self, children: EmptyCollection()) }

        var playgroundDescription: Any { return description }
+
+        private func lockedTerminate() {
+            receiveSubscription = nil
+            receiveOutput       = nil
+            receiveCompletion   = nil
+            receiveCancel       = nil
+            receiveRequest      = nil
+            status = .terminal
+        }
    }
 }
@@ -6,10 +6,33 @@

 extension Publisher {

-    /// Ingores all upstream elements, but passes along a completion
-    /// state (finished or failed).
+    /// Ingores all upstream elements, but passes along a completion state (finished or
+    /// failed).
+    ///
+    /// Use the `ignoreOutput(`` operator to determine if a publisher is able to complete
+    /// successfully or would fail.
+    ///
+    /// In the example below, the array publisher (`numbers`) delivers the first five of
+    /// its elements successfully, as indicated by the `ignoreOutput()` operator.
+    /// The operator consumes, but doesn’t republish the elements downstream. However,
+    /// the sixth element, `0`, causes the error throwing closure to catch
+    /// a `NoZeroValuesAllowedError` that terminates the stream.
+    ///
+    ///     struct NoZeroValuesAllowedError: Error {}
+    ///     let numbers = [1, 2, 3, 4, 5, 0, 6, 7, 8, 9]
+    ///     cancellable = numbers.publisher
+    ///         .tryFilter({ anInt in
+    ///             guard anInt != 0 else { throw NoZeroValuesAllowedError() }
+    ///             return anInt < 20
+    ///         })
+    ///         .ignoreOutput()
+    ///         .sink(receiveCompletion: {print("completion: \($0)")},
+    ///               receiveValue: {print("value \($0)")})
+    ///
+    ///     // Prints: "completion: failure(NoZeroValuesAllowedError())"
    ///
    /// The output type of this publisher is `Never`.
+    ///
    /// - Returns: A publisher that ignores all upstream elements.
    public func ignoreOutput() -> Publishers.IgnoreOutput<Self> {
        return .init(upstream: self)
@@ -41,44 +64,27 @@ extension Publishers {
 }

 extension Publishers.IgnoreOutput {
-    private final class Inner<Downstream: Subscriber>
+    private struct Inner<Downstream: Subscriber>
        : Subscriber,
-          Subscription,
          CustomStringConvertible,
          CustomReflectable,
          CustomPlaygroundDisplayConvertible
        where Downstream.Input == Never, Downstream.Failure == Upstream.Failure
    {
-        // NOTE: This class has been audited for thread safety
-
        typealias Input = Upstream.Output

        typealias Failure = Upstream.Failure

        private let downstream: Downstream

-        private var status = SubscriptionStatus.awaitingSubscription
-
-        private let lock = UnfairLock.allocate()
+        let combineIdentifier = CombineIdentifier()

        fileprivate init(downstream: Downstream) {
            self.downstream = downstream
        }

-        deinit {
-            lock.deallocate()
-        }
-
        func receive(subscription: Subscription) {
-            lock.lock()
-            guard case .awaitingSubscription = status else {
-                lock.unlock()
-                subscription.cancel()
-                return
-            }
-            status = .subscribed(subscription)
-            lock.unlock()
-            downstream.receive(subscription: self)
+            downstream.receive(subscription: subscription)
            subscription.request(.unlimited)
        }

@@ -87,42 +93,13 @@ extension Publishers.IgnoreOutput {
        }

        func receive(completion: Subscribers.Completion<Failure>) {
-            lock.lock()
-            guard case .subscribed = status else {
-                lock.unlock()
-                return
-            }
-            status = .terminal
-            lock.unlock()
            downstream.receive(completion: completion)
        }

-        func request(_ demand: Subscribers.Demand) {
-            // ignore and requests from downstream since we'll never send
-            // any values
-        }
-
-        func cancel() {
-            lock.lock()
-            guard case let .subscribed(subscription) = status else {
-                lock.unlock()
-                return
-            }
-            status = .terminal
-            lock.unlock()
-            subscription.cancel()
-        }
-
        var description: String { return "IgnoreOutput" }

        var customMirror: Mirror {
-            lock.lock()
-            defer { lock.unlock() }
-            let children: [Mirror.Child] = [
-                ("downstream", downstream),
-                ("status", status)
-            ]
-            return Mirror(self, children: children)
+            return Mirror(self, children: EmptyCollection())
        }

        var playgroundDescription: Any { return description }
@@ -7,33 +7,82 @@

 extension Publisher {

-    /// Only publishes the last element of a stream, after the stream finishes.
+    /// Publishes the last element of a stream, after the stream finishes.
+    ///
+    /// Use `last()` when you need to emit only the last element from an upstream
+    /// publisher.
+    ///
+    /// In the example below, the range publisher only emits the last element from
+    /// the sequence publisher, `10`, then finishes normally.
+    ///
+    ///     let numbers = (-10...10)
+    ///     cancellable = numbers.publisher
+    ///         .last()
+    ///         .sink { print("\($0)") }
+    ///
+    ///     // Prints: "10"
+    ///
    /// - Returns: A publisher that only publishes the last element of a stream.
    public func last() -> Publishers.Last<Self> {
        return .init(upstream: self)
    }

-    /// Only publishes the last element of a stream that satisfies a predicate closure,
-    /// after the stream finishes.
+    /// Publishes the last element of a stream that satisfies a predicate closure,
+    /// after upstream finishes.
+    ///
+    /// Use `last(where:)` when you need to republish only the last element of a stream
+    /// that satisfies a closure you specify.
+    ///
+    /// In the example below, a range publisher emits the last element that satisfies
+    /// the closure’s criteria, then finishes normally:
+    ///
+    ///     let numbers = (-10...10)
+    ///     cancellable = numbers.publisher
+    ///         .last { $0 < 6 }
+    ///         .sink { print("\($0)") }
+    ///
+    ///     // Prints: "5"
    ///
    /// - Parameter predicate: A closure that takes an element as its parameter and
-    ///   returns a Boolean value indicating whether to publish the element.
-    /// - Returns: A publisher that only publishes the last element satisfying
-    ///   the given predicate.
+    ///   returns a Boolean value that indicates whether to publish the element.
+    /// - Returns: A publisher that only publishes the last element satisfying the given
+    ///   predicate.
    public func last(
        where predicate: @escaping (Output) -> Bool
    ) -> Publishers.LastWhere<Self> {
        return .init(upstream: self, predicate: predicate)
    }

-    /// Only publishes the last element of a stream that satisfies an error-throwing
-    /// predicate closure, after the stream finishes.
+    /// Publishes the last element of a stream that satisfies an error-throwing predicate
+    /// closure, after the stream finishes.
+    ///
+    /// Use `tryLast(where:)` when you need to republish the last element that satisfies
+    /// an error-throwing closure you specify. If the predicate closure throws an error,
+    /// the publisher fails.
+    ///
+    /// In the example below, a publisher emits the last element that satisfies
+    /// the error-throwing closure, then finishes normally:
+    ///
+    ///     struct RangeError: Error {}
+    ///
+    ///     let numbers = [-62, 1, 6, 10, 9, 22, 41, -1, 5]
+    ///     cancellable = numbers.publisher
+    ///         .tryLast {
+    ///             guard 0 != 0  else {throw RangeError()}
+    ///             return true
+    ///         }
+    ///         .sink(
+    ///             receiveCompletion: { print ("completion: \($0)", terminator: " ") },
+    ///             receiveValue: { print ("\($0)", terminator: " ") }
+    ///         )
+    ///     // Prints: "5 completion: finished"
+    ///     // If instead the numbers array had contained a `0`, the `tryLast` operator
+    ///     // would terminate publishing with a RangeError."
    ///
-    /// If the predicate closure throws, the publisher fails with the thrown error.
    /// - Parameter predicate: A closure that takes an element as its parameter and
-    ///   returns a Boolean value indicating whether to publish the element.
-    /// - Returns: A publisher that only publishes the last element satisfying
-    ///   the given predicate.
+    ///   returns a Boolean value that indicates whether to publish the element.
+    /// - Returns: A publisher that only publishes the last element satisfying the given
+    ///   predicate.
    public func tryLast(
        where predicate: @escaping (Output) throws -> Bool
    ) -> Publishers.TryLastWhere<Self> {
@@ -9,6 +9,34 @@ extension Publisher where Failure == Never {

    /// Creates a connectable wrapper around the publisher.
    ///
+    /// In the following example, `makeConnectable()` wraps its upstream publisher
+    /// (an instance of `Publishers.Share`) with a `ConnectablePublisher`. Without this,
+    /// the first sink subscriber would receive all the elements from the sequence
+    /// publisher and cause it to complete before the second subscriber attaches.
+    /// By making the publisher connectable, the publisher doesn’t produce any elements
+    /// until after the `connect()` call.
+    ///
+    ///      let subject = Just<String>("Sent")
+    ///      let pub = subject
+    ///          .share()
+    ///          .makeConnectable()
+    ///      cancellable1 = pub.sink { print ("Stream 1 received: \($0)")  }
+    ///
+    ///      // For example purposes, use DispatchQueue to add a second subscriber
+    ///      // a second later, and then connect to the publisher a second after that.
+    ///      DispatchQueue.main.asyncAfter(deadline: .now() + 1) {
+    ///          self.cancellable2 = pub.sink { print ("Stream 2 received: \($0)") }
+    ///      }
+    ///      DispatchQueue.main.asyncAfter(deadline: .now() + 2) {
+    ///          self.connectable = pub.connect()
+    ///      }
+    ///      // Prints:
+    ///      // Stream 2 received: Sent
+    ///      // Stream 1 received: Sent
+    ///
+    ///  > Note: The `connect()` operator returns a `Cancellable` instance that you must
+    ///  retain. You can also use this instance to cancel publishing.
+    ///
    /// - Returns: A `ConnectablePublisher` wrapping this publisher.
    public func makeConnectable() -> Publishers.MakeConnectable<Self> {
        return .init(upstream: self)
@@ -17,6 +45,15 @@ extension Publisher where Failure == Never {

 extension Publishers {

+    /// A publisher that provides explicit connectability to another publisher.
+    ///
+    /// `Publishers.MakeConnectable` is a `ConnectablePublisher`, which allows you to
+    /// perform configuration before publishing any elements. Call `connect()` on this
+    /// publisher when you want to attach to its upstream publisher and start producing
+    /// elements.
+    ///
+    /// Use the `makeConnectable()` operator to wrap an upstream publisher with
+    /// an instance of this publisher.
    public struct MakeConnectable<Upstream: Publisher>: ConnectablePublisher {

        public typealias Output = Upstream.Output
@@ -25,6 +62,9 @@ extension Publishers {

        private let inner: Multicast<Upstream, PassthroughSubject<Output, Failure>>

+        /// Creates a connectable publisher, attached to the provide upstream publisher.
+        ///
+        /// - Parameter upstream: The publisher from which to receive elements.
        public init(upstream: Upstream) {
            inner = upstream.multicast(subject: .init())
        }
@@ -9,6 +9,30 @@ extension Publisher {

    /// Transforms all elements from the upstream publisher with a provided closure.
    ///
+    /// OpenCombine’s `map(_:)` operator performs a function similar to that of `map(_:)`
+    /// in the Swift standard library: it uses a closure to transform each element it
+    /// receives from the upstream publisher. You use `map(_:)` to transform from one kind
+    /// of element to another.
+    ///
+    /// The following example uses an array of numbers as the source for a collection
+    /// based publisher. A `map(_:)` operator consumes each integer from the publisher and
+    /// uses a dictionary to transform it from its Arabic numeral to a Roman equivalent,
+    /// as a `String`.
+    /// If the `map(_:)`’s closure fails to look up a Roman numeral, it returns the string
+    /// `(unknown)`.
+    ///
+    ///     let numbers = [5, 4, 3, 2, 1, 0]
+    ///     let romanNumeralDict: [Int : String] =
+    ///        [1:"I", 2:"II", 3:"III", 4:"IV", 5:"V"]
+    ///     cancellable = numbers.publisher
+    ///         .map { romanNumeralDict[$0] ?? "(unknown)" }
+    ///         .sink { print("\($0)", terminator: " ") }
+    ///
+    ///     // Prints: "V IV III II I (unknown)"
+    ///
+    /// If your closure can throw an error, use OpenCombine’s `tryMap(_:)` operator
+    /// instead.
+    ///
    /// - Parameter transform: A closure that takes one element as its parameter and
    ///   returns a new element.
    /// - Returns: A publisher that uses the provided closure to map elements from
@@ -19,14 +43,47 @@ extension Publisher {
        return Publishers.Map(upstream: self, transform: transform)
    }

-    /// Transforms all elements from the upstream publisher with a provided
-    /// error-throwing closure.
+    /// Transforms all elements from the upstream publisher with a provided error-throwing
+    /// closure.
    ///
-    /// If the `transform` closure throws an error, the publisher fails with the thrown
-    /// error.
+    /// OpenCombine’s `tryMap(_:)` operator performs a function similar to that of
+    /// `map(_:)` in the Swift standard library: it uses a closure to transform each
+    /// element it receives from the upstream publisher. You use `tryMap(_:)` to transform
+    /// from one kind of element to another, and to terminate publishing when the map’s
+    /// closure throws an error.
+    ///
+    /// The following example uses an array of numbers as the source for a collection
+    /// based publisher. A `tryMap(_:)` operator consumes each integer from the publisher
+    /// and uses a dictionary to transform it from its Arabic numeral to a Roman
+    /// equivalent, as a `String`.
+    /// If the `tryMap(_:)`’s closure fails to look up a Roman numeral, it throws
+    /// an error. The `tryMap(_:)` operator catches this error and terminates publishing,
+    /// sending a `Subscribers.Completion.failure(_:)` that wraps the error.
+    ///
+    ///     struct ParseError: Error {}
+    ///     func romanNumeral(from:Int) throws -> String {
+    ///         let romanNumeralDict: [Int : String] =
+    ///             [1:"I", 2:"II", 3:"III", 4:"IV", 5:"V"]
+    ///         guard let numeral = romanNumeralDict[from] else {
+    ///             throw ParseError()
+    ///         }
+    ///         return numeral
+    ///     }
+    ///     let numbers = [5, 4, 3, 2, 1, 0]
+    ///     cancellable = numbers.publisher
+    ///         .tryMap { try romanNumeral(from: $0) }
+    ///         .sink(
+    ///             receiveCompletion: { print ("completion: \($0)") },
+    ///             receiveValue: { print ("\($0)", terminator: " ") }
+    ///          )
+    ///
+    ///     // Prints: "V IV III II I completion: failure(ParseError())"
+    ///
+    /// If your closure doesn’t throw, use `map(_:)` instead.
    ///
    /// - Parameter transform: A closure that takes one element as its parameter and
-    ///   returns a new element.
+    ///   returns a new element. If the closure throws an error, the publisher fails with
+    ///   the thrown error.
    /// - Returns: A publisher that uses the provided closure to map elements from
    ///   the upstream publisher to new elements that it then publishes.
    public func tryMap<Result>(
@@ -34,6 +91,30 @@ extension Publisher {
    ) -> Publishers.TryMap<Self, Result> {
        return Publishers.TryMap(upstream: self, transform: transform)
    }
+
+    /// Replaces `nil` elements in the stream with the provided element.
+    ///
+    /// The `replaceNil(with:)` operator enables replacement of `nil` values in a stream
+    /// with a substitute value. In the example below, a collection publisher contains
+    /// a `nil` value. The `replaceNil(with:)` operator replaces this with `0.0`.
+    ///
+    ///     let numbers: [Double?] = [1.0, 2.0, nil, 3.0]
+    ///     numbers.publisher
+    ///         .replaceNil(with: 0.0)
+    ///         .sink { print("\($0)", terminator: " ") }
+    ///
+    ///     // Prints: "Optional(1.0) Optional(2.0) Optional(0.0) Optional(3.0)"
+    ///
+    /// - Parameter output: The element to use when replacing `nil`.
+    /// - Returns: A publisher that replaces `nil` elements from the upstream publisher
+    ///   with the provided element.
+    public func replaceNil<ElementOfResult>(
+        with output: ElementOfResult
+    ) -> Publishers.Map<Self, ElementOfResult>
+        where Output == ElementOfResult?
+    {
+        return Publishers.Map(upstream: self) { $0 ?? output }
+    }
 }

 extension Publishers {
@@ -218,13 +299,7 @@ extension Publishers.TryMap {
                return try downstream.receive(map(input))
            } catch {
                lock.lock()
-                let subscription: Subscription?
-                switch status {
-                case let .subscribed(upstreamSubscription):
-                    subscription = upstreamSubscription
-                case .awaitingSubscription, .terminal:
-                    subscription = nil
-                }
+                let subscription = status.subscription
                status = .terminal
                lock.unlock()
                subscription?.cancel()
@@ -45,13 +45,44 @@ extension Publisher {

    /// Converts any failure from the upstream publisher into a new error.
    ///
-    /// Until the upstream publisher finishes normally or fails with an error,
-    /// the returned publisher republishes all the elements it receives.
+    /// Use the `mapError(_:)` operator when you need to replace one error type with
+    /// another, or where a downstream operator needs the error types of its inputs to
+    /// match.
    ///
-    /// - Parameter transform: A closure that takes the upstream failure as a
-    /// parameter and returns a new error for the publisher to terminate with.
-    /// - Returns: A publisher that replaces any upstream failure with a
-    /// new error produced by the `transform` closure.
+    /// The following example uses a `tryMap(_:)` operator to divide `1` by each element
+    /// produced by a sequence publisher. When the publisher produces a `0`,
+    /// the `tryMap(_:)` fails with a `DivisionByZeroError`. The `mapError(_:)` operator
+    /// converts this into a `MyGenericError`.
+    ///
+    ///     struct DivisionByZeroError: Error {}
+    ///     struct MyGenericError: Error { var wrappedError: Error }
+    ///
+    ///     func myDivide(_ dividend: Double, _ divisor: Double) throws -> Double {
+    ///         guard divisor != 0 else { throw DivisionByZeroError() }
+    ///         return dividend / divisor
+    ///     }
+    ///
+    ///     let divisors: [Double] = [5, 4, 3, 2, 1, 0]
+    ///     divisors.publisher
+    ///         .tryMap { try myDivide(1, $0) }
+    ///         .mapError { MyGenericError(wrappedError: $0) }
+    ///         .sink(
+    ///             receiveCompletion: { print ("completion: \($0)") ,
+    ///             receiveValue: { print ("value: \($0)") }
+    ///          )
+    ///
+    ///     // Prints:
+    ///     //   value: 0.2
+    ///     //   value: 0.25
+    ///     //   value: 0.3333333333333333
+    ///     //   value: 0.5
+    ///     //   value: 1.0
+    ///     //   completion: failure(MyGenericError(wrappedError: DivisionByZeroError()))"
+    ///
+    /// - Parameter transform: A closure that takes the upstream failure as a parameter
+    ///   and returns a new error for the publisher to terminate with.
+    /// - Returns: A publisher that replaces any upstream failure with a new error
+    ///   produced by the `transform` closure.
    public func mapError<NewFailure: Error>(
        _ transform: @escaping (Failure) -> NewFailure
    ) -> Publishers.MapError<Self, NewFailure>
@@ -12,6 +12,10 @@ from gyb_opencombine_support import (
    list_with_suffix_variadic
 )

+import random
+
+RNG = random.Random(0)
+
 instantiations = [(1, '', ''),
                  (2, 'two', 'second '),
                  (3, 'three', 'third ')]
@@ -41,18 +45,66 @@ extension Publisher {
 %
 %     publisher_name = make_publisher_name(arity)
 %
-%     doc_cardinal = 'a keyt path' if arity == 1 else cardinal + ' key paths'
-
-    /// Returns a publisher that publishes the values of ${doc_cardinal} as a tuple.
+%     doc_cardinal = 'a key path' if arity == 1 else cardinal + ' key paths'
+%
+%     doc_comment_suffix = 'value of the key path' \
+%         if arity == 1 else 'values of {} key paths as a tuple'.format(cardinal)
+%
+%     doc_output_values = 'the value of this `Int`' \
+%         if arity == 1 else 'these {} values (as an `({})` tuple)' \
+%             .format(cardinal, ', '.join(['Int'] * arity))
+%
+%     example_dice_roll_properties = ['die'] \
+%         if arity == 1 else ['die{}'.format(i + 1) for i in range(arity)]
+%     enumerated_example_dice_roll_properties = '`die` member' \
+%         if arity == 1 else ('`die1` and `die2`' if arity == 2 else \
+%             ''.join(['`{}`, '.format(s) for s in example_dice_roll_properties[:-1]]) + \
+%                 'and `{}`'.format(example_dice_roll_properties[-1])) + ' members'
+%
+%     dice_roll_init = ',\n    ///                                 ' \
+%         .join([s + ': Int.random(in: 1...6)' for s in example_dice_roll_properties])
+    /// Publishes the ${doc_comment_suffix}.
+    ///
+    /// In the following example, the `map(${'_:' * arity})` operator uses the Swift
+    /// key path syntax to access the ${enumerated_example_dice_roll_properties}
+    /// of the `DiceRoll` structure published by the `Just` publisher.
+    ///
+    /// The downstream sink subscriber receives only
+    /// ${doc_output_values},
+    /// not the entire `DiceRoll`.
+    ///
+    ///     struct DiceRoll {
+%     for prop in example_dice_roll_properties:
+    ///         let ${prop}: Int
+%     end
+    ///     }
+    ///
+    ///     cancellable = Just(DiceRoll(${dice_roll_init}))
+    ///         .map(${', '.join(['\.' + s for s in example_dice_roll_properties])})
+%     if arity == 1:
+    ///         .sink {
+    ///             print ("Rolled: \($0)")
+    ///         }
+%     else:
+%         closure_args = ['values.{}'.format(i) for i in range(arity)]
+    ///         .sink { values in
+    ///             print("""
+    ///             Rolled: ${', '.join(['\\({})'.format(s) for s in closure_args])} \
+    ///             (total \(${' + '.join(closure_args)}))
+    ///             """)
+    ///         }
+%     end
+%     random_numbers = [RNG.randint(1, 6) for _ in range(arity)]
+%     random_numbers_printed = ', '.join([str(i) for i in random_numbers]) + \
+%         (' (or some other random value)' if arity == 1 \
+%             else ' (total: {})" (or other random values)'.format(sum(random_numbers)))
+    ///     // Prints "Rolled: ${random_numbers_printed}.
    ///
    /// - Parameters:
 %     for i in range(arity):
-%         ordinal = 'another ' if i == 1 else 'a ' + instantiations[i][2]
-    ///    - ${key_path_var(i, arity)}: The key path of ${ordinal}property on `Output`
+%         ordinal = 'another ' if i == 1 and arity < 3 else 'a ' + instantiations[i][2]
+    ///    - ${key_path_var(i, arity)}: The key path of ${ordinal}property on `Output`.
 %     end
-%
-%   doc_comment_suffix = 'value of the key path' \
-%       if arity == 1 else 'values of {} key paths as a tuple'.format(cardinal)
    /// - Returns: A publisher that publishes the ${doc_comment_suffix}.
    public func map<${cs_result_types}>(
        ${method_args_joined}
@@ -10,11 +10,30 @@ extension Publisher {
    /// Measures and emits the time interval between events received from an upstream
    /// publisher.
    ///
-    /// The output type of the returned scheduler is the time interval of the provided
+    /// Use `measureInterval(using:options:)` to measure the time between events delivered
+    /// from an upstream publisher.
+    ///
+    /// In the example below, a 1-second `Timer` is used as the data source for an event
+    /// publisher; the `measureInterval(using:options:)` operator reports the elapsed time
+    /// between the reception of events on the main run loop:
+    ///
+    ///     cancellable = Timer.publish(every: 1, on: .main, in: .default)
+    ///         .autoconnect()
+    ///         .measureInterval(using: RunLoop.main)
+    ///         .sink { print("\($0)", terminator: "\n") }
+    ///
+    ///     // Prints:
+    ///     //      Stride(magnitude: 1.0013610124588013)
+    ///     //      Stride(magnitude: 0.9992760419845581)
+    ///
+    /// The output type of the returned publisher is the time interval of the provided
    /// scheduler.
    ///
+    /// This operator uses the provided scheduler’s `now` property to measure intervals
+    /// between events.
+    ///
    /// - Parameters:
-    ///   - scheduler: The scheduler on which to deliver elements.
+    ///   - scheduler: A scheduler to use for tracking the timing of events.
    ///   - options: Options that customize the delivery of elements.
    /// - Returns: A publisher that emits elements representing the time interval between
    ///   the elements it receives.
@@ -39,9 +58,15 @@ extension Publishers {
        /// The publisher from which this publisher receives elements.
        public let upstream: Upstream

-        /// The scheduler on which to deliver elements.
+        /// The scheduler used for tracking the timing of events.
        public let scheduler: Context

+        /// Creates a publisher that measures and emits the time interval between events
+        /// received from an upstream publisher.
+        ///
+        /// - Parameters:
+        ///   - upstream: The publisher from which this publisher receives elements.
+        ///   - scheduler: A scheduler to use for tracking the timing of events.
        public init(upstream: Upstream, scheduler: Context) {
            self.upstream = upstream
            self.scheduler = scheduler
@@ -51,7 +76,7 @@ extension Publishers {
            where Upstream.Failure == Downstream.Failure,
                  Downstream.Input == Context.SchedulerTimeType.Stride
        {
-            upstream.subscribe(Inner(self, downstream: subscriber))
+            upstream.subscribe(Inner(scheduler: scheduler, downstream: subscriber))
        }
    }
 }
@@ -66,27 +91,22 @@ extension Publishers.MeasureInterval {
        where Downstream.Input == Context.SchedulerTimeType.Stride,
              Downstream.Failure == Upstream.Failure
    {
-        // NOTE: This class has been audited for thread safety
-
        typealias Input = Upstream.Output
        typealias Failure = Upstream.Failure

-        typealias MeasureInterval = Publishers.MeasureInterval<Upstream, Context>
-
-        private enum State {
-            case ready(MeasureInterval, Downstream)
-            case subscribed(MeasureInterval, Downstream, Subscription)
-            case terminal
-        }
-
        private let lock = UnfairLock.allocate()

-        private var state: State
+        private let downstream: Downstream
+
+        private let scheduler: Context
+
+        private var state = SubscriptionStatus.awaitingSubscription

        private var last: Context.SchedulerTimeType?

-        init(_ measureInterval: MeasureInterval, downstream: Downstream) {
-            state = .ready(measureInterval, downstream)
+        init(scheduler: Context, downstream: Downstream) {
+            self.downstream = downstream
+            self.scheduler = scheduler
        }

        deinit {
@@ -95,25 +115,26 @@ extension Publishers.MeasureInterval {

        func receive(subscription: Subscription) {
            lock.lock()
-            guard case let .ready(measureInterval, downstream) = state else {
+            guard case .awaitingSubscription = state else {
                lock.unlock()
                subscription.cancel()
                return
            }
-            state = .subscribed(measureInterval, downstream, subscription)
-            last = measureInterval.scheduler.now
+            state = .subscribed(subscription)
+            last = scheduler.now
            lock.unlock()
            downstream.receive(subscription: self)
        }

        func receive(_: Input) -> Subscribers.Demand {
            lock.lock()
-            guard case let .subscribed(measureInterval, downstream, subscription) = state,
-                  let previousTime = last else {
+            guard case let .subscribed(subscription) = state,
+                  let previousTime = last else
+            {
                lock.unlock()
                return .none
            }
-            let now = measureInterval.scheduler.now
+            let now = scheduler.now
            last = now
            lock.unlock()
            let newDemand = downstream.receive(previousTime.distance(to: now))
@@ -125,7 +146,7 @@ extension Publishers.MeasureInterval {

        func receive(completion: Subscribers.Completion<Failure>) {
            lock.lock()
-            guard case let .subscribed(_, downstream, _) = state else {
+            guard case .subscribed = state else {
                lock.unlock()
                return
            }
@@ -137,7 +158,7 @@ extension Publishers.MeasureInterval {

        func request(_ demand: Subscribers.Demand) {
            lock.lock()
-            guard case let .subscribed(_, _, subscription) = state else {
+            guard case let .subscribed(subscription) = state else {
                lock.unlock()
                return
            }
@@ -147,7 +168,7 @@ extension Publishers.MeasureInterval {

        func cancel() {
            lock.lock()
-            guard case let .subscribed(_, _, subscription) = state else {
+            guard case let .subscribed(subscription) = state else {
                lock.unlock()
                return
            }
@@ -11,10 +11,45 @@ extension Publisher {
    ///
    /// Use a multicast publisher when you have multiple downstream subscribers, but you
    /// want upstream publishers to only process one `receive(_:)` call per event.
-    /// In contrast with `multicast(subject:)`, this method produces a publisher that
-    /// creates a separate Subject for each subscriber.
+    /// This is useful when upstream publishers are doing expensive work you don’t want
+    /// to duplicate, like performing network requests.
    ///
-    /// - Parameter createSubject: A closure to create a new Subject each time
+    /// In contrast with `multicast(subject:)`, this method produces a publisher that
+    /// creates a separate `Subject` for each subscriber.
+    ///
+    /// The following example uses a sequence publisher as a counter to publish three
+    /// random numbers, generated by a `map(_:)` operator.
+    /// It uses a `multicast(_:)` operator whose closure creates a `PassthroughSubject`
+    /// to share the same random number to each of two subscribers. Because the multicast
+    /// publisher is a `ConnectablePublisher`, publishing only begins after a call to
+    /// `connect()`.
+    ///
+    ///     let pub = ["First", "Second", "Third"].publisher
+    ///         .map( { return ($0, Int.random(in: 0...100)) } )
+    ///         .print("Random")
+    ///         .multicast { PassthroughSubject<(String, Int), Never>() }
+    ///
+    ///     cancellable1 = pub
+    ///        .sink { print ("Stream 1 received: \($0)")}
+    ///     cancellable2 = pub
+    ///        .sink { print ("Stream 2 received: \($0)")}
+    ///     pub.connect()
+    ///
+    ///     // Prints:
+    ///     // Random: receive value: (("First", 9))
+    ///     // Stream 2 received: ("First", 9)
+    ///     // Stream 1 received: ("First", 9)
+    ///     // Random: receive value: (("Second", 46))
+    ///     // Stream 2 received: ("Second", 46)
+    ///     // Stream 1 received: ("Second", 46)
+    ///     // Random: receive value: (("Third", 26))
+    ///     // Stream 2 received: ("Third", 26)
+    ///     // Stream 1 received: ("Third", 26)
+    ///
+    /// In this example, the output shows that the `print(_:to:)` operator receives each
+    /// random value only one time, and then sends the value to both subscribers.
+    ///
+    /// - Parameter createSubject: A closure to create a new `Subject` each time
    ///   a subscriber attaches to the multicast publisher.
    public func multicast<SubjectType: Subject>(
        _ createSubject: @escaping () -> SubjectType
@@ -28,8 +63,42 @@ extension Publisher {
    ///
    /// Use a multicast publisher when you have multiple downstream subscribers, but you
    /// want upstream publishers to only process one `receive(_:)` call per event.
-    /// In contrast with `multicast(_:)`, this method produces a publisher shares
-    /// the provided Subject among all the downstream subscribers.
+    /// This is useful when upstream publishers are doing expensive work you don’t want
+    /// to duplicate, like performing network requests.
+    ///
+    /// In contrast with `multicast(_:)`, this method produces a publisher that shares
+    /// the provided `Subject` among all the downstream subscribers.
+    ///
+    /// The following example uses a sequence publisher as a counter to publish three
+    /// random numbers, generated by a `map(_:)` operator.
+    /// It uses a `multicast(subject:)` operator with a `PassthroughSubject` to share
+    /// the same random number to each of two subscribers. Because the multicast publisher
+    /// is a `ConnectablePublisher`, publishing only begins after a call to `connect()`.
+    ///
+    ///     let pub = ["First", "Second", "Third"].publisher
+    ///         .map( { return ($0, Int.random(in: 0...100)) } )
+    ///         .print("Random")
+    ///         .multicast(subject: PassthroughSubject<(String, Int), Never>())
+    ///
+    ///     cancellable1 = pub
+    ///         .sink { print ("Stream 1 received: \($0)")}
+    ///     cancellable2 = pub
+    ///         .sink { print ("Stream 2 received: \($0)")}
+    ///     pub.connect()
+    ///
+    ///     // Prints:
+    ///     // Random: receive value: (("First", 78))
+    ///     // Stream 2 received: ("First", 78)
+    ///     // Stream 1 received: ("First", 78)
+    ///     // Random: receive value: (("Second", 98))
+    ///     // Stream 2 received: ("Second", 98)
+    ///     // Stream 1 received: ("Second", 98)
+    ///     // Random: receive value: (("Third", 61))
+    ///     // Stream 2 received: ("Third", 61)
+    ///     // Stream 1 received: ("Third", 61)
+    ///
+    /// In this example, the output shows that the `print(_:to:)` operator receives each
+    /// random value only one time, and then sends the value to both subscribers.
    ///
    /// - Parameter subject: A subject to deliver elements to downstream subscribers.
    public func multicast<SubjectType: Subject>(
@@ -44,18 +113,19 @@ extension Publisher {
 extension Publishers {

    /// A publisher that uses a subject to deliver elements to multiple subscribers.
+    ///
+    /// Use a multicast publisher when you have multiple downstream subscribers, but you
+    /// want upstream publishers to only process one `receive(_:)` call per event.
    public final class Multicast<Upstream: Publisher, SubjectType: Subject>
        : ConnectablePublisher
        where Upstream.Failure == SubjectType.Failure,
              Upstream.Output == SubjectType.Output
    {
-        // NOTE: This class has been audited for thread safety
-
        public typealias Output = Upstream.Output

        public typealias Failure = Upstream.Failure

-        /// The publisher from which this publisher receives elements.
+        /// The publisher that this publisher receives elements from.
        public let upstream: Upstream

        /// A closure to create a new Subject each time a subscriber attaches
@@ -79,12 +149,10 @@ extension Publishers {
            return subject
        }

-        /// Creates a multicast publisher that applies a closure to create a subject
-        /// that delivers elements to subscribers.
+        /// Creates a multicast publisher that applies a closure to create a subject that
+        /// delivers elements to subscribers.
        ///
-        /// - Parameter upstream: The publisher from which this publisher receives
-        ///   elements.
-        /// - Parameter createSubject: A closure to create a new Subject each time
+        /// - Parameter createSubject: A closure that returns a `Subject` each time
        ///   a subscriber attaches to the multicast publisher.
        public init(upstream: Upstream, createSubject: @escaping () -> SubjectType) {
            self.upstream = upstream
@@ -9,6 +9,19 @@ extension Publisher {

    /// Republishes elements up to the specified maximum count.
    ///
+    /// Use `prefix(_:)` to limit the number of elements republished to the downstream
+    /// subscriber.
+    ///
+    /// In the example below, the `prefix(_:)` operator limits its output to the first
+    /// two elements before finishing normally:
+    ///
+    ///     let numbers = (0...10)
+    ///     cancellable = numbers.publisher
+    ///         .prefix(2)
+    ///         .sink { print("\($0)", terminator: " ") }
+    ///
+    ///     // Prints: "0 1"
+    ///
    /// - Parameter maxLength: The maximum number of elements to republish.
    /// - Returns: A publisher that publishes up to the specified number of elements
    ///   before completing.
@@ -22,8 +35,20 @@ extension Publisher {
    /// Publishes a specific element, indicated by its index in the sequence of published
    /// elements.
    ///
-    /// If the publisher completes normally or with an error before publishing
-    /// the specified element, then the publisher doesn’t produce any elements.
+    /// Use `output(at:)` when you need to republish a specific element specified by
+    /// its position in the stream. If the publisher completes normally or with an error
+    /// before publishing the specified element, then the publisher doesn’t produce any
+    /// elements.
+    ///
+    /// In the example below, the array publisher emits the fifth element in the sequence
+    /// of published elements:
+    ///
+    ///     let numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
+    ///     numbers.publisher
+    ///         .output(at: 5)
+    ///         .sink { print("\($0)") }
+    ///
+    ///     // Prints: "6"
    ///
    /// - Parameter index: The index that indicates the element to publish.
    /// - Returns: A publisher that publishes a specific indexed element.
@@ -33,10 +58,21 @@ extension Publisher {

    /// Publishes elements specified by their range in the sequence of published elements.
    ///
-    /// After all elements are published, the publisher finishes normally.
+    /// Use `output(in:)` to republish a range indices you specify in the published
+    /// stream. After publishing all elements, the publisher finishes normally.
    /// If the publisher completes normally or with an error before producing all
    /// the elements in the range, it doesn’t publish the remaining elements.
    ///
+    /// In the example below, an array publisher emits the subset of elements at
+    /// the indices in the specified range:
+    ///
+    ///     let numbers = [1, 1, 2, 2, 2, 3, 4, 5, 6]
+    ///     numbers.publisher
+    ///         .output(in: (3...5))
+    ///         .sink { print("\($0)", terminator: " ") }
+    ///
+    ///     // Prints: "2 2 3"
+    ///
    /// - Parameter range: A range that indicates which elements to publish.
    /// - Returns: A publisher that publishes elements specified by a range.
    public func output<Range: RangeExpression>(in range: Range) -> Publishers.Output<Self>
@@ -10,10 +10,22 @@ extension Publisher {
    /// Republishes elements while a predicate closure indicates publishing should
    /// continue.
    ///
-    /// The publisher finishes when the closure returns `false`.
+    /// Use `prefix(while:)` to emit values while elements from the upstream publishe
+    /// meet a condition you specify. The publisher finishes when the closure returns
+    /// `false`.
+    ///
+    /// In the example below, the `prefix(while:)` operator emits values while the element
+    /// it receives is less than five:
+    ///
+    ///     let numbers = (0...10)
+    ///     numbers.publisher
+    ///         .prefix { $0 < 5 }
+    ///         .sink { print("\($0)", terminator: " ") }
+    ///
+    ///     // Prints: "0 1 2 3 4"
    ///
    /// - Parameter predicate: A closure that takes an element as its parameter and
-    ///   returns a Boolean value indicating whether publishing should continue.
+    ///   returns a Boolean value that indicates whether publishing should continue.
    /// - Returns: A publisher that passes through elements until the predicate indicates
    ///   publishing should finish.
    public func prefix(
@@ -22,11 +34,28 @@ extension Publisher {
        return .init(upstream: self, predicate: predicate)
    }

-    /// Republishes elements while a error-throwing predicate closure indicates publishing
-    /// should continue.
+    /// Republishes elements while an error-throwing predicate closure indicates
+    /// publishing should continue.
    ///
-    /// The publisher finishes when the closure returns `false`. If the closure throws,
-    /// the publisher fails with the thrown error.
+    /// Use `tryPrefix(while:)` to emit values from the upstream publisher that meet
+    /// a condition you specify in an error-throwing closure.
+    /// The publisher finishes when the closure returns `false`. If the closure throws
+    /// an error, the publisher fails with that error.
+    ///
+    ///     struct OutOfRangeError: Error {}
+    ///
+    ///     let numbers = (0...10).reversed()
+    ///     cancellable = numbers.publisher
+    ///         .tryPrefix {
+    ///             guard $0 != 0 else { throw OutOfRangeError() }
+    ///             return $0 <= numbers.max()!
+    ///         }
+    ///         .sink(
+    ///             receiveCompletion: { print ("completion: \($0)", terminator: " ") },
+    ///             receiveValue: { print ("\($0)", terminator: " ") }
+    ///         )
+    ///
+    ///     // Prints: "10 9 8 7 6 5 4 3 2 1 completion: failure(OutOfRangeError()) "
    ///
    /// - Parameter predicate: A closure that takes an element as its parameter and
    ///   returns a Boolean value indicating whether publishing should continue.
@@ -9,8 +9,27 @@ extension Publisher {

    /// Prints log messages for all publishing events.
    ///
-    /// - Parameter prefix: A string with which to prefix all log messages. Defaults to
-    ///   an empty string.
+    /// Use `print(_:to:)` to log messages the console.
+    ///
+    /// In the example below, log messages are printed on the console:
+    ///
+    ///     let integers = (1...2)
+    ///     cancellable = integers.publisher
+    ///        .print("Logged a message", to: nil)
+    ///        .sink { _ in }
+    ///
+    ///     // Prints:
+    ///     //  Logged a message: receive subscription: (1..<2)
+    ///     //  Logged a message: request unlimited
+    ///     //  Logged a message: receive value: (1)
+    ///     //  Logged a message: receive finished
+    ///
+    /// - Parameters:
+    ///   - prefix: A string — which defaults to empty — with which to prefix all log
+    ///     messages.
+    ///   - stream: A stream for text output that receives messages, and which directs
+    ///     output to the console by default.  A custom stream can be used to log messages
+    ///     to other destinations.
    /// - Returns: A publisher that prints log messages for all publishing events.
    public func print(_ prefix: String = "",
                      to stream: TextOutputStream? = nil) -> Publishers.Print<Self> {
@@ -24,11 +43,12 @@ extension Publishers {
    /// prefixed with a given string.
    ///
    /// This publisher prints log messages when receiving the following events:
-    /// * subscription
-    /// * value
-    /// * normal completion
-    /// * failure
-    /// * cancellation
+    ///
+    /// - subscription
+    /// - value
+    /// - normal completion
+    /// - failure
+    /// - cancellation
    public struct Print<Upstream: Publisher>: Publisher {

        public typealias Output = Upstream.Output
@@ -8,12 +8,15 @@
 extension Publisher {
    /// Specifies the scheduler on which to receive elements from the publisher.
    ///
-    /// You use the `receive(on:options:)` operator to receive results on a specific
-    /// scheduler, such as performing UI work on the main run loop.
-    /// In contrast with `subscribe(on:options:)`, which affects upstream messages,
+    /// You use the `receive(on:options:)` operator to receive results and completion on
+    /// a specific scheduler, such as performing UI work on the main run loop. In contrast
+    /// with `subscribe(on:options:)`, which affects upstream messages,
    /// `receive(on:options:)` changes the execution context of downstream messages.
-    /// In the following example, requests to `jsonPublisher` are performed on
-    /// `backgroundQueue`, but elements received from it are performed on `RunLoop.main`.
+    ///
+    /// In the following example, the `subscribe(on:options:)` operator causes
+    /// `jsonPublisher` to receive requests on `backgroundQueue`, while
+    /// the `receive(on:options:)` causes `labelUpdater` to receive elements and
+    /// completion on `RunLoop.main`.
    ///
    ///     // Some publisher.
    ///     let jsonPublisher = MyJSONLoaderPublisher()
@@ -23,12 +26,31 @@ extension Publisher {
    ///
    ///     jsonPublisher
    ///         .subscribe(on: backgroundQueue)
-    ///         .receiveOn(on: RunLoop.main)
+    ///         .receive(on: RunLoop.main)
    ///         .subscribe(labelUpdater)
    ///
+    ///
+    /// Prefer `receive(on:options:)` over explicit use of dispatch queues when performing
+    /// work in subscribers. For example, instead of the following pattern:
+    ///
+    ///     pub.sink {
+    ///         DispatchQueue.main.async {
+    ///             // Do something.
+    ///         }
+    ///     }
+    ///
+    /// Use this pattern instead:
+    ///
+    ///     pub.receive(on: DispatchQueue.main).sink {
+    ///         // Do something.
+    ///     }
+    ///
+    ///  > Note: `receive(on:options:)` doesn’t affect the scheduler used to cal
+    ///  the subscriber’s `receive(subscription:)` method.
+    ///
    /// - Parameters:
-    ///   - scheduler: The scheduler the publisher is to use for element delivery.
-    ///   - options: Scheduler options that customize the element delivery.
+    ///   - scheduler: The scheduler the publisher uses for element delivery.
+    ///   - options: Scheduler options used to customize element delivery.
    /// - Returns: A publisher that delivers elements using the specified scheduler.
    public func receive<Context: Scheduler>(
        on scheduler: Context,
@@ -69,7 +91,10 @@ extension Publishers {
            where Upstream.Failure == Downstream.Failure,
                  Upstream.Output == Downstream.Input
        {
-            upstream.subscribe(Inner(self, downstream: subscriber))
+            let inner = Inner(scheduler: scheduler,
+                              options: options,
+                              downstream: subscriber)
+            upstream.subscribe(inner)
        }
    }
 }
@@ -84,23 +109,21 @@ extension Publishers.ReceiveOn {
        where Downstream.Input == Upstream.Output, Downstream.Failure == Upstream.Failure
    {
        typealias Input = Upstream.Output
-
        typealias Failure = Upstream.Failure

-        typealias ReceiveOn = Publishers.ReceiveOn<Upstream, Context>
-
-        private enum State {
-            case ready(ReceiveOn, Downstream)
-            case subscribed(ReceiveOn, Downstream, Subscription)
-            case terminal
-        }
-
        private let lock = UnfairLock.allocate()
-        private var state: State
+        private let downstream: Downstream
+        private let scheduler: Context
+        private let options: Context.SchedulerOptions?
+        private var state = SubscriptionStatus.awaitingSubscription
        private let downstreamLock = UnfairRecursiveLock.allocate()

-        init(_ receiveOn: ReceiveOn, downstream: Downstream) {
-            state = .ready(receiveOn, downstream)
+        init(scheduler: Context,
+             options: Context.SchedulerOptions?,
+             downstream: Downstream) {
+            self.downstream = downstream
+            self.scheduler = scheduler
+            self.options = options
        }

        deinit {
@@ -110,62 +133,65 @@ extension Publishers.ReceiveOn {

        func receive(subscription: Subscription) {
            lock.lock()
-            guard case let .ready(receiveOn, downstream) = state else {
+            guard case .awaitingSubscription = state else {
                lock.unlock()
                subscription.cancel()
                return
            }
-            state = .subscribed(receiveOn, downstream, subscription)
+            state = .subscribed(subscription)
            lock.unlock()
            downstreamLock.lock()
            downstream.receive(subscription: self)
            downstreamLock.unlock()
        }

-        func receive(_ input: Upstream.Output) -> Subscribers.Demand {
+        func receive(_ input: Input) -> Subscribers.Demand {
            lock.lock()
-            guard case let .subscribed(receiveOn, downstream, _) = state else {
+            guard case .subscribed = state else {
                lock.unlock()
                return .none
            }
            lock.unlock()
-            receiveOn.scheduler.schedule(options: receiveOn.options) {
-                self.scheduledReceive(input, downstream: downstream)
+            scheduler.schedule(options: options) {
+                self.scheduledReceive(input)
            }
            return .none
        }

-        private func scheduledReceive(_ input: Upstream.Output, downstream: Downstream) {
+        private func scheduledReceive(_ input: Input) {
+            lock.lock()
+            guard state.subscription != nil else {
+                lock.unlock()
+                return
+            }
+            lock.unlock()
            downstreamLock.lock()
            let newDemand = downstream.receive(input)
            downstreamLock.unlock()
-            guard newDemand > 0 else {
-                return
-            }
+            if newDemand == .none { return }
            lock.lock()
-            guard case let .subscribed(_, _, subscription) = state else {
-                lock.unlock()
-                return
-            }
+            let subscription = state.subscription
            lock.unlock()
-            subscription.request(newDemand)
+            subscription?.request(newDemand)
        }

-        func receive(completion: Subscribers.Completion<Upstream.Failure>) {
+        func receive(completion: Subscribers.Completion<Failure>) {
            lock.lock()
-            guard case let .subscribed(receiveOn, downstream, _) = state else {
+            guard case let .subscribed(subscription) = state else {
                lock.unlock()
                return
            }
+            state = .pendingTerminal(subscription)
+            lock.unlock()
+            scheduler.schedule(options: options) {
+                self.scheduledReceive(completion: completion)
+            }
+        }
+
+        private func scheduledReceive(completion: Subscribers.Completion<Failure>) {
+            lock.lock()
            state = .terminal
            lock.unlock()
-            receiveOn.scheduler.schedule(options: receiveOn.options) {
-                self.scheduledReceive(completion: completion, downstream: downstream)
-            }
-        }
-
-        private func scheduledReceive(completion: Subscribers.Completion<Failure>,
-                                      downstream: Downstream) {
            downstreamLock.lock()
            downstream.receive(completion: completion)
            downstreamLock.unlock()
@@ -173,7 +199,7 @@ extension Publishers.ReceiveOn {

        func request(_ demand: Subscribers.Demand) {
            lock.lock()
-            guard case let .subscribed(_, _, subscription) = state else {
+            guard case let .subscribed(subscription) = state else {
                lock.unlock()
                return
            }
@@ -183,7 +209,7 @@ extension Publishers.ReceiveOn {

        func cancel() {
            lock.lock()
-            guard case let .subscribed(_, _, subscription) = state else {
+            guard case let .subscribed(subscription) = state else {
                lock.unlock()
                return
            }
@@ -7,15 +7,32 @@

 extension Publisher {

-    /// Applies a closure that accumulates each element of a stream and publishes
-    /// a final result upon completion.
+    /// Applies a closure that collects each element of a stream and publishes a final
+    /// result upon completion.
+    ///
+    /// Use `reduce(_:_:)` to collect a stream of elements and produce an accumulated
+    /// value based on a closure you provide.
+    ///
+    /// In the following example, the `reduce(_:_:)` operator collects all the integer
+    /// values it receives from its upstream publisher:
+    ///
+    ///     let numbers = (0...10)
+    ///     cancellable = numbers.publisher
+    ///         .reduce(0, { accum, next in accum + next })
+    ///         .sink { print("\($0)") }
+    ///
+    ///     // Prints: "55"
    ///
    /// - Parameters:
-    ///   - initialResult: The value the closure receives the first time it is called.
-    ///   - nextPartialResult: A closure that takes the previously-accumulated value and
-    ///     the next element from the upstream publisher to produce a new value.
+    ///   - initialResult: The value that the closure receives the first time it’s called.
+    ///   - nextPartialResult: A closure that produces a new value by taking
+    ///     the previously-accumulated value and the next element it receives from
+    ///     the upstream publisher.
    /// - Returns: A publisher that applies the closure to all received elements and
    ///   produces an accumulated value when the upstream publisher finishes.
+    ///   If `reduce(_:_:)` receives an error from the upstream publisher, the operator
+    ///   delivers it to the downstream subscriber, the publisher terminates and publishes
+    ///   no value.
    public func reduce<Accumulator>(
        _ initialResult: Accumulator,
        _ nextPartialResult: @escaping (Accumulator, Output) -> Accumulator
@@ -25,17 +42,36 @@ extension Publisher {
                     nextPartialResult: nextPartialResult)
    }

-    /// Applies an error-throwing closure that accumulates each element of a stream and
+    /// Applies an error-throwing closure that collects each element of a stream and
    /// publishes a final result upon completion.
    ///
-    /// If the closure throws an error, the publisher fails, passing the error
-    /// to its subscriber.
+    /// Use `tryReduce(_:_:)` to collect a stream of elements and produce an accumulated
+    /// value based on an error-throwing closure you provide.
+    /// If the closure throws an error, the publisher fails and passes the error to its
+    /// subscriber.
    ///
+    /// In the example below, the publisher’s `0` element causes the `myDivide(_:_:)`
+    /// function to throw an error and publish the `Double.nan` result:
+    ///
+    ///     struct DivisionByZeroError: Error {}
+    ///     func myDivide(_ dividend: Double, _ divisor: Double) throws -> Double {
+    ///         guard divisor != 0 else { throw DivisionByZeroError() }
+    ///         return dividend / divisor
+    ///     }
+    ///
+    ///     var numbers: [Double] = [5, 4, 3, 2, 1, 0]
+    ///     numbers.publisher
+    ///         .tryReduce(numbers.first!, { accum, next in try myDivide(accum, next) })
+    ///         .catch({ _ in Just(Double.nan) })
+    ///         .sink { print("\($0)") }
+    ///
+
    /// - Parameters:
-    ///   - initialResult: The value the closure receives the first time it is called.
+    ///   - initialResult: The value that the closure receives the first time it’s called.
    ///   - nextPartialResult: An error-throwing closure that takes
    ///     the previously-accumulated value and the next element from the upstream
    ///     publisher to produce a new value.
+    ///
    /// - Returns: A publisher that applies the closure to all received elements and
    ///   produces an accumulated value when the upstream publisher finishes.
    public func tryReduce<Accumulator>(
@@ -9,7 +9,23 @@ extension Publisher where Output: Equatable {

    /// Publishes only elements that don’t match the previous element.
    ///
-    /// - Returns: A publisher that consumes — rather than publishes — duplicate elements.
+    /// Use `removeDuplicates()` to remove repeating elements from an upstream publisher.
+    /// This operator has a two-element memory: the operator uses the current and
+    /// previously published elements as the basis for its comparison.
+    ///
+    /// In the example below, `removeDuplicates()` triggers on the doubled, tripled, and
+    /// quadrupled occurrences of `1`, `3`, and `4` respectively. Because the two-element
+    /// memory considers only the current element and the previous element, the operator
+    /// prints the final `0` in the example data since its immediate predecessor is `4`.
+    ///
+    ///     let numbers = [0, 1, 2, 2, 3, 3, 3, 4, 4, 4, 4, 0]
+    ///     cancellable = numbers.publisher
+    ///         .removeDuplicates()
+    ///         .sink { print("\($0)", terminator: " ") }
+    ///
+    ///     // Prints: "0 1 2 3 4 0"
+    ///
+    /// - Returns: A publisher that consumes — rather than publishes — duplicate elements.
    public func removeDuplicates() -> Publishers.RemoveDuplicates<Self> {
        return removeDuplicates(by: ==)
    }
@@ -20,9 +36,39 @@ extension Publisher {
    /// Publishes only elements that don’t match the previous element, as evaluated by
    /// a provided closure.
    ///
+    /// Use `removeDuplicates(by:)` to remove repeating elements from an upstream
+    /// publisher based upon the evaluation of the current and previously published
+    /// elements using a closure you provide.
+    ///
+    /// Use the `removeDuplicates(by:)` operator when comparing types that don’t
+    /// themselves implement `Equatable`, or if you need to compare values differently
+    /// than the type’s `Equatable` implementation.
+    ///
+    /// In the example below, the `removeDuplicates(by:)` functionality triggers when
+    /// the `x` property of the current and previous elements are equal, otherwise
+    /// the operator publishes the current `Point` to the downstream subscriber:
+    ///
+    ///     struct Point {
+    ///         let x: Int
+    ///         let y: Int
+    ///     }
+    ///
+    ///     let points = [Point(x: 0, y: 0), Point(x: 0, y: 1),
+    ///                   Point(x: 1, y: 1), Point(x: 2, y: 1)]
+    ///     cancellable = points.publisher
+    ///         .removeDuplicates { prev, current in
+    ///             // Considers points to be duplicate if the x coordinate
+    ///             // is equal, and ignores the y coordinate
+    ///             prev.x == current.x
+    ///         }
+    ///         .sink { print("\($0)", terminator: " ") }
+    ///
+    ///     // Prints: Point(x: 0, y: 0) Point(x: 1, y: 1) Point(x: 2, y: 1)
+    ///
    /// - Parameter predicate: A closure to evaluate whether two elements are equivalent,
    ///   for purposes of filtering. Return `true` from this closure to indicate that
    ///   the second element is a duplicate of the first.
+    /// - Returns: A publisher that consumes — rather than publishes — duplicate elements.
    public func removeDuplicates(
        by predicate: @escaping (Output, Output) -> Bool
    ) -> Publishers.RemoveDuplicates<Self> {
@@ -32,10 +78,37 @@ extension Publisher {
    /// Publishes only elements that don’t match the previous element, as evaluated by
    /// a provided error-throwing closure.
    ///
+    /// Use `tryRemoveDuplicates(by:)` to remove repeating elements from an upstream
+    /// publisher based upon the evaluation of elements using an error-throwing closure
+    /// you provide. If your closure throws an error, the publisher terminates with
+    /// the error.
+    ///
+    /// In the example below, the closure provided to `tryRemoveDuplicates(by:)` returns
+    /// `true` when two consecutive elements are equal, thereby filtering out `0`,
+    /// `1`, `2`, and `3`. However, the closure throws an error when it encounters `4`.
+    /// The publisher then terminates with this error.
+    ///
+    ///     struct BadValuesError: Error {}
+    ///     let numbers = [0, 0, 0, 0, 1, 2, 2, 3, 3, 3, 4, 4, 4, 4]
+    ///     cancellable = numbers.publisher
+    ///         .tryRemoveDuplicates { first, second -> Bool in
+    ///             if (first == 4 && second == 4) {
+    ///                 throw BadValuesError()
+    ///             }
+    ///             return first == second
+    ///         }
+    ///         .sink(
+    ///             receiveCompletion: { print ("\($0)") },
+    ///             receiveValue: { print ("\($0)", terminator: " ") }
+    ///          )
+    ///
+    ///      // Prints: "0 1 2 3 4 failure(BadValuesError()"
+    ///
    /// - Parameter predicate: A closure to evaluate whether two elements are equivalent,
    ///   for purposes of filtering. Return `true` from this closure to indicate that
    ///   the second element is a duplicate of the first. If this closure throws an error,
    ///   the publisher terminates with the thrown error.
+    /// - Returns: A publisher that consumes — rather than publishes — duplicate elements.
    public func tryRemoveDuplicates(
        by predicate: @escaping (Output, Output) throws -> Bool
    ) -> Publishers.TryRemoveDuplicates<Self> {
@@ -55,11 +128,10 @@ extension Publishers {
        /// The publisher from which this publisher receives elements.
        public let upstream: Upstream

-        /// A closure to evaluate whether two elements are equivalent,
-        /// for purposes of filtering.
+        /// The predicate closure used to evaluate whether two elements are duplicates.
        public let predicate: (Output, Output) -> Bool

-        /// Creates a publisher that publishes only elements that don’t match the previou
+        /// Creates a publisher that publishes only elements that don’t match the previous
        /// element, as evaluated by a provided closure.
        ///
        /// - Parameter upstream: The publisher from which this publisher receives
@@ -82,7 +154,7 @@ extension Publishers {

    /// A publisher that publishes only elements that don’t match the previous element,
    /// as evaluated by a provided error-throwing closure.
-    public struct TryRemoveDuplicates<Upstream: Publisher>: Publisher{
+    public struct TryRemoveDuplicates<Upstream: Publisher>: Publisher {

        public typealias Output = Upstream.Output

@@ -128,8 +200,6 @@ extension Publishers.RemoveDuplicates {
                         (Output, Output) -> Bool>
        where Downstream.Input == Upstream.Output, Downstream.Failure == Upstream.Failure
    {
-        // NOTE: This class has been audited for thread-safety
-
        private var last: Upstream.Output?

        override func receive(
@@ -163,8 +233,6 @@ extension Publishers.TryRemoveDuplicates {
                         (Output, Output) throws -> Bool>
        where Downstream.Input == Upstream.Output, Downstream.Failure == Error
    {
-        // NOTE: This class has been audited for thread-safety
-
        private var last: Upstream.Output?

        override func receive(
@@ -9,12 +9,33 @@ extension Publisher {

    /// Replaces an empty stream with the provided element.
    ///
-    /// If the upstream publisher finishes without producing any elements,
-    /// this publisher emits the provided element, then finishes normally.
-    /// - Parameter output: An element to emit when the upstream publisher
-    ///                     finishes without emitting any elements.
-    /// - Returns: A publisher that replaces an empty stream with
-    ///            the provided output element.
+    /// Use `replaceEmpty(with:)` to provide a replacement element if the upstream
+    /// publisher finishes without producing any elements.
+    ///
+    /// In the example below, the empty `Double` array publisher doesn’t produce any
+    /// elements, so `replaceEmpty(with:)` publishes `Double.nan` and finishes normally.
+    ///
+    ///     let numbers: [Double] = []
+    ///     cancellable = numbers.publisher
+    ///         .replaceEmpty(with: Double.nan)
+    ///         .sink { print("\($0)", terminator: " ") }
+    ///
+    ///     // Prints "(nan)".
+    ///
+    /// Conversely, providing a non-empty publisher publishes all elements and
+    /// the publisher then terminates normally:
+    ///
+    ///     let otherNumbers: [Double] = [1.0, 2.0, 3.0]
+    ///     cancellable2 = otherNumbers.publisher
+    ///         .replaceEmpty(with: Double.nan)
+    ///         .sink { print("\($0)", terminator: " ") }
+    ///
+    ///     // Prints: 1.0 2.0 3.0
+    ///
+    /// - Parameter output: An element to emit when the upstream publisher finishes
+    ///   without emitting any elements.
+    /// - Returns: A publisher that replaces an empty stream with the provided output
+    ///   element.
    public func replaceEmpty(with output: Output) -> Publishers.ReplaceEmpty<Self> {
        return .init(upstream: self, output: output)
    }
@@ -6,13 +6,36 @@
 //

 extension Publisher {
+
    /// Replaces any errors in the stream with the provided element.
    ///
    /// If the upstream publisher fails with an error, this publisher emits the provided
    /// element, then finishes normally.
+    ///
+    /// In the example below, a publisher of strings fails with a `MyError` instance,
+    /// which sends a failure completion downstream. The `replaceError(with:)` operator
+    /// handles the failure by publishing the string `(replacement element)` and
+    /// completing normally.
+    ///
+    ///     struct MyError: Error {}
+    ///     let fail = Fail<String, MyError>(error: MyError())
+    ///     cancellable = fail
+    ///         .replaceError(with: "(replacement element)")
+    ///         .sink(
+    ///             receiveCompletion: { print ("\($0)") },
+    ///             receiveValue: { print ("\($0)", terminator: " ") }
+    ///         )
+    ///
+    ///     // Prints: "(replacement element) finished".
+    ///
+    /// This `replaceError(with:)` functionality is useful when you want to handle
+    /// an error by sending a single replacement element and end the stream.
+    /// Use `catch(_:)` to recover from an error and provide a replacement publisher
+    /// to continue providing elements to the downstream subscriber.
+    ///
    /// - Parameter output: An element to emit when the upstream publisher fails.
    /// - Returns: A publisher that replaces an error from the upstream publisher with
-    ///            the provided output element.
+    ///   the provided output element.
    public func replaceError(with output: Output) -> Publishers.ReplaceError<Self> {
        return .init(upstream: self, output: output)
    }
@@ -73,8 +96,6 @@ extension Publishers.ReplaceError {
          CustomPlaygroundDisplayConvertible
        where Upstream.Output == Downstream.Input
    {
-        // NOTE: this class has been audited for thread safety.
-
        typealias Input = Upstream.Output
        typealias Failure = Upstream.Failure

@@ -104,12 +125,12 @@ extension Publishers.ReplaceError {
            status = .subscribed(subscription)
            let pendingDemand = self.pendingDemand
            lock.unlock()
-            if pendingDemand > 0 {
+            if pendingDemand != .none {
                subscription.request(pendingDemand)
            }
        }

-        func receive(_ input: Upstream.Output) -> Subscribers.Demand {
+        func receive(_ input: Input) -> Subscribers.Demand {
            lock.lock()
            guard case .subscribed = status else {
                lock.unlock()
@@ -118,7 +139,7 @@ extension Publishers.ReplaceError {
            pendingDemand -= 1
            lock.unlock()
            let demand = downstream.receive(input)
-            guard demand > 0 else {
+            if demand == .none {
                return .none
            }
            lock.lock()
@@ -127,20 +148,27 @@ extension Publishers.ReplaceError {
            return demand
        }

-        func receive(completion: Subscribers.Completion<Upstream.Failure>) {
+        func receive(completion: Subscribers.Completion<Failure>) {
+            lock.lock()
+            guard case .subscribed = status else {
+                lock.unlock()
+                return
+            }
            switch completion {
            case .finished:
+                status = .terminal
+                lock.unlock()
                downstream.receive(completion: .finished)
            case .failure:
-                lock.lock()
                // If there was no demand from downstream,
                // ReplaceError does not forward the value that
                // replaces the error until it is requested.
-                guard pendingDemand > 0 else {
+                if pendingDemand == .none {
                    terminated = true
                    lock.unlock()
                    return
                }
+                status = .terminal
                lock.unlock()
                _ = downstream.receive(output)
                downstream.receive(completion: .finished)
@@ -1,22 +0,0 @@
-//
-//  Publishers.ReplaceNil.swift
-//  
-//
-//  Created by Joseph Spadafora on 7/4/19.
-//
-
-extension Publisher {
-
-    /// Replaces nil elements in the stream with the proviced element.
-    ///
-    /// - Parameter output: The element to use when replacing `nil`.
-    /// - Returns: A publisher that replaces `nil` elements from
-    /// the upstream publisher with the provided element.
-    public func replaceNil<ElementOfResult>(
-        with output: ElementOfResult
-    ) -> Publishers.Map<Self, ElementOfResult>
-        where Output == ElementOfResult?
-    {
-        return Publishers.Map(upstream: self) { $0 ?? output }
-    }
-}
@@ -6,15 +6,20 @@

 extension Publisher {

-    /// Transforms elements from the upstream publisher by providing the current element
-    /// to a closure along with the last value returned by the closure.
+    /// Transforms elements from the upstream publisher by providing the current
+    /// element to a closure along with the last value returned by the closure.
    ///
-    ///     let pub = (0...5)
-    ///         .publisher
-    ///         .scan(0, { return $0 + $1 })
-    ///         .sink(receiveValue: { print ("\($0)", terminator: " ") })
-    ///      // Prints "0 1 3 6 10 15 ".
+    /// Use `scan(_:_:)` to accumulate all previously-published values into a single
+    /// value, which you then combine with each newly-published value.
    ///
+    /// The following example logs a running total of all values received
+    /// from the sequence publisher.
+    ///
+    ///     let range = (0...5)
+    ///     cancellable = range.publisher
+    ///         .scan(0) { return $0 + $1 }
+    ///         .sink { print ("\($0)", terminator: " ") }
+    ///      // Prints: "0 1 3 6 10 15 ".
    ///
    /// - Parameters:
    ///   - initialResult: The previous result returned by the `nextPartialResult`
@@ -37,7 +42,37 @@ extension Publisher {
    /// Transforms elements from the upstream publisher by providing the current element
    /// to an error-throwing closure along with the last value returned by the closure.
    ///
+    /// Use `tryScan(_:_:)` to accumulate all previously-published values into a single
+    /// value, which you then combine with each newly-published value.
+    /// If your accumulator closure throws an error, the publisher terminates with
+    /// the error.
+    ///
+    /// In the example below, `tryScan(_:_:)` calls a division function on elements of
+    /// a collection publisher. The `Publishers.TryScan` publisher publishes each result
+    /// until the function encounters a `DivisionByZeroError`, which terminates
+    /// the publisher.
+    ///
+    ///     struct DivisionByZeroError: Error {}
+    ///
+    ///     /// A function that throws a DivisionByZeroError if `current` provided by the
+    ///     /// TryScan publisher is zero.
+    ///     func myThrowingFunction(_ lastValue: Int, _ currentValue: Int) throws -> Int {
+    ///         guard currentValue != 0 else { throw DivisionByZeroError() }
+    ///         return (lastValue + currentValue) / currentValue
+    ///      }
+    ///
+    ///     let numbers = [1,2,3,4,5,0,6,7,8,9]
+    ///     cancellable = numbers.publisher
+    ///         .tryScan(10) { try myThrowingFunction($0, $1) }
+    ///         .sink(
+    ///             receiveCompletion: { print ("\($0)") },
+    ///             receiveValue: { print ("\($0)", terminator: " ") }
+    ///          )
+    ///
+    ///     // Prints: "11 6 3 1 1 -1 failure(DivisionByZeroError())".
+    ///
    /// If the closure throws an error, the publisher fails with the error.
+    ///
    /// - Parameters:
    ///   - initialResult: The previous result returned by the `nextPartialResult`
    ///     closure.
@@ -29,13 +29,13 @@ extension Publishers {
            where Failure == Downstream.Failure,
                  Elements.Element == Downstream.Input
        {
-            var iterator = sequence.makeIterator()
-            if iterator.next() != nil {
-                let inner = Inner(downstream: subscriber, sequence: sequence)
-                subscriber.receive(subscription: inner)
-            } else {
+            let inner = Inner(downstream: subscriber, sequence: sequence)
+            if inner.isExhausted {
                subscriber.receive(subscription: Subscriptions.empty)
                subscriber.receive(completion: .finished)
+                inner.cancel()
+            } else {
+                subscriber.receive(subscription: inner)
            }
        }
    }
@@ -51,7 +51,6 @@ extension Publishers.Sequence {
        where Downstream.Input == Elements.Element,
              Downstream.Failure == Failure
    {
-        // NOTE: This class has been audited for thread-safety

        typealias Iterator = Elements.Iterator
        typealias Element = Elements.Element
@@ -75,6 +74,10 @@ extension Publishers.Sequence {
            lock.deallocate()
        }

+        fileprivate var isExhausted: Bool {
+            return next == nil
+        }
+
        var description: String {
            return sequence.map(String.init(describing:)) ?? "Sequence"
        }
@@ -48,9 +48,30 @@ extension Publisher where Failure == Never {

    /// Changes the failure type declared by the upstream publisher.
    ///
-    /// The publisher returned by this method cannot actually fail
-    /// with the specified type and instead just finishes normally. Instead, you use
-    /// this method when you need to match the error types of two mismatched publishers.
+    /// Use `setFailureType(to:)` when you need set the error type of a publisher that
+    /// cannot fail.
+    ///
+    /// Conversely, if the upstream can fail, you would use `mapError(_:)` to provide
+    /// instructions on converting the error types to needed by the downstream publisher’s
+    /// inputs.
+    ///
+    /// The following example has two publishers with mismatched error types: `pub1`’s
+    /// error type is `Never`, and `pub2`’s error type is `Error`. Because of
+    /// the mismatch, the `combineLatest(_:)` operator requires that `pub1` use
+    /// `setFailureType(to:)` to make it appear that `pub1` can produce the `Error` type,
+    /// like `pub2` can.
+    ///
+    ///     let pub1 = [0, 1, 2, 3, 4, 5].publisher
+    ///     let pub2 = CurrentValueSubject<Int, Error>(0)
+    ///     let cancellable = pub1
+    ///         .setFailureType(to: Error.self)
+    ///         .combineLatest(pub2)
+    ///         .sink(
+    ///             receiveCompletion: { print ("completed: \($0)") },
+    ///             receiveValue: { print ("value: \($0)")}
+    ///          )
+    ///
+    ///     // Prints: "value: (5, 0)".
    ///
    /// - Parameter failureType: The `Failure` type presented by this publisher.
    /// - Returns: A publisher that appears to send the specified failure type.
@@ -7,13 +7,53 @@

 extension Publisher {

-    /// Returns a publisher as a class instance.
+    /// Shares the output of an upstream publisher with multiple subscribers.
    ///
-    /// The downstream subscriber receieves elements and completion states unchanged from
-    /// the upstream publisher. Use this operator when you want to use
-    /// reference semantics, such as storing a publisher instance in a property.
+    /// The publisher returned by this operator supports multiple subscribers, all of whom
+    /// receive unchanged elements and completion states from the upstream publisher.
    ///
-    /// - Returns: A class instance that republishes its upstream publisher.
+    ///  - Tip: `Publishers.Share` is effectively a combination of
+    ///  the `Publishers.Multicast` and `PassthroughSubject` publishers, with an implicit
+    ///  `autoconnect()`.
+    ///
+    /// The following example uses a sequence publisher as a counter to publish three
+    /// random numbers, generated by a `map(_:)` operator. It uses a `share()` operator
+    /// to share the same random number to each of two subscribers. This example uses
+    /// a `delay(for:tolerance:scheduler:options:)` operator only to prevent the first
+    /// subscriber from exhausting the sequence publisher immediately; an asynchronous
+    /// publisher wouldn’t need this.
+    ///
+    ///     let pub = (1...3).publisher
+    ///         .delay(for: 1, scheduler: DispatchQueue.main)
+    ///         .map( { _ in return Int.random(in: 0...100) } )
+    ///         .print("Random")
+    ///         .share()
+    ///
+    ///     cancellable1 = pub
+    ///         .sink { print ("Stream 1 received: \($0)")}
+    ///     cancellable2 = pub
+    ///         .sink { print ("Stream 2 received: \($0)")}
+    ///
+    ///     // Prints:
+    ///     // Random: receive value: (20)
+    ///     // Stream 1 received: 20
+    ///     // Stream 2 received: 20
+    ///     // Random: receive value: (85)
+    ///     // Stream 1 received: 85
+    ///     // Stream 2 received: 85
+    ///     // Random: receive value: (98)
+    ///     // Stream 1 received: 98
+    ///     // Stream 2 received: 98
+    ///
+    /// Without the `share()` operator, stream 1 receives three random values,
+    /// followed by stream 2 receiving three different random values.
+    ///
+    /// Also note that `Publishers.Share` is a `class` rather than a `struct` like most
+    /// other publishers. This means you can use this operator to create a publisher
+    /// instance that uses reference semantics.
+    ///
+    /// - Returns: A class instance that shares elements received from its upstream to
+    ///   multiple subscribers.
    public func share() -> Publishers.Share<Self> {
        return .init(upstream: self)
    }
@@ -21,8 +61,19 @@ extension Publisher {

 extension Publishers {

-    /// A publisher implemented as a class, which otherwise behaves like its upstream
-    /// publisher.
+    /// A publisher that shares the output of an upstream publisher with multiple
+    /// subscribers.
+    ///
+    /// This publisher type supports multiple subscribers, all of whom receive unchanged
+    /// elements and completion states from the upstream publisher.
+    ///
+    ///  - Tip: `Publishers.Share` is effectively a combination of
+    ///  the `Publishers.Multicast` and `PassthroughSubject` publishers,
+    ///  with an implicit `autoconnect()`.
+    ///
+    /// Also note that `Publishers.Share` is a `class` rather than a `struct` like most
+    /// other publishers. Use this type when you need a publisher instance that uses
+    /// reference semantics.
    public final class Share<Upstream: Publisher>: Publisher, Equatable {

        public typealias Output = Upstream.Output
@@ -11,20 +11,27 @@ extension Publisher {
    /// operations.
    ///
    /// In contrast with `receive(on:options:)`, which affects downstream messages,
-    /// `subscribe(on:)` changes the execution context of upstream messages.
-    /// In the following example, requests to `jsonPublisher` are performed on
-    /// `backgroundQueue`, but elements received from it are performed on `RunLoop.main`.
+    /// `subscribe(on:options:)` changes the execution context of upstream messages.
+    ///
+    /// In the following example, the `subscribe(on:options:)` operator causes
+    /// `ioPerformingPublisher` to receive requests on `backgroundQueue`, while
+    /// the `receive(on:options:)` causes `uiUpdatingSubscriber` to receive elements and
+    /// completion on `RunLoop.main`.
    ///
    ///     let ioPerformingPublisher == // Some publisher.
    ///     let uiUpdatingSubscriber == // Some subscriber that updates the UI.
    ///
    ///     ioPerformingPublisher
    ///         .subscribe(on: backgroundQueue)
-    ///         .receiveOn(on: RunLoop.main)
+    ///         .receive(on: RunLoop.main)
    ///         .subscribe(uiUpdatingSubscriber)
    ///
+    ///
+    /// Using `subscribe(on:options:)` also causes the upstream publisher to perform
+    /// `cancel()` using the specfied scheduler.
+    ///
    /// - Parameters:
-    ///   - scheduler: The scheduler on which to receive upstream messages.
+    ///   - scheduler: The scheduler used to send messages to upstream publishers.
    ///   - options: Options that customize the delivery of elements.
    /// - Returns: A publisher which performs upstream operations on the specified
    ///   scheduler.
@@ -67,8 +74,11 @@ extension Publishers {
            where Upstream.Failure == Downstream.Failure,
                  Upstream.Output == Downstream.Input
        {
+            let inner = Inner(scheduler: scheduler,
+                              options: options,
+                              downstream: subscriber)
            scheduler.schedule(options: options) {
-                self.upstream.subscribe(Inner(self, downstream: subscriber))
+                self.upstream.subscribe(inner)
            }
        }
    }
@@ -87,20 +97,19 @@ extension Publishers.SubscribeOn {

        typealias Failure = Upstream.Failure

-        typealias SubscribeOn = Publishers.SubscribeOn<Upstream, Context>
-
-       private enum State {
-            case ready(SubscribeOn, Downstream)
-            case subscribed(SubscribeOn, Downstream, Subscription)
-            case terminal
-        }
-
        private let lock = UnfairLock.allocate()
-        private var state: State
+        private let downstream: Downstream
+        private let scheduler: Context
+        private let options: Context.SchedulerOptions?
+        private var state = SubscriptionStatus.awaitingSubscription
        private let upstreamLock = UnfairLock.allocate()

-        init(_ subscribeOn: SubscribeOn, downstream: Downstream) {
-            state = .ready(subscribeOn, downstream)
+        init(scheduler: Context,
+             options: Context.SchedulerOptions?,
+             downstream: Downstream) {
+            self.downstream = downstream
+            self.scheduler = scheduler
+            self.options = options
        }

        deinit {
@@ -110,19 +119,19 @@ extension Publishers.SubscribeOn {

        func receive(subscription: Subscription) {
            lock.lock()
-            guard case let .ready(subscribeOn, downstream) = state else {
+            guard case .awaitingSubscription = state else {
                lock.unlock()
                subscription.cancel()
                return
            }
-            state = .subscribed(subscribeOn, downstream, subscription)
+            state = .subscribed(subscription)
            lock.unlock()
            downstream.receive(subscription: self)
        }

-        func receive(_ input: Upstream.Output) -> Subscribers.Demand {
+        func receive(_ input: Input) -> Subscribers.Demand {
            lock.lock()
-            guard case let .subscribed(_, downstream, _) = state else {
+            guard case .subscribed = state else {
                lock.unlock()
                return .none
            }
@@ -130,9 +139,9 @@ extension Publishers.SubscribeOn {
            return downstream.receive(input)
        }

-        func receive(completion: Subscribers.Completion<Upstream.Failure>) {
+        func receive(completion: Subscribers.Completion<Failure>) {
            lock.lock()
-            guard case let .subscribed(_, downstream, _) = state else {
+            guard case .subscribed = state else {
                lock.unlock()
                return
            }
@@ -143,13 +152,13 @@ extension Publishers.SubscribeOn {

        func request(_ demand: Subscribers.Demand) {
            lock.lock()
-            guard case let .subscribed(subscribeOn, _, subscription) = state else {
+            guard case let .subscribed(subscription) = state else {
                lock.unlock()
                return
            }
            lock.unlock()
-            subscribeOn.scheduler.schedule(options: subscribeOn.options) { [weak self] in
-                self?.scheduledRequest(demand, subscription: subscription)
+            scheduler.schedule(options: options) {
+                self.scheduledRequest(demand, subscription: subscription)
            }
        }

@@ -162,14 +171,14 @@ extension Publishers.SubscribeOn {

        func cancel() {
            lock.lock()
-            guard case let .subscribed(subscribeOn, _, subscription) = state else {
+            guard case let .subscribed(subscription) = state else {
                lock.unlock()
                return
            }
            state = .terminal
            lock.unlock()
-            subscribeOn.scheduler.schedule(options: subscribeOn.options) { [weak self] in
-                self?.scheduledCancel(subscription)
+            scheduler.schedule(options: options) {
+                self.scheduledCancel(subscription)
            }
        }

@@ -7,31 +7,144 @@

 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.
+    /// Republishes elements sent by the most recently received publisher.
    ///
-    /// 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.
+    /// This operator works with an upstream publisher of publishers, flattening
+    /// the stream of elements to appear as if they were coming from a single stream of
+    /// elements. It switches the inner publisher as new ones arrive but keeps the outer
+    /// publisher constant for downstream subscribers.
+    ///
+    /// For example, given the type `AnyPublisher<URLSession.DataTaskPublisher, NSError>`,
+    /// calling `switchToLatest()` results in the type
+    /// `SwitchToLatest<(Data, URLResponse), URLError>`.
+    /// The downstream subscriber sees a continuous stream of `(Data, URLResponse)`
+    /// elements from what looks like a single `URLSession.DataTaskPublisher` even though
+    /// the elements are coming from different upstream publishers.
+    ///
+    /// When this operator receives a new publisher from the upstream publisher, it
+    /// cancels its previous subscription. Use this feature to prevent earlier publishers
+    /// from performing unnecessary work, such as creating network request publishers from
+    /// frequently updating user interface publishers.
+    ///
+    /// The following example updates a `PassthroughSubject` with a new value every
+    /// `0.1` seconds. A `map(_:)` operator receives the new value and uses it to create
+    /// a new `URLSession.DataTaskPublisher`. By using the `switchToLatest()` operator,
+    /// the downstream sink subscriber receives the `(Data, URLResponse)` output type from
+    /// the data task publishers, rather than the `URLSession.DataTaskPublisher` type
+    /// produced by the `map(_:)` operator. Furthermore, creating each new data task
+    /// publisher cancels the previous data task publisher.
+    ///
+    ///     let subject = PassthroughSubject<Int, Never>()
+    ///     cancellable = subject
+    ///         .setFailureType(to: URLError.self)
+    ///         .map() { index -> URLSession.DataTaskPublisher in
+    ///             let url = URL(string: "https://example.org/get?index=\(index)")!
+    ///             return URLSession.shared.dataTaskPublisher(for: url)
+    ///         }
+    ///         .switchToLatest()
+    ///         .sink(receiveCompletion: { print("Complete: \($0)") },
+    ///               receiveValue: { (data, response) in
+    ///                 guard let url = response.url else {
+    ///                     print("Bad response.")
+    ///                     return
+    ///                 }
+    ///                 print("URL: \(url)")
+    ///         })
+    ///
+    ///     for index in 1...5 {
+    ///         DispatchQueue.main.asyncAfter(deadline: .now() + TimeInterval(index/10)) {
+    ///             subject.send(index)
+    ///         }
+    ///     }
+    ///
+    ///     // Prints "URL: https://example.org/get?index=5"
+    ///
+    /// The exact behavior of this example depends on the value of `asyncAfter` and
+    /// the speed of the network connection. If the delay value is longer, or the network
+    /// connection is fast, the earlier data tasks may complete before `switchToLatest()`
+    /// can cancel them. If this happens, the output includes multiple URLs whose tasks
+    /// complete before cancellation.
    public func switchToLatest() -> Publishers.SwitchToLatest<Output, Self> {
        return .init(upstream: self)
    }
 }

+extension Publisher where Output: Publisher, Failure == Never {
+
+    /// Republishes elements sent by the most recently received publisher.
+    ///
+    /// This operator works with an upstream publisher of publishers, flattening
+    /// the stream of elements to appear as if they were coming from a single stream of
+    /// elements. It switches the inner publisher as new ones arrive but keeps the outer
+    /// publisher constant for downstream subscribers.
+    ///
+    /// When this operator receives a new publisher from the upstream publisher, it
+    /// cancels its previous subscription. Use this feature to prevent earlier publishers
+    /// from performing unnecessary work, such as creating network request publishers from
+    /// frequently updating user interface publishers.
+    public func switchToLatest() -> Publishers.SwitchToLatest<
+        Output, Publishers.SetFailureType<Self, Output.Failure>
+    > {
+        return setFailureType(to: Output.Failure.self).switchToLatest()
+    }
+}
+
+extension Publisher where Output: Publisher, Failure == Never, Output.Failure == Never {
+
+    /// Republishes elements sent by the most recently received publisher.
+    ///
+    /// This operator works with an upstream publisher of publishers, flattening
+    /// the stream of elements to appear as if they were coming from a single stream of
+    /// elements. It switches the inner publisher as new ones arrive but keeps the outer
+    /// publisher constant for downstream subscribers.
+    ///
+    /// When this operator receives a new publisher from the upstream publisher, it
+    /// cancels its previous subscription. Use this feature to prevent earlier publishers
+    /// from performing unnecessary work, such as creating network request publishers from
+    /// frequently updating user interface publishers.
+    public func switchToLatest() -> Publishers.SwitchToLatest<Output, Self> {
+        return .init(upstream: self)
+    }
+}
+
+extension Publisher where Output: Publisher, Output.Failure == Never {
+
+    /// Republishes elements sent by the most recently received publisher.
+    ///
+    /// This operator works with an upstream publisher of publishers, flattening
+    /// the stream of elements to appear as if they were coming from a single stream of
+    /// elements. It switches the inner publisher as new ones arrive but keeps the outer
+    /// publisher constant for downstream subscribers.
+    ///
+    /// When this operator receives a new publisher from the upstream publisher, it
+    /// cancels its previous subscription. Use this feature to prevent earlier publishers
+    /// from performing unnecessary work, such as creating network request publishers from
+    /// frequently updating user interface publishers.
+    public func switchToLatest() -> Publishers.SwitchToLatest<
+        Publishers.SetFailureType<Output, Failure>,
+        Publishers.Map<Self, Publishers.SetFailureType<Output, Failure>>
+    > {
+        return map { $0.setFailureType(to: Failure.self) }.switchToLatest()
+    }
+}
+
 extension Publishers {

-    /// A publisher that “flattens” nested 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.
+    /// Given a publisher that publishes `Publisher` instances,
+    /// the `Publishers.SwitchToLatest` publisher produces a sequence of events from only
+    /// the most recent one. For example, given the type
+    /// `AnyPublisher<URLSession.DataTaskPublisher, NSError>`, calling `switchToLatest()`
+    /// results in the type `SwitchToLatest<(Data, URLResponse), URLError>`.
+    /// The downstream subscriber sees a continuous stream of `(Data, URLResponse)`
+    /// elements from what looks like a single `URLSession.DataTaskPublisher` even though
+    /// the elements are coming from different upstream publishers.
    ///
-    /// 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.
+    /// When `Publishers.SwitchToLatest` receives a new publisher from the upstream
+    /// publisher, it cancels its previous subscription. Use this feature to prevent
+    /// earlier publishers from performing unnecessary work, such as creating network
+    /// request publishers from frequently-updating user interface publishers.
    public struct SwitchToLatest<NestedPublisher: Publisher, Upstream: Publisher>
        : Publisher
        where Upstream.Output == NestedPublisher,
@@ -0,0 +1,301 @@
+//
+//  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.
+    ///
+    /// Use `timeout(_:scheduler:options:customError:)` to terminate a publisher if
+    /// an element isn’t delivered within a timeout interval you specify.
+    ///
+    /// In the example below, a `PassthroughSubject` publishes `String` elements and is
+    /// configured to time out if no new elements are received within its `TIME_OUT`
+    /// window of 5 seconds. A single value is published after the specified 2-second
+    /// `WAIT_TIME`, after which no more elements are available; the publisher then times
+    /// out and completes normally.
+    ///
+    ///     var WAIT_TIME : Int = 2
+    ///     var TIMEOUT_TIME : Int = 5
+    ///
+    ///     let subject = PassthroughSubject<String, Never>()
+    ///     let cancellable = subject
+    ///         .timeout(.seconds(TIMEOUT_TIME),
+    ///                  scheduler: DispatchQueue.main,
+    ///                  options: nil,
+    ///                  customError: nil)
+    ///         .sink(
+    ///               receiveCompletion: { print ("completion: \($0) at \(Date())") },
+    ///               receiveValue: { print ("value: \($0) at \(Date())") }
+    ///          )
+    ///
+    ///     DispatchQueue.main.asyncAfter(
+    ///         deadline: .now() + .seconds(WAIT_TIME),
+    ///         execute: {
+    ///             subject.send("Some data - sent after a delay of \(WAIT_TIME) seconds")
+    ///         }
+    ///     )
+    ///
+    ///     // Prints:
+    ///     //   value: Some data - sent after a delay of 2 seconds at
+    ///     //          2020-03-10 23:47:59 +0000
+    ///     //   completion: finished at 2020-03-10 23:48:04 +0000
+    ///
+    /// If `customError` is `nil`, the publisher completes normally; if you provide
+    /// a closure for the `customError` argument, the upstream publisher is instead
+    /// terminated upon timeout, and the error is delivered to the downstream.
+    ///
+    /// - 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: (() -> 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?
+
+        private var initialDemand = false
+
+        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()
+        }
+
+        func receive(_ input: Input) -> 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.scheduledReceive(input)
+            }
+            return .none
+        }
+
+        func receive(completion: Subscribers.Completion<Failure>) {
+            lock.lock()
+            guard case .subscribed = state else {
+                lock.unlock()
+                return
+            }
+            timer?.cancel()
+            lock.unlock()
+            scheduler.schedule(options: options) {
+                self.scheduledReceive(completion: completion)
+            }
+        }
+
+        func request(_ demand: Subscribers.Demand) {
+            lock.lock()
+            guard case let .subscribed(subscription) = state else {
+                lock.unlock()
+                return
+            }
+            if !initialDemand {
+                timer = timeoutClock()
+                initialDemand = true
+            }
+            lock.unlock()
+            subscription.request(demand)
+        }
+
+        func cancel() {
+            lock.lock()
+            guard case let .subscribed(subscription) = state else {
+                lock.unlock()
+                return
+            }
+            state = .terminal
+            lock.unlock()
+            timer?.cancel()
+            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,
+                          timedOut)
+            return AnyCancellable(cancellable.cancel)
+        }
+
+        private func scheduledReceive(_ input: Input) {
+            lock.lock()
+            guard !didTimeout, case let .subscribed(subscription) = state else {
+                lock.unlock()
+                return
+            }
+            lock.unlock()
+            downstreamLock.lock()
+            let newDemand = downstream.receive(input)
+            downstreamLock.unlock()
+            if newDemand != .none {
+                subscription.request(newDemand)
+            }
+        }
+
+        private func scheduledReceive(completion: Subscribers.Completion<Failure>) {
+            lock.lock()
+            guard case .subscribed = state else {
+                lock.unlock()
+                return
+            }
+            state = .terminal
+            lock.unlock()
+            downstreamLock.lock()
+            downstream.receive(completion: completion)
+            downstreamLock.unlock()
+        }
+    }
+}
@@ -5,9 +5,9 @@
 //  Created by Sergej Jaskiewicz on 14.06.2019.
 //

-/// A namespace for types related to the Publisher protocol.
+/// A namespace for types that serve as publishers.
 ///
 /// The various operators defined as extensions on `Publisher` implement their
-/// functionality as classes or structures that extend this enumeration. For example,
-/// the `contains()` operator returns a `Publishers.Contains` instance.
+/// functionality as classes or structures that extend this enumeration.
+/// For example, the `contains(_:)` operator returns a `Publishers.Contains` instance.
 public enum Publishers {}
@@ -12,19 +12,30 @@ public struct Record<Output, Failure: Error>: Publisher {
    /// The recorded output and completion.
    public let recording: Recording

-    /// Interactively record a series of outputs and a completion.
+    /// Creates a publisher to interactively record a series of outputs and a completion.
+    ///
+    /// - Parameter record: A recording instance that can be retrieved after completion
+    ///   to create new record publishers to replay the recording.
    public init(record: (inout Recording) -> Void) {
        var recording = Recording()
        record(&recording)
        self.init(recording: recording)
    }

-    /// Initialize with a recording.
+    /// Creates a record publisher from an existing recording.
+    ///
+    /// - Parameter recording: A previously-recorded recording of published elements
+    ///   and a completion.
    public init(recording: Recording) {
        self.recording = recording
    }

-    /// Set up a complete recording with the specified output and completion.
+    /// Creates a record publisher to publish the provided elements, followed by
+    /// the provided completion value.
+    ///
+    /// - Parameters:
+    ///   - output: An array of output elements to publish.
+    ///   - completion: The completion value with which to end publishing.
    public init(output: [Output], completion: Subscribers.Completion<Failure>) {
        self.init(recording: Recording(output: output, completion: completion))
    }
@@ -43,7 +54,7 @@ public struct Record<Output, Failure: Error>: Publisher {
        }
    }

-    /// A recorded set of `Output` and a `Subscribers.Completion`.
+    /// A recorded sequence of outputs, followed by a completion value.
    public struct Recording {

        public typealias Input = Output
@@ -8,22 +8,32 @@
 /// A protocol that provides a scheduler with an expression for relative time.
 public protocol SchedulerTimeIntervalConvertible {

+    /// Converts the specified number of seconds into an instance of this scheduler time
+    /// type.
    static func seconds(_ s: Int) -> Self

+    /// Converts the specified number of seconds, as a floating-point value, into
+    /// an instance of this scheduler time type.
    static func seconds(_ s: Double) -> Self

+    /// Converts the specified number of milliseconds into an instance of this scheduler
+    /// time type.
    static func milliseconds(_ ms: Int) -> Self

+    /// Converts the specified number of microseconds into an instance of this scheduler
+    /// time type.
    static func microseconds(_ us: Int) -> Self

+    /// Converts the specified number of nanoseconds into an instance of this scheduler
+    /// time type.
    static func nanoseconds(_ ns: Int) -> Self
 }

 /// A protocol that defines when and how to execute a closure.
 ///
-/// A scheduler used to execute code as soon as possible, or after a future date.
+/// You can use a scheduler to execute code as soon as possible, or after a future date.
 /// Individual scheduler implementations use whatever time-keeping system makes sense
-/// for them. Schdedulers express this as their `SchedulerTimeType`. Since this type
+/// for them. Schedulers express this as their `SchedulerTimeType`. Since this type
 /// conforms to `SchedulerTimeIntervalConvertible`, you can always express these times
 /// with the convenience functions like `.milliseconds(500)`. Schedulers can accept
 /// options to control how they execute the actions passed to them. These options may
@@ -40,10 +50,10 @@ public protocol Scheduler {
    /// take a `Scheduler` parameter will also take `SchedulerOptions`.
    associatedtype SchedulerOptions

-    /// Returns this scheduler's definition of the current moment in time.
+    /// This scheduler’s definition of the current moment in time.
    var now: SchedulerTimeType { get }

-    /// Returns the minimum tolerance allowed by the scheduler.
+    /// The minimum tolerance allowed by the scheduler.
    var minimumTolerance: SchedulerTimeType.Stride { get }

    /// Performs the action at the next possible opportunity.
@@ -68,6 +78,8 @@ extension Scheduler {

    /// Performs the action at some time after the specified date, using the scheduler’s
    /// minimum tolerance.
+    ///
+    /// The immediate scheduler ignores `date` and performs the action immediately.
    @inlinable
    public func schedule(after date: SchedulerTimeType,
                         _ action: @escaping () -> Void) {
@@ -81,6 +93,8 @@ extension Scheduler {
    }

    /// Performs the action at some time after the specified date.
+    ///
+    /// The immediate scheduler ignores `date` and performs the action immediately.
    @inlinable
    public func schedule(after date: SchedulerTimeType,
                         tolerance: SchedulerTimeType.Stride,
@@ -90,6 +104,8 @@ extension Scheduler {

    /// Performs the action at some time after the specified date, at the specified
    /// frequency, taking into account tolerance if possible.
+    ///
+    /// The immediate scheduler ignores `date` and performs the action immediately.
    @inlinable
    public func schedule(after date: SchedulerTimeType,
                         interval: SchedulerTimeType.Stride,
@@ -104,6 +120,8 @@ extension Scheduler {

    /// Performs the action at some time after the specified date, at the specified
    /// frequency, using minimum tolerance possible for this Scheduler.
+    ///
+    /// The immediate scheduler ignores `date` and performs the action immediately.
    @inlinable
    public func schedule(after date: SchedulerTimeType,
                         interval: SchedulerTimeType.Stride,
@@ -23,14 +23,22 @@ public protocol Subject: AnyObject, Publisher {
    ///   has finished normally or failed with an error.
    func send(completion: Subscribers.Completion<Failure>)

-    /// Provides this Subject an opportunity to establish demand for any new upstream
-    /// subscriptions (say, via `Publisher.subscribe<S: Subject>(_: Subject)`)
+    /// Sends a subscription to the subscriber.
+    ///
+    /// This call provides the `Subject` an opportunity to establish demand for any new
+    /// upstream subscriptions.
+    ///
+    /// - Parameter subscription: The subscription instance through which the subscriber
+    ///   can request elements.
    func send(subscription: Subscription)
 }

 extension Subject where Output == Void {

-    /// Signals subscribers.
+    /// Sends a void value to the subscriber.
+    ///
+    /// Use `Void` inputs and outputs when you want to signal that an event has occurred,
+    /// but don’t need to send the event itself.
    public func send() {
        send(())
    }
@@ -6,6 +6,28 @@
 //

 /// A protocol that declares a type that can receive input from a publisher.
+///
+/// A `Subscriber` instance receives a stream of elements from a `Publisher`, along with
+/// life cycle events describing changes to their relationship. A given subscriber’s
+/// `Input` and `Failure` associated types must match the `Output` and `Failure` of its
+/// corresponding publisher.
+///
+/// You connect a subscriber to a publisher by calling the publisher’s `subscribe(_:)`
+/// method.  After making this call, the publisher invokes the subscriber’s
+/// `receive(subscription:)` method. This gives the subscriber a `Subscription` instance,
+/// which it uses to demand elements from the publisher, and to optionally cancel
+/// the subscription. After the subscriber makes an initial demand, the publisher calls
+/// `receive(_:)`, possibly asynchronously, to deliver newly-published elements.
+/// If the publisher stops publishing, it calls `receive(completion:)`, using a parameter
+/// of type `Subscribers.Completion` to indicate whether publishing completes normally or
+/// with an error.
+///
+/// OpenCombine provides the following subscribers as operators on the `Publisher` type:
+///
+/// - `sink(receiveCompletion:receiveValue:)` executes arbitrary closures when
+///   it receives a completion signal and each time it receives a new element.
+/// - `assign(to:on:)` writes each newly-received value to a property identified by
+///   a key path on a given instance.
 public protocol Subscriber: CustomCombineIdentifierConvertible {

    /// The kind of values this subscriber receives.
@@ -27,20 +49,27 @@ public protocol Subscriber: CustomCombineIdentifierConvertible {
    /// Tells the subscriber that the publisher has produced an element.
    ///
    /// - Parameter input: The published element.
-    /// - Returns: A `Demand` instance indicating how many more elements the subcriber
-    ///   expects to receive.
+    /// - Returns: A `Subscribers.Demand` instance indicating how many more elements
+    ///   the subscriber expects to receive.
    func receive(_ input: Input) -> Subscribers.Demand

    /// Tells the subscriber that the publisher has completed publishing, either normally
    /// or with an error.
    ///
-    /// - Parameter completion: A `Completion` case indicating whether publishing
-    ///   completed normally or with an error.
+    /// - Parameter completion: A `Subscribers.Completion` case indicating whether
+    ///   publishing completed normally or with an error.
    func receive(completion: Subscribers.Completion<Failure>)
 }

 extension Subscriber where Input == Void {

+    /// Tells the subscriber that a publisher of void elements is ready to receive further
+    /// requests.
+    ///
+    /// Use `Void` inputs and outputs when you want to signal that an event has occurred,
+    /// but don’t need to send the event itself.
+    /// - Returns: A `Subscribers.Demand` instance indicating how many more elements
+    /// the subscriber expects to receive.
    public func receive() -> Subscribers.Demand {
        return receive(())
    }
@@ -7,25 +7,34 @@

 extension Subscribers {

+    /// A simple subscriber that assigns received elements to a property indicated by
+    /// a key path.
    public final class Assign<Root, Input>: Subscriber,
                                            Cancellable,
                                            CustomStringConvertible,
                                            CustomReflectable,
                                            CustomPlaygroundDisplayConvertible
    {
-        // NOTE: this class has been audited for thread safety.
-        // Combine doesn't use any locking here.
-
        public typealias Failure = Never

+        private let lock = UnfairLock.allocate()
+
+        /// The object that contains the property to assign.
+        ///
+        /// The subscriber holds a strong reference to this object until the upstream
+        /// publisher calls `Subscriber.receive(completion:)`, at which point
+        /// the subscriber sets this property to `nil`.
        public private(set) var object: Root?

+        /// The key path that indicates the property to assign.
        public let keyPath: ReferenceWritableKeyPath<Root, Input>

        private var status = SubscriptionStatus.awaitingSubscription

+        /// A textual representation of this subscriber.
        public var description: String { return "Assign \(Root.self)." }

+        /// A mirror that reflects the subscriber.
        public var customMirror: Mirror {
            let children: [Mirror.Child] = [
                ("object", object as Any),
@@ -37,56 +46,117 @@ extension Subscribers {

        public var playgroundDescription: Any { return description }

+        /// Creates a subscriber to assign the value of a property indicated by
+        /// a key path.
+        ///
+        /// - Parameters:
+        ///   - object: The object that contains the property. The subscriber assigns
+        ///     the object’s property every time it receives a new value.
+        ///   - keyPath: A key path that indicates the property to assign. See
+        ///     [Key-Path Expression](https://docs.swift.org/swift-book/ReferenceManual/Expressions.html#ID563)
+        ///     in _The Swift Programming Language_ to learn how to use key paths to
+        ///     specify a property of an object.
        public init(object: Root, keyPath: ReferenceWritableKeyPath<Root, Input>) {
            self.object = object
            self.keyPath = keyPath
        }

+        deinit {
+            lock.deallocate()
+        }
+
        public func receive(subscription: Subscription) {
-            switch status {
-            case .subscribed, .terminal:
+            lock.lock()
+            guard case .awaitingSubscription = status else {
+                lock.unlock()
                subscription.cancel()
-            case .awaitingSubscription:
-                status = .subscribed(subscription)
-                subscription.request(.unlimited)
+                return
            }
+            status = .subscribed(subscription)
+            lock.unlock()
+            subscription.request(.unlimited)
        }

        public func receive(_ value: Input) -> Subscribers.Demand {
-            switch status {
-            case .subscribed:
-                object?[keyPath: keyPath] = value
-            case .awaitingSubscription, .terminal:
-                break
+            lock.lock()
+            guard case .subscribed = status, let object = self.object else {
+                lock.unlock()
+                return .none
            }
+            lock.unlock()
+            object[keyPath: keyPath] = value
            return .none
        }

        public func receive(completion: Subscribers.Completion<Never>) {
-            cancel()
+            lock.lock()
+            guard case .subscribed = status else {
+                lock.unlock()
+                return
+            }
+            terminateAndConsumeLock()
        }

        public func cancel() {
+            lock.lock()
            guard case let .subscribed(subscription) = status else {
+                lock.unlock()
                return
            }
+            terminateAndConsumeLock()
            subscription.cancel()
+        }
+
+        private func terminateAndConsumeLock() {
+#if DEBUG
+            lock.assertOwner()
+#endif
            status = .terminal
            object = nil
+            lock.unlock()
        }
    }
 }

 extension Publisher where Failure == Never {

-    /// Assigns each element from a Publisher to a property on an object.
+    /// Assigns each element from a publisher to a property on an object.
+    ///
+    /// Use the `assign(to:on:)` subscriber when you want to set a given property each
+    /// time a publisher produces a value.
+    ///
+    /// In this example, the `assign(to:on:)` sets the value of the `anInt` property on
+    /// an instance of `MyClass`:
+    ///
+    ///     class MyClass {
+    ///         var anInt: Int = 0 {
+    ///             didSet {
+    ///                 print("anInt was set to: \(anInt)", terminator: "; ")
+    ///             }
+    ///         }
+    ///     }
+    ///
+    ///     var myObject = MyClass()
+    ///     let myRange = (0...2)
+    ///     cancellable = myRange.publisher
+    ///         .assign(to: \.anInt, on: myObject)
+    ///
+    ///     // Prints: "anInt was set to: 0; anInt was set to: 1; anInt was set to: 2"
+    ///
+    ///  > Important: The `Subscribers.Assign` instance created by this operator maintains
+    ///  a strong reference to `object`, and sets it to `nil` when the upstream publisher
+    ///  completes (either normally or with an error).
    ///
    /// - Parameters:
-    ///   - keyPath: The key path of the property to assign.
-    ///   - object: The object on which to assign the value.
-    /// - Returns: A cancellable instance; used when you end assignment
-    ///   of the received value. Deallocation of the result will tear down
-    ///   the subscription stream.
+    ///   - keyPath: A key path that indicates the property to assign. See
+    ///     [Key-Path Expression](https://docs.swift.org/swift-book/ReferenceManual/Expressions.html#ID563)
+    ///     in _The Swift Programming Language_ to learn how to use key paths to specify
+    ///     a property of an object.
+    ///   - object: The object that contains the property. The subscriber assigns
+    ///     the object’s property every time it receives a new value.
+    /// - Returns: An `AnyCancellable` instance. Call `cancel()` on this instance when you
+    ///   no longer want the publisher to automatically assign the property.
+    ///   Deinitializing this instance will also cancel automatic assignment.
    public func assign<Root>(to keyPath: ReferenceWritableKeyPath<Root, Output>,
                             on object: Root) -> AnyCancellable {
        let subscriber = Subscribers.Assign(object: object, keyPath: keyPath)
@@ -7,15 +7,14 @@

 extension Subscribers {

-    /// A signal that a publisher doesn’t produce additional elements, either due
-    /// to normal completion or an error.
-    ///
-    /// - `finished`: The publisher finished normally.
-    /// - `failure`: The publisher stopped publishing due to the indicated error.
+    /// A signal that a publisher doesn’t produce additional elements, either due to
+    /// normal completion or an error.
    public enum Completion<Failure: Error> {

+        /// The publisher finished normally.
        case finished

+        /// The publisher stopped publishing due to the indicated error.
        case failure(Failure)
    }
 }
@@ -9,11 +9,8 @@

 extension Subscribers {

-    /// A requested number of items, sent to a publisher from a subscriber via
+    /// A requested number of items, sent to a publisher from a subscriber through
    /// the subscription.
-    ///
-    /// - `unlimited`: A request for an unlimited number of items.
-    /// - `max`: A request for a maximum number of items.
    public struct Demand: Equatable,
                          Comparable,
                          Hashable,
@@ -29,23 +26,27 @@ extension Subscribers {
            self.rawValue = min(UInt(Int.max) + 1, rawValue)
        }

-        /// Requests as many values as the `Publisher` can produce.
+        /// A request for as many values as the publisher can produce.
        @inline(__always)
        @inlinable
        public static var unlimited: Demand {
            return Demand(rawValue: .max)
        }

-        /// A demand for no items.
+        /// A request for no elements from the publisher.
        ///
        /// This is equivalent to `Demand.max(0)`.
        @inline(__always)
        @inlinable
        public static var none: Demand { return .max(0) }

-        /// Limits the maximum number of values.
-        /// The `Publisher` may send fewer than the requested number.
-        /// Negative values will result in a `fatalError`.
+        /// Creates a demand for the given maximum number of elements.
+        ///
+        /// The publisher is free to send fewer than the requested maximum number of
+        /// elements.
+        ///
+        /// - Parameter value: The maximum number of elements. Providing a negative value
+        /// for this parameter results in a fatal error.
        @inline(__always)
        @inlinable
        public static func max(_ value: Int) -> Demand {
@@ -437,7 +438,9 @@ extension Subscribers {
            return lhs.rawValue == rhs.rawValue
        }

-        /// The number of requested values, or nil if `.unlimited`.
+        /// The number of requested values.
+        ///
+        /// The value is `nil` if the demand is `Subscribers.Demand.unlimited`.
        @inlinable public var max: Int? {
            if self == .unlimited {
                return nil
@@ -446,10 +449,17 @@ extension Subscribers {
            }
        }

+        /// Creates a demand instance from a decoder.
+        ///
+        /// - Parameter decoder: The decoder of a previously-encoded `Subscribers.Demand`
+        ///   instance.
        public init(from decoder: Decoder) throws {
            try self.init(rawValue: decoder.singleValueContainer().decode(UInt.self))
        }

+        /// Encodes the demand to the provided encoder.
+        ///
+        /// - Parameter encoder: An encoder instance.
        public func encode(to encoder: Encoder) throws {
            var container = encoder.singleValueContainer()
            try container.encode(rawValue)
@@ -15,17 +15,17 @@ extension Subscribers {
          CustomReflectable,
          CustomPlaygroundDisplayConvertible
    {
-        // NOTE: this class has been audited for thread safety.
-        // Combine doesn't use any locking here.

        /// The closure to execute on receipt of a value.
-        public let receiveValue: (Input) -> Void
+        public var receiveValue: (Input) -> Void

        /// The closure to execute on completion.
-        public let receiveCompletion: (Subscribers.Completion<Failure>) -> Void
+        public var receiveCompletion: (Subscribers.Completion<Failure>) -> Void

        private var status = SubscriptionStatus.awaitingSubscription

+        private let lock = UnfairLock.allocate()
+
        public var description: String { return "Sink" }

        public var customMirror: Mirror {
@@ -47,32 +47,55 @@ extension Subscribers {
            self.receiveValue = receiveValue
        }

+        deinit {
+            lock.deallocate()
+        }
+
        public func receive(subscription: Subscription) {
-            switch status {
-            case .subscribed, .terminal:
+            lock.lock()
+            guard case .awaitingSubscription = status else {
+                lock.unlock()
                subscription.cancel()
-            case .awaitingSubscription:
-                status = .subscribed(subscription)
-                subscription.request(.unlimited)
+                return
            }
+            status = .subscribed(subscription)
+            lock.unlock()
+            subscription.request(.unlimited)
        }

        public func receive(_ value: Input) -> Subscribers.Demand {
+            lock.lock()
+            let receiveValue = self.receiveValue
+            lock.unlock()
            receiveValue(value)
            return .none
        }

        public func receive(completion: Subscribers.Completion<Failure>) {
+            lock.lock()
+            let receiveCompletion = self.receiveCompletion
+            terminateAndConsumeLock()
            receiveCompletion(completion)
-            status = .terminal
        }

        public func cancel() {
+            lock.lock()
            guard case let .subscribed(subscription) = status else {
+                lock.unlock()
                return
            }
+            terminateAndConsumeLock()
            subscription.cancel()
+        }
+
+        private func terminateAndConsumeLock() {
+#if DEBUG
+            lock.assertOwner()
+#endif
            status = .terminal
+            receiveValue = { _ in }
+            receiveCompletion = { _ in }
+            lock.unlock()
        }
    }
 }
@@ -81,14 +104,36 @@ extension Publisher {

    /// Attaches a subscriber with closure-based behavior.
    ///
+    /// Use `sink(receiveCompletion:receiveValue:)` to observe values received by
+    /// the publisher and process them using a closure you specify.
+    ///
+    /// In this example, a `Range` publisher publishes integers to
+    /// a `sink(receiveCompletion:receiveValue:)` operator’s `receiveValue` closure that
+    /// prints them to the console. Upon completion
+    /// the `sink(receiveCompletion:receiveValue:)` operator’s `receiveCompletion` closure
+    /// indicates the successful termination of the stream.
+    ///
+    ///     let myRange = (0...3)
+    ///     cancellable = myRange.publisher
+    ///         .sink(receiveCompletion: { print ("completion: \($0)") },
+    ///               receiveValue: { print ("value: \($0)") })
+    ///
+    ///     // Prints:
+    ///     //  value: 0
+    ///     //  value: 1
+    ///     //  value: 2
+    ///     //  value: 3
+    ///     //  completion: finished
+    ///
    /// This method creates the subscriber and immediately requests an unlimited number
    /// of values, prior to returning the subscriber.
+    /// The return value should be held, otherwise the stream will be canceled.
    ///
    /// - parameter receiveComplete: The closure to execute on completion.
    /// - parameter receiveValue: The closure to execute on receipt of a value.
-    /// - Returns: A cancellable instance; used when you end assignment of
-    ///   the received value. Deallocation of the result will tear down
-    ///   the subscription stream.
+    /// - Returns: A cancellable instance, which you use when you end assignment of
+    ///   the received value. Deallocation of the result will tear down the subscription
+    ///   stream.
    public func sink(
        receiveCompletion: @escaping (Subscribers.Completion<Failure>) -> Void,
        receiveValue: @escaping ((Output) -> Void)
@@ -104,15 +149,33 @@ extension Publisher {

 extension Publisher where Failure == Never {

-    /// Attaches a subscriber with closure-based behavior.
+    /// Attaches a subscriber with closure-based behavior to a publisher that never fails.
    ///
-    /// This method creates the subscriber and immediately requests an unlimited number
-    /// of values, prior to returning the subscriber.
+    /// Use `sink(receiveValue:)` to observe values received by the publisher and print
+    /// them to the console. This operator can only be used when the stream doesn’t fail,
+    /// that is, when the publisher’s `Failure` type is `Never`.
+    ///
+    /// In this example, a `Range` publisher publishes integers to a `sink(receiveValue:)`
+    /// operator’s `receiveValue` closure that prints them to the console:
+    ///
+    ///     let integers = (0...3)
+    ///     integers.publisher
+    ///         .sink { print("Received \($0)") }
+    ///
+    ///     // Prints:
+    ///     //  Received 0
+    ///     //  Received 1
+    ///     //  Received 2
+    ///     //  Received 3
+    ///
+    /// This method creates the subscriber and immediately requests an unlimited number of
+    /// values, prior to returning the subscriber.
+    /// The return value should be held, otherwise the stream will be canceled.
    ///
    /// - parameter receiveValue: The closure to execute on receipt of a value.
-    /// - Returns: A cancellable instance; used when you end assignment of
-    ///   the received value. Deallocation of the result will tear down
-    ///   the subscription stream.
+    /// - Returns: A cancellable instance, which you use when you end assignment of
+    ///   the received value. Deallocation of the result will tear down the subscription
+    ///   stream.
    public func sink(
        receiveValue: @escaping (Output) -> Void
    ) -> AnyCancellable {
--- a/Show More
+++ b/Show More
Author	SHA1	Message	Date
Sergej Jaskiewicz	9bba508134	Bump the version to 0.11.0	2020-10-30 00:53:48 +03:00
Sergej Jaskiewicz	2d857d6d66	Remove the note about the project being in early development	2020-10-30 00:53:48 +03:00
Sergej Jaskiewicz	7286336b28	Enable URLSession tests on non-Darwin platforms since Swift 5.3 Closes #169.	2020-10-29 19:49:34 +03:00
Sergej Jaskiewicz	8a0bb6f846	[Xcode 12] Update Publishers.Debounce implementation and tests The tests now pass in compatibility mode when run against iOS 14 Combine.	2020-10-29 19:49:34 +03:00
Sergej Jaskiewicz	142811c500	[Xcode 12] Update @Published implementation and tests The tests now pass in compatibility mode when run against iOS 14 Combine.	2020-10-29 19:49:34 +03:00
Sergej Jaskiewicz	3b1cff9337	[Xcode 12] Implement Optional.publisher property	2020-10-29 19:49:34 +03:00
Sergej Jaskiewicz	efb4369c74	[Xcode 12] Add new flatMap(maxPublishers:_:) overloads	2020-10-29 19:49:34 +03:00
Sergej Jaskiewicz	9d87a3b4ea	[Xcode 12] Update Publishers.Buffer implementation and tests The tests now pass in compatibility mode when run against iOS 14 Combine.	2020-10-29 19:49:34 +03:00
Sergej Jaskiewicz	4714b80631	[Xcode 12] Update Publishers.Drop implementation and tests The tests now pass in compatibility mode when run against iOS 14 Combine.	2020-10-29 19:49:34 +03:00
Sergej Jaskiewicz	3cba7363b4	Better API for the Atomic test helper	2020-10-29 19:49:34 +03:00
Sergej Jaskiewicz	b2f592611d	Enable tests on Ubuntu using Swift 5.0 and make them pass	2020-10-29 19:49:34 +03:00
Sergej Jaskiewicz	8786d0860a	Use custom script for test discovery Standard test discovery is not available in Swift 5.0, but we want to support this version of Swift. Besides, standard test discovery doesn't work very well with CircleCI: https://bugs.swift.org/browse/SR-10783	2020-10-29 19:49:34 +03:00
Sergej Jaskiewicz	9100ccafb3	Enable testing with Swift 5.1 and Swift 5.3	2020-10-29 19:49:34 +03:00
Sergej Jaskiewicz	3fa048ddca	Make DispatchQueue scheduler tests pass in release mode. In the new version of the Swift compiler, due to a new optimization the cancellation token was deallocated immediately, resulting in the closure never being executed. Fix this by extending the token's lifetime.	2020-10-29 19:49:34 +03:00
Sergej Jaskiewicz	8832baa05b	[Xcode 12] Update Publishers.ReplaceError implementation and tests The tests now pass in compatibility mode when run against iOS 14 Combine.	2020-10-29 19:49:34 +03:00
Sergej Jaskiewicz	11fdf7eaf3	[Xcode 12] Update Publishers.[Encode\|Decode] implementation and tests The tests now pass in compatibility mode when run against iOS 14 Combine.	2020-10-29 19:49:34 +03:00
Sergej Jaskiewicz	059a86d393	[Xcode 12] Update Publishers.CompactMap implementation and tests The tests now pass in compatibility mode when run against iOS 14 Combine.	2020-10-29 19:49:34 +03:00
Sergej Jaskiewicz	c1b4d93a0f	[Xcode 12] Update Publishers.Filter implementation and tests The tests now pass in compatibility mode when run against iOS 14 Combine.	2020-10-29 19:49:34 +03:00
Sergej Jaskiewicz	fb184ceebe	[Xcode 12] Add new switchToLatest() overloads	2020-10-29 19:49:34 +03:00
Sergej Jaskiewicz	69b50074ff	[Xcode 12] Update AnyPublisher and AnySubscriber initializers The new version avoids double boxing.	2020-10-29 19:49:34 +03:00
Sergej Jaskiewicz	7351cd671c	[Xcode 12] Update documentation	2020-10-29 19:49:34 +03:00
Sergej Jaskiewicz	26e112f894	[Xcode 12] Implement _Introspection functionality _Introspection allows to observe the subscription graph and data flow.	2020-10-29 19:49:34 +03:00
Sergej Jaskiewicz	6c732515d8	[Xcode 12] Update Publishers.Delay implementation and tests The tests now pass in compatibility mode when run against iOS 14 Combine.	2020-10-29 19:49:34 +03:00
Sergej Jaskiewicz	6892923743	[Xcode 12] Update Timer publisher implementation and tests The tests now pass in compatibility mode when run against iOS 14 Combine.	2020-10-29 19:49:34 +03:00
Sergej Jaskiewicz	f8809ffac4	[Xcode 12] Update Publishers.HandleEvents implementation and tests The tests now pass in compatibility mode when run against iOS 14 Combine.	2020-10-29 19:49:34 +03:00
Sergej Jaskiewicz	5da402bb2c	Fix a deprecation warning in Publishers.Breakpoint implementation	2020-10-29 19:49:34 +03:00
Sergej Jaskiewicz	e962ce1e3b	[Xcode 12] Update OperationQueue scheduler implementation and tests The tests now pass in compatibility mode when run against iOS 14 Combine.	2020-10-29 19:49:34 +03:00
Sergej Jaskiewicz	19df744bf1	[Xcode 12] Update Publishers.ReceiveOn implementation and tests The tests now pass in compatibility mode when run against iOS 14 Combine.	2020-10-29 19:49:34 +03:00
Sergej Jaskiewicz	69cc5a92c2	[Xcode 12] Update Publishers.Timeout implementation and tests The tests now pass in compatibility mode when run against iOS 14 Combine.	2020-10-29 19:49:34 +03:00
Sergej Jaskiewicz	cb22cc98f2	[Xcode 12] Update Publishers.Sequence implementation and tests The tests now pass in compatibility mode when run against iOS 14 Combine.	2020-10-29 19:49:34 +03:00
Sergej Jaskiewicz	22cf5b69ba	[Xcode 12] Update Publishers.SubscribeOn implementation and tests The tests now pass in compatibility mode when run against iOS 14 Combine.	2020-10-29 19:49:34 +03:00
Sergej Jaskiewicz	a3fecd18e6	[Xcode 12] Update RunLoop scheduler implementation and tests The tests now pass in compatibility mode when run against iOS 14 Combine.	2020-10-29 19:49:34 +03:00
Sergej Jaskiewicz	48c6f2999b	[Xcode 12] Update Publishers.MeasureInterval implementation and tests The tests now pass in compatibility mode when run against iOS 14 Combine.	2020-10-29 19:49:34 +03:00
Sergej Jaskiewicz	91297ae63a	[Xcode 12] Update Publishers.IgnoreOutput implementation and tests The tests now pass in compatibility mode when run against iOS 14 Combine.	2020-10-29 19:49:34 +03:00
Sergej Jaskiewicz	6f61dcc083	[Xcode 12] Update Subscribers.Assign implementation and tests The tests now pass in compatibility mode when run against iOS 14 Combine.	2020-10-29 19:49:34 +03:00
Sergej Jaskiewicz	24eca2fab8	[Xcode 12] Update Publishers.Breakpoint implementation and tests The tests now pass in compatibility mode when run against iOS 14 Combine.	2020-10-29 19:49:34 +03:00
Sergej Jaskiewicz	9b3c36124f	[Xcode 12] Update Subscribers.Sink implementation and tests The tests now pass in compatibility mode when run against iOS 14 Combine.	2020-10-29 19:49:34 +03:00
Sergej Jaskiewicz	1b017e1dfc	[Xcode 12] Update DispatchQueue.SchedulerTimeType.Stride implementation and tests The tests now pass in compatibility mode when run against iOS 14 Combine.	2020-10-29 19:49:34 +03:00
Sergej Jaskiewicz	32b3aeb94c	[Xcode 12] Update CombineIdentifier implementation and tests The tests now pass in compatibility mode when run against iOS 14 Combine.	2020-10-29 19:49:34 +03:00
Sergej Jaskiewicz	579d174288	[Xcode 12] Update Publishers.Concatenate implementation and tests The tests now pass in compatibility mode when run against iOS 14 Combine.	2020-10-29 19:49:34 +03:00
Sergej Jaskiewicz	1c850fc6bb	Make it buildable with Xcode 12 toolchain	2020-10-29 19:49:34 +03:00
Sergej Jaskiewicz	b35ecf8356	Enable compatibility testing against iOS 14 Combine	2020-10-29 19:49:34 +03:00
Sergej Jaskiewicz	302f663a3f	Don't track file and line in test methods Since Xcode 12 those are tracked automatically. No need to use #file and #line tokens in method arguments.	2020-10-29 19:49:34 +03:00
Sergej Jaskiewicz	c805f0f5aa	Bump the version to 0.10.2 (#188 )	2020-10-23 15:29:02 +03:00
Sergej Jaskiewicz	cb99f8b298	Make downstreamSubject a weak property in SubjectSubscriber (#187 ) This is how it is in Combine. Fixes #186.	2020-10-23 13:48:43 +03:00
Maximilian Wendel	915a7efaf5	Prepare for 0.10.1 (#185 )	2020-10-04 15:01:09 +03:00
Sergej Jaskiewicz	024e576b0f	Add link to generated interfaces for Combine	2020-10-01 13:21:33 +03:00
Max Desiatov	f4a611e95f	Run compatibility tests on iOS 13.6/Xcode 11.6 (#181 )	2020-08-31 12:12:38 +01:00
Maximilian Wendel	c09e47f792	Fix OperationQueue scheduler on non-Darwin platforms before Swift 5.1 (#177 )	2020-07-29 16:26:50 +03:00
Maximilian Wendel	dd6be33016	Don't use PropertyListEncoder on non-Darwin platforms before Swift 5.1 (#176 ) PropertyListEncoder and PropertyListDecoder are both unavailable prior to Swift 5.1, causing a build error for Swift 5.0.	2020-07-29 16:24:28 +03:00
dependabot[bot]	5af4fb6ba4	Bump json from 2.2.0 to 2.3.1 (#175 )	2020-07-28 07:22:03 +00:00
Adam Leonard	0ca4c7658f	Fix a build error on linux: `kCFStringEncodingUTF8` is not defined. (#173 ) Instead, use `CFStringBuiltInEncodings.UTF8.rawValue`. Also fix a type error I was getting in a unit test. Co-authored-by: adaml <adam@seesaw.me>	2020-07-21 16:05:48 +03:00
Alexey Salangin	8cf59d6d2a	Fix some typos (#172 )	2020-07-14 08:48:35 +03:00
Sergej Jaskiewicz	f3d068d6f2	Bump the version to 0.10.0 (#171 )	2020-06-28 20:39:03 +03:00
Sergej Jaskiewicz	1cfb4a2eae	Implement Publishers.Debounce (#133 )	2020-06-28 19:50:45 +03:00
Sergej Jaskiewicz	2b64b7981d	Implement Publishers.Timeout (#164 )	2020-06-28 14:31:15 +03:00
Sergej Jaskiewicz	ad95dfdc8c	Update CircleCI badge	2020-06-26 17:25:12 +03:00
Sergej Jaskiewicz	988644159e	Update badges after migrating to an organization	2020-06-26 16:32:22 +03:00
Sergej Jaskiewicz	a9fa1ed4f4	Update the repository URL after migrating to an organization	2020-06-26 16:19:42 +03:00
Sergej Jaskiewicz	3f125b30e1	Implement OperationQueue scheduler (#165 )	2020-06-26 15:40:15 +03:00
Sergej Jaskiewicz	c9e7293a2a	Fix behavior of CurrentValueSubject when setting new value after completion	2020-06-26 11:38:57 +03:00
Sergej Jaskiewicz	f5d2c39c58	Add a test for CurrentValueSUbject	2020-06-26 11:17:32 +03:00
Sergej Jaskiewicz	70bf8e8bb3	Run compatibility tests on iOS 13.5/Xcode 11.5	2020-06-26 00:11:34 +03:00
Sergej Jaskiewicz	f04053e1eb	A more efficient and correct implementation of Future	2020-06-26 00:11:34 +03:00
Sergej Jaskiewicz	af510706d7	A more efficient and correct implementation of CurrentValueSubject	2020-06-26 00:11:34 +03:00
Sergej Jaskiewicz	29fbf7de31	A more efficient and correct implementation of PassthroughSubject	2020-06-26 00:11:34 +03:00
Sergej Jaskiewicz	102eef88a0	Implement ConduitList	2020-06-26 00:11:34 +03:00
Sergej Jaskiewicz	b34d4652d3	Make TimerPublisher tests more stable (#167 )	2020-06-24 16:09:15 +03:00
Max Desiatov	fcc2a4350a	Add TimerPublisher and Timer.publish (#156 ) Co-authored-by: Sergej Jaskiewicz <jaskiewiczs@icloud.com>	2020-06-23 20:55:20 +03:00