experimenting with RemoteAudioSource

* Bumps version

* Bumps version

Co-authored-by: Dimitrios C <dimitrisc@DimitrisC-Macbook-Pro.local>

AudioStreaming.podspec

@@ -1,6 +1,6 @@
 Pod::Spec.new do |s|
   s.name = 'AudioStreaming'
-  s.version = '0.4.0'
+  s.version = '0.7.0'
   s.license = 'MIT'
   s.summary = 'An AudioPlayer/Streaming library for iOS written in Swift using AVAudioEngine.'
   s.homepage = 'https://github.com/dimitris-c/AudioStreaming'

AudioStreaming.xcodeproj/project.pbxproj

@@ -52,6 +52,7 @@
 		B59DF1A32493E90C0043C498 /* AudioFileStream+Helpers.swift in Sources */ = {isa = PBXBuildFile; fileRef = B59DF1A22493E90C0043C498 /* AudioFileStream+Helpers.swift */; };
 		B5AEDBB824744153007D8101 /* AudioStreaming.framework in Frameworks */ = {isa = PBXBuildFile; fileRef = B5AEDBAE24744153007D8101 /* AudioStreaming.framework */; };
 		B5AEDBBF24744153007D8101 /* AudioStreaming.h in Headers */ = {isa = PBXBuildFile; fileRef = B5AEDBB124744153007D8101 /* AudioStreaming.h */; settings = {ATTRIBUTES = (Public, ); }; };
+		B5B36E432655A32200DC96F5 /* FrameFilterProcessor.swift in Sources */ = {isa = PBXBuildFile; fileRef = B5B36E422655A32200DC96F5 /* FrameFilterProcessor.swift */; };
 		B5B3B7CC248647ED00656828 /* AudioPlayerState.swift in Sources */ = {isa = PBXBuildFile; fileRef = B5B3B7CB248647ED00656828 /* AudioPlayerState.swift */; };
 		B5D4A40925D9321400E1450C /* IcycastHeaderParser.swift in Sources */ = {isa = PBXBuildFile; fileRef = B5D4A40825D9321400E1450C /* IcycastHeaderParser.swift */; };
 		B5D4A41025D948EF00E1450C /* IcycastHeadersProcessor.swift in Sources */ = {isa = PBXBuildFile; fileRef = B5D4A40B25D9445600E1450C /* IcycastHeadersProcessor.swift */; };
@@ -144,6 +145,7 @@
 		B5AEDBB224744153007D8101 /* Info.plist */ = {isa = PBXFileReference; lastKnownFileType = text.plist.xml; path = Info.plist; sourceTree = "<group>"; };
 		B5AEDBB724744153007D8101 /* AudioStreamingTests.xctest */ = {isa = PBXFileReference; explicitFileType = wrapper.cfbundle; includeInIndex = 0; path = AudioStreamingTests.xctest; sourceTree = BUILT_PRODUCTS_DIR; };
 		B5AEDBBE24744153007D8101 /* Info.plist */ = {isa = PBXFileReference; lastKnownFileType = text.plist.xml; path = Info.plist; sourceTree = "<group>"; };
+		B5B36E422655A32200DC96F5 /* FrameFilterProcessor.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = FrameFilterProcessor.swift; sourceTree = "<group>"; };
 		B5B3B7CB248647ED00656828 /* AudioPlayerState.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = AudioPlayerState.swift; sourceTree = "<group>"; };
 		B5D4A40825D9321400E1450C /* IcycastHeaderParser.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = IcycastHeaderParser.swift; sourceTree = "<group>"; };
 		B5D4A40B25D9445600E1450C /* IcycastHeadersProcessor.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = IcycastHeadersProcessor.swift; sourceTree = "<group>"; };
@@ -259,6 +261,7 @@
 		B55CEAC024855AA20001C498 /* Processors */ = {
 			isa = PBXGroup;
 			children = (
+				B5B36E422655A32200DC96F5 /* FrameFilterProcessor.swift */,
 				B5667A8F2499018D00D93F85 /* AudioFileStreamProcessor.swift */,
 				B5667B3D249BC43000D93F85 /* AudioPlayerRenderProcessor.swift */,
 				B55CE97024810DE20001C498 /* MetadataStreamProcessor.swift */,
@@ -615,6 +618,7 @@
 				B5667A902499018D00D93F85 /* AudioFileStreamProcessor.swift in Sources */,
 				B59D0B6F255C904900D6CCE5 /* FileAudioSource.swift in Sources */,
 				B5EF9555247E9393003E8FF8 /* AudioEntry.swift in Sources */,
+				B5B36E432655A32200DC96F5 /* FrameFilterProcessor.swift in Sources */,
 				B51FE0C02488F67C00F2A4D2 /* Queue.swift in Sources */,
 				B5667A922499063D00D93F85 /* AudioPlayerContext.swift in Sources */,
 				B55CE97124810DE20001C498 /* MetadataStreamProcessor.swift in Sources */,
@@ -794,6 +798,7 @@
 			buildSettings = {
 				CLANG_ENABLE_MODULES = YES;
 				CODE_SIGN_STYLE = Automatic;
+				CURRENT_PROJECT_VERSION = 2;
 				DEFINES_MODULE = YES;
 				DYLIB_COMPATIBILITY_VERSION = 1;
 				DYLIB_CURRENT_VERSION = 1;
@@ -806,7 +811,7 @@
 					"@executable_path/Frameworks",
 					"@loader_path/Frameworks",
 				);
-				MARKETING_VERSION = 0.4.0;
+				MARKETING_VERSION = 0.7.0;
 				OTHER_LDFLAGS = "-ObjC";
 				PRODUCT_BUNDLE_IDENTIFIER = com.decimal.AudioStreaming;
 				PRODUCT_NAME = "$(TARGET_NAME:c99extidentifier)";
@@ -824,6 +829,7 @@
 			buildSettings = {
 				CLANG_ENABLE_MODULES = YES;
 				CODE_SIGN_STYLE = Automatic;
+				CURRENT_PROJECT_VERSION = 2;
 				DEFINES_MODULE = YES;
 				DYLIB_COMPATIBILITY_VERSION = 1;
 				DYLIB_CURRENT_VERSION = 1;
@@ -836,7 +842,7 @@
 					"@executable_path/Frameworks",
 					"@loader_path/Frameworks",
 				);
-				MARKETING_VERSION = 0.4.0;
+				MARKETING_VERSION = 0.7.0;
 				OTHER_LDFLAGS = "-ObjC";
 				PRODUCT_BUNDLE_IDENTIFIER = com.decimal.AudioStreaming;
 				PRODUCT_NAME = "$(TARGET_NAME:c99extidentifier)";

AudioStreaming/Core/Extensions/AVAudioFormat+Convenience.swift

@@ -9,7 +9,7 @@ extension AVAudioFormat {
     /// The underlying audio stream description.
     ///
     /// This exposes the `pointee` value of the `UsafePointer<AudioStreamBasicDescription>`
-    var basicStreamDescription: AudioStreamBasicDescription {
+    public var basicStreamDescription: AudioStreamBasicDescription {
         return streamDescription.pointee
     }
 }

AudioStreaming/Core/Network/NetworkDataStream.swift

@@ -33,6 +33,7 @@ internal final class NetworkDataStream {
         let error: Error?
     }
 
+    private var lock = UnfairLock()
     private var streamCallback: StreamCompletion?
 
     /// The serial queue for all internal async actions.
@@ -66,6 +67,7 @@ internal final class NetworkDataStream {
 
     @discardableResult
     func responseStream(completion: @escaping StreamCompletion) -> Self {
+        lock.lock(); defer { lock.unlock() }
         streamCallback = completion
         return self
     }
@@ -79,6 +81,7 @@ internal final class NetworkDataStream {
     }
 
     func cancel() {
+        lock.lock(); defer { lock.unlock() }
         guard state.canBecome(.cancelled) else { return }
         state = .cancelled
         streamCallback = nil

AudioStreaming/Streaming/Audio Entry/AudioEntryProvider.swift

@@ -46,10 +46,10 @@ final class AudioEntryProvider: AudioEntryProviding {
         FileAudioSource(url: url, underlyingQueue: underlyingQueue)
     }
 
-    func source(for url: URL, headers _: [String: String]) -> CoreAudioStreamSource {
+    func source(for url: URL, headers: [String: String]) -> CoreAudioStreamSource {
         guard !url.isFileURL else {
             return provideFileAudioSource(url: url)
         }
-        return provideAudioSource(url: url, headers: [:])
+        return provideAudioSource(url: url, headers: headers)
     }
 }

AudioStreaming/Streaming/Audio Source/RemoteAudioSource.swift

@@ -44,7 +44,6 @@ public class RemoteAudioSource: AudioStreamSource {
     }
 
     internal let underlyingQueue: DispatchQueue
-    internal let streamOperationQueue: OperationQueue
     internal let netStatusService: NetStatusProvider
     internal var waitingForNetwork = false
     internal let retrierTimeout: Retrier
@@ -67,12 +66,7 @@ public class RemoteAudioSource: AudioStreamSource {
         supportsSeek = false
         netStatusService = netStatusProvider
         self.icycastHeadersProcessor = icycastHeadersProcessor
-        self.underlyingQueue = underlyingQueue
-        streamOperationQueue = OperationQueue()
-        streamOperationQueue.underlyingQueue = underlyingQueue
-        streamOperationQueue.maxConcurrentOperationCount = 1
-        streamOperationQueue.isSuspended = true
-        streamOperationQueue.name = "remote.audio.source.data.stream.queue"
+        self.underlyingQueue = DispatchQueue(label: "remote.audio.source.queue", target: underlyingQueue)
         retrierTimeout = retrier
         startNetworkService()
     }
@@ -110,8 +104,6 @@ public class RemoteAudioSource: AudioStreamSource {
     func close() {
         retrierTimeout.cancel()
         netStatusService.stop()
-        streamOperationQueue.isSuspended = true
-        streamOperationQueue.cancelAllOperations()
         if let streamTask = streamRequest {
             streamTask.cancel()
             networkingClient.remove(task: streamTask)
@@ -139,12 +131,10 @@ public class RemoteAudioSource: AudioStreamSource {
 
     func suspend() {
         streamRequest?.suspend()
-        streamOperationQueue.isSuspended = true
     }
 
     func resume() {
         streamRequest?.resume()
-        streamOperationQueue.isSuspended = false
     }
 
     // MARK: Private
@@ -166,7 +156,9 @@ public class RemoteAudioSource: AudioStreamSource {
         let request = networkingClient.stream(request: urlRequest)
             .responseStream { [weak self] event in
                 guard let self = self else { return }
-                self.handleResponse(event: event)
+                self.underlyingQueue.sync {
+                    self.handleResponse(event: event)
+                }
             }
             .resume()
 
@@ -180,17 +172,13 @@ public class RemoteAudioSource: AudioStreamSource {
         switch event {
         case let .response(urlResponse):
             parseResponseHeader(response: urlResponse)
-            streamOperationQueue.isSuspended = false
         case let .stream(event):
             handleStreamEvent(event: event)
         case let .complete(event):
             if let error = event.error {
                 delegate?.errorOccured(source: self, error: error)
             } else {
-                addCompletionOperation { [weak self] in
-                    guard let self = self else { return }
-                    self.delegate?.endOfFileOccured(source: self)
-                }
+                delegate?.endOfFileOccured(source: self)
             }
         }
     }
@@ -199,26 +187,23 @@ public class RemoteAudioSource: AudioStreamSource {
         switch event {
         case let .success(value):
             if let audioData = value.data {
-                addStreamOperation { [weak self] in
-                    guard let self = self else { return }
-                    if self.shouldTryParsingIcycastHeaders {
-                        let (header, extractedAudio) = self.icycastHeadersProcessor.proccess(data: audioData)
-                        if let header = header {
-                            self.shouldTryParsingIcycastHeaders = false
-                            let parser = IcycastHeaderParser()
-                            self.parsedHeaderOutput = parser.parse(input: header)
-                            if let metadataStep = self.parsedHeaderOutput?.metadataStep {
-                                self.metadataStreamProcessor.metadataAvailable(step: metadataStep)
-                            }
-
-                            let audioCount = self.processAudio(data: extractedAudio)
-                            self.relativePosition += audioCount
-                            return
+                if shouldTryParsingIcycastHeaders {
+                    let (header, extractedAudio) = icycastHeadersProcessor.proccess(data: audioData)
+                    if let header = header {
+                        shouldTryParsingIcycastHeaders = false
+                        let parser = IcycastHeaderParser()
+                        parsedHeaderOutput = parser.parse(input: header)
+                        if let metadataStep = parsedHeaderOutput?.metadataStep {
+                            metadataStreamProcessor.metadataAvailable(step: metadataStep)
                         }
+
+                        let audioCount = processAudio(data: extractedAudio)
+                        relativePosition += audioCount
+                        return
                     }
-                    let audioCount = self.processAudio(data: audioData)
-                    self.relativePosition += audioCount
                 }
+                let audioCount = processAudio(data: audioData)
+                relativePosition += audioCount
             }
         case .failure:
             if !netStatusService.isConnected {
@@ -281,7 +266,7 @@ public class RemoteAudioSource: AudioStreamSource {
         var urlRequest = URLRequest(url: url)
         urlRequest.networkServiceType = .avStreaming
         urlRequest.cachePolicy = .reloadIgnoringLocalCacheData
-        urlRequest.timeoutInterval = 60
+        urlRequest.timeoutInterval = 240
 
         for header in additionalRequestHeaders {
             urlRequest.addValue(header.value, forHTTPHeaderField: header.key)
@@ -302,25 +287,6 @@ public class RemoteAudioSource: AudioStreamSource {
             self.seek(at: self.position)
         }
     }
-
-    // MARK: - Network Stream Operation Queue
-
-    /// Schedules the given block on the stream operation queue
-    ///
-    /// - Parameter block: A closure to be executed
-    private func addStreamOperation(_ block: @escaping () -> Void) {
-        let operation = BlockOperation(block: block)
-        streamOperationQueue.addOperation(operation)
-    }
-
-    /// Schedules the given block on the stream operation queue as a completion
-    ///
-    /// - Parameter block: A closure to be executed
-    private func addCompletionOperation(_ block: @escaping () -> Void) {
-        let operation = BlockOperation(block: block)
-        operation.queuePriority = .veryLow
-        streamOperationQueue.addOperation(operation)
-    }
 }
 
 extension RemoteAudioSource: MetadataStreamSourceDelegate {

AudioStreaming/Streaming/AudioPlayer/AudioPlayer.swift

@@ -6,7 +6,7 @@
 import AVFoundation
 import CoreAudio
 
-public final class AudioPlayer {
+open class AudioPlayer {
     public weak var delegate: AudioPlayerDelegate?
 
     public var muted: Bool {
@@ -86,6 +86,19 @@ public final class AudioPlayer {
     /// The current configuration of the player.
     public let configuration: AudioPlayerConfiguration
 
+    /// A Boolean value that indicates whether the audio engine is running.
+    /// `true` if the engine is running, otherwise, `false`
+    public var isEngineRunning: Bool { audioEngine.isRunning }
+
+    /// The `AVAudioMixerNode` as created by the underlying audio engine
+    public var mainMixerNode: AVAudioMixerNode {
+        audioEngine.mainMixerNode
+    }
+
+    public var frameFiltering: FrameFiltering {
+        frameFilterProcessor
+    }
+
     /// An `AVAudioFormat` object for the canonical audio stream
     private var outputAudioFormat: AVAudioFormat = {
         AVAudioFormat(commonFormat: .pcmFormatFloat32, sampleRate: 44100.0, channels: 2, interleaved: true)!
@@ -95,25 +108,22 @@ public final class AudioPlayer {
     private var stateBeforePaused: InternalState = .initial
 
     /// The underlying `AVAudioEngine` object
-    let audioEngine = AVAudioEngine()
+    private let audioEngine = AVAudioEngine()
     /// An `AVAudioUnit` object that represents the audio player
     private(set) var player = AVAudioUnit()
     /// An `AVAudioUnitTimePitch` that controls the playback rate of the audio engine
-    let rateNode = AVAudioUnitTimePitch()
-
-    /// A Boolean value that indicates whether the audio engine is running.
-    /// `true` if the engine is running, otherwise, `false`
-    var isEngineRunning: Bool { audioEngine.isRunning }
+    private let rateNode = AVAudioUnitTimePitch()
 
     /// An object representing the context of the audio render.
     /// Holds the audio buffer and in/out lists as required by the audio rendering
-    let rendererContext: AudioRendererContext
+    private let rendererContext: AudioRendererContext
     /// An object representing the context of the player.
     /// Holds the player's state, current playing and reading entries.
-    let playerContext: AudioPlayerContext
+    private let playerContext: AudioPlayerContext
 
-    let fileStreamProcessor: AudioFileStreamProcessor
-    let playerRenderProcessor: AudioPlayerRenderProcessor
+    private let fileStreamProcessor: AudioFileStreamProcessor
+    private let playerRenderProcessor: AudioPlayerRenderProcessor
+    private let frameFilterProcessor: FrameFilterProcessor
 
     private let audioReadSource: DispatchTimerSource
     private let serializationQueue: DispatchQueue
@@ -142,6 +152,8 @@ public final class AudioPlayer {
                                                        rendererContext: rendererContext,
                                                        outputAudioFormat: outputAudioFormat.basicStreamDescription)
 
+        frameFilterProcessor = FrameFilterProcessor(mixerNode: audioEngine.mainMixerNode)
+
         playerRenderProcessor = AudioPlayerRenderProcessor(playerContext: playerContext,
                                                            rendererContext: rendererContext,
                                                            outputAudioFormat: outputAudioFormat.basicStreamDescription)
@@ -306,6 +318,8 @@ public final class AudioPlayer {
         startReadProcessFromSourceIfNeeded()
     }
 
+    /// Seeks the audio to the specified time.
+    /// - Parameter time: A `Double` value specifing the time of the requested seek in seconds
     public func seek(to time: Double) {
         guard let playingEntry = playerContext.audioPlayingEntry else {
             return
@@ -328,10 +342,16 @@ public final class AudioPlayer {
         }
     }
 
+    /// Attaches the given `AVAudioNode` to the engine
+    /// - Note: The node will be added after the default rate node
+    /// - Parameter node: An instance of `AVAudioNode`
     public func attach(node: AVAudioNode) {
         attach(nodes: [node])
     }
 
+    /// Attaches the given `AVAudioNode`s to the engine
+    /// - Note: The nodes will be added after the default rate node
+    /// - Parameter node: An array of `AVAudioNode` instances
     public func attach(nodes: [AVAudioNode]) {
         nodes.forEach { node in
             customAttachedNodes.append(node)
@@ -341,6 +361,8 @@ public final class AudioPlayer {
         reattachCustomNodes()
     }
 
+    /// Detaches the given `AVAudioNode` from the engine
+    /// - Parameter node: An instance of `AVAudioNode`
     public func detach(node: AVAudioNode) {
         guard customAttachedNodes.contains(node) else {
             return
@@ -350,6 +372,8 @@ public final class AudioPlayer {
         reattachCustomNodes()
     }
 
+    /// Detaches the given `AVAudioNode`s from the engine
+    /// - Parameter node: An array of `AVAudioNode` instances
     public func detachCustomAttachedNodes() {
         customAttachedNodes.forEach { node in
             audioEngine.detach(node)
@@ -511,10 +535,11 @@ public final class AudioPlayer {
     /// This calls `processSource` method every `500 ms`
     private func startReadProcessFromSourceIfNeeded() {
         guard audioReadSource.state != .activated else { return }
-        audioReadSource.add { [weak self] in
-            self?.processSource()
-        }
-        audioReadSource.activate()
+        // TODO: this might be needed after all...
+//        audioReadSource.add { [weak self] in
+//            self?.processSource()
+//        }
+//        audioReadSource.activate()
     }
 
     /// Stops and removes the handler from the timer, @see `audioReadSource`

AudioStreaming/Streaming/AudioPlayer/Processors/FrameFilterProcessor.swift

@@ -0,0 +1,177 @@
+//
+//  Created by Dimitrios C on 19/05/2021.
+//  Copyright © 2021 Decimal. All rights reserved.
+//
+
+import AVFoundation
+
+///
+/// - parameter buffer: A buffer of audio captured from the output of an AVAudioNode.
+/// - parameter when: The time the buffer was captured.
+///
+public typealias FilterCallback = (_ buffer: AVAudioPCMBuffer,
+                                   _ when: AVAudioTime) -> Void
+
+/// A value type whose instances are used for frame filter
+/// - Note:
+/// The filter block will be called from a thread other than the main thread
+public struct FilterEntry: Equatable {
+    /// A string value indicating the name of the filter
+    public let name: String
+
+    /// A block in which you apply any filtering
+    public let filter: FilterCallback
+
+    public init(name: String, filter: @escaping FilterCallback) {
+        self.name = name
+        self.filter = filter
+    }
+
+    public static func == (lhs: FilterEntry, rhs: FilterEntry) -> Bool {
+        lhs.name == rhs.name
+    }
+}
+
+public protocol FrameFiltering {
+
+    /// A Boolean value indicating whether there are filter entries
+    var hasEntries: Bool { get }
+
+    /// Adds a filter entry at the end of the queue
+    /// - Parameter entry: An instance of `FilterEntry`
+    func add(entry: FilterEntry)
+
+    /// Adds a filter entry after the specified name of another entry
+    /// - Parameters:
+    ///   - entry: An instance of `FilterEntry`
+    ///   - named: The name of a previously added filter
+    func add(entry: FilterEntry, afterEntry named: String)
+
+    /// Adds a filter entry with the given parameters
+    /// - Parameters:
+    ///   - named: The name of the entry to be added
+    ///   - filter: The block for the filter hanlding
+    func add(entry named: String, filter: @escaping FilterCallback)
+
+    /// Adds a filter entry with the given parameters
+    /// - Parameters:
+    ///   - name: The name for the new entry
+    ///   - filterName: The name of a previously added filters
+    ///   - filter: The block for the filter hanlding
+    func add(entry named: String, after filterName: String, filter: @escaping FilterCallback)
+
+    /// Removes a filter entry
+    /// - Parameter entry: An instance of `FilterEntry` to be removed
+    func remove(entry: FilterEntry)
+
+    /// Attemps to remove a filter entry by its name
+    /// - Parameter named: A `String` representing the name of the filter entry
+    func remove(entry named: String)
+
+    /// Removes all filter entries
+    func removeAll()
+}
+
+final class FrameFilterProcessor: NSObject, FrameFiltering {
+
+    public var hasEntries: Bool {
+        lock.lock(); defer { lock.unlock() }
+        return !entries.isEmpty
+    }
+
+    private let lock = UnfairLock()
+    private let mixerNode: AVAudioMixerNode
+
+    private(set) var entries: [FilterEntry] = []
+
+    private var hasInstalledTap: Bool = false
+
+    init(mixerNode: AVAudioMixerNode) {
+        self.mixerNode = mixerNode
+    }
+
+    public func add(entry: FilterEntry) {
+        lock.lock(); defer { lock.unlock() }
+        entries.append(entry)
+        installTapIfNeeded()
+    }
+
+    public func add(entry: FilterEntry, afterEntry named: String) {
+        lock.lock(); defer { lock.unlock() }
+        guard let entryIndex = entries.firstIndex(where: { $0.name == named }) else {
+            return
+        }
+        if entryIndex.advanced(by: 1) > entries.count {
+            entries.append(entry)
+        } else {
+            entries.insert(entry, at: entryIndex + 1)
+        }
+        installTapIfNeeded()
+    }
+
+    public func add(entry named: String, filter: @escaping FilterCallback) {
+        lock.lock(); defer { lock.unlock() }
+        entries.append(FilterEntry(name: named, filter: filter))
+        installTapIfNeeded()
+    }
+
+    func add(entry named: String, after filterName: String, filter: @escaping FilterCallback) {
+        let entry = FilterEntry(name: named, filter: filter)
+        add(entry: entry, afterEntry: filterName)
+    }
+
+    public func remove(entry: FilterEntry) {
+        lock.lock(); defer { lock.unlock() }
+        guard let entryIndex = entries.firstIndex(where: { $0 == entry }) else {
+            return
+        }
+        entries.remove(at: entryIndex)
+        if entries.isEmpty {
+            removeTap()
+        }
+    }
+
+    public func remove(entry named: String) {
+        lock.lock(); defer { lock.unlock() }
+        guard let entryIndex = entries.firstIndex(where: { $0.name == named }) else {
+            return
+        }
+        entries.remove(at: entryIndex)
+        if entries.isEmpty {
+            removeTap()
+        }
+    }
+
+    public func removeAll() {
+        lock.lock(); defer { lock.unlock() }
+        entries.removeAll()
+        removeTap()
+    }
+
+    private func process(buffer: AVAudioPCMBuffer, when: AVAudioTime) {
+        lock.lock(); defer { lock.unlock() }
+        guard !entries.isEmpty else { return }
+        for entry in entries {
+            entry.filter(buffer, when)
+        }
+    }
+
+    private func installTapIfNeeded() {
+        guard !hasInstalledTap else { return }
+        hasInstalledTap = true
+        let format = mixerNode.outputFormat(forBus: 0)
+        mixerNode.installTap(onBus: 0, bufferSize: 1024, format: format) { [weak self] buffer, when in
+            guard let self = self else { return }
+            guard self.hasEntries else { return }
+            self.process(
+                buffer: buffer,
+                when: when
+            )
+        }
+    }
+
+    private func removeTap() {
+        hasInstalledTap = false
+        mixerNode.removeTap(onBus: 0)
+    }
+}

README.md

@@ -3,7 +3,7 @@
 # AudioStreaming
 An AudioPlayer/Streaming library for iOS written in Swift, allows playback of online audio streaming, local file as well as gapless queueing.
 
-Under the hood `AudioStreaming` uses `AVAudioEngine` and `CoreAudio` for playback and provides an easy way of applying real-time [audio enhancements](https://developer.apple.com/documentation/avfoundation/audio_playback_recording_and_processing/avaudioengine/audio_units?language=swift).
+Under the hood `AudioStreaming` uses `AVAudioEngine` and `CoreAudio` for playback and provides an easy way of applying real-time [audio enhancements](https://developer.apple.com/documentation/avfaudio/audio_engine/audio_units).
 
 #### Supported audio
 - Online streaming (Shoutcast/ICY streams) with metadata parsing 
@@ -124,6 +124,41 @@ player.detachCustomAttachedNodes()
 
 The example project shows an example of adding a custom `AVAudioUnitEQ` node for adding equaliser to the `AudioPlayer`
 
+### Adding custom frame filter for recording and observation of audio data
+
+`AudioStreaming` allow for custom frame fliters to be added so that recording or other observation for audio that's playing.
+
+You add a frame filter by using the `AudioPlayer`'s property `frameFiltering`.
+
+```
+let player = AudioPlayer()
+let format = player.mainMixerNode.outputFormat(forBus: 0)
+
+let settings = [
+    AVFormatIDKey: kAudioFormatMPEG4AAC,
+    AVSampleRateKey: format.sampleRate,
+    AVNumberOfChannelsKey: format.channelCount
+] as [String : Any]
+
+var audioFile = try? AVAudioFile(
+        forWriting: outputUrl,
+        settings: settings,
+        commonFormat: format.commonFormat,
+        interleaved: format.isInterleaved)
+
+let record = FilterEntry(name: "record") { buffer, when in
+    try? audioFile?.write(from: buffer)
+}
+
+player.frameFiltering.add(entry: record)
+```
+See the `FrameFiltering` protocol for more ways of adding and removing frame filters. 
+The callback in which you observe a filter will be run on a thread other than the main thread. 
+
+Under the hood the concrete class for frame filters, `FrameFilterProcessor` installs a tap on the `mainMixerNode` of  `AVAudioEngine` in which all the added fitler will be called from.
+
+**Note** since the `mainMixerNode` is publicly exposed extra care should be taken to not install a tap directly and also use frame filters, this result in an exception because only one tap can be installed on an output node, as per Apple's documention. 
+
 # Installation
 
 ### Cocoapods