Support for AudioSession to play while in the background

Author: mickael-menu

Sources/Navigator/Audiobook/AudioNavigator.swift

@@ -23,11 +23,28 @@ open class _AudioNavigator: _MediaNavigator, _AudioSessionUser, Loggable {
 
     private let publication: Publication
     private let initialLocation: Locator?
-
-    public init(publication: Publication, initialLocation: Locator? = nil) {
+    public let audioConfiguration: _AudioSession.Configuration
+
+    public init(
+        publication: Publication,
+        initialLocation: Locator? = nil,
+        audioConfig: _AudioSession.Configuration = .init(
+            category: .playback,
+            mode: .default,
+            routeSharingPolicy: {
+                if #available(iOS 11.0, *) {
+                    return .longForm
+                } else {
+                    return .default
+                }
+            }(),
+            options: []
+        )
+    ) {
         self.publication = publication
         self.initialLocation = initialLocation
             ?? publication.readingOrder.first.flatMap { publication.locate($0) }
+        self.audioConfiguration = audioConfig
 
         let durations = publication.readingOrder.map { $0.duration ?? 0 }
         self.durations = durations

Sources/Navigator/TTS/AVTTSEngine.swift

@@ -38,8 +38,17 @@ public class AVTTSEngine: NSObject, TTSEngine, AVSpeechSynthesizerDelegate, Logg
     /// Creates a new `AVTTSEngine` instance.
     ///
     /// - Parameters:
+    ///   - audioSessionConfig: AudioSession configuration used while playing utterances. If `nil`, utterances won't
+    ///     play when the app is in the background.
     ///   - debug: Print the state machine transitions.
-    public init(debug: Bool = false) {
+    public init(
+        audioSessionConfig: _AudioSession.Configuration? = .init(
+            category: .playback,
+            mode: .spokenAudio,
+            options: .mixWithOthers
+        ),
+        debug: Bool = false
+    ) {
         let config = TTSConfiguration(
             defaultLanguage: Language(code: .bcp47(AVSpeechSynthesisVoice.currentLanguageCode())),
             rate: avRateRange.percentageForValue(Double(AVSpeechUtteranceDefaultSpeechRate)),
@@ -49,6 +58,7 @@ public class AVTTSEngine: NSObject, TTSEngine, AVSpeechSynthesizerDelegate, Logg
         self.defaultConfig = config
         self.config = config
         self.debug = debug
+        self.audioSessionUser = audioSessionConfig.map { AudioSessionUser(config: $0) }
 
         super.init()
         synthesizer.delegate = self
@@ -283,7 +293,11 @@ public class AVTTSEngine: NSObject, TTSEngine, AVSpeechSynthesizerDelegate, Logg
 
         case let .play(utterance):
             synthesizer.speak(avUtterance(from: utterance))
-            
+
+            if let user = audioSessionUser {
+                _AudioSession.shared.start(with: user)
+            }
+
         case .stop:
             synthesizer.stopSpeaking(at: .immediate)
 
@@ -313,6 +327,24 @@ public class AVTTSEngine: NSObject, TTSEngine, AVSpeechSynthesizerDelegate, Logg
             return AVSpeechSynthesisVoice(language: language)
         }
     }
+
+    // MARK: - Audio session
+
+    private let audioSessionUser: AudioSessionUser?
+
+    private final class AudioSessionUser: R2Shared._AudioSessionUser {
+        let audioConfiguration: _AudioSession.Configuration
+
+        init(config: _AudioSession.Configuration) {
+            self.audioConfiguration = config
+        }
+
+        deinit {
+            _AudioSession.shared.end(for: self)
+        }
+
+        func play() {}
+    }
 }
 
 private extension TTSVoice {

Sources/Shared/Toolkit/Media/AudioSession.swift

@@ -13,7 +13,6 @@ import AVFoundation
 import Foundation
 
 /// An user of the `AudioSession`, for example a media player object.
-@available(iOS 10.0, *)
 public protocol _AudioSessionUser: AnyObject {
 
     /// Audio session configuration to use for this user.
@@ -25,28 +24,31 @@ public protocol _AudioSessionUser: AnyObject {
 
 }
 
-@available(iOS 10.0, *)
 public extension _AudioSessionUser {
-    
     var audioConfiguration: _AudioSession.Configuration { .init() }
-    
 }
 
 /// Manages an activated `AVAudioSession`.
 /// 
 /// **WARNING:** This API is experimental and may change or be removed in a future release without
 /// notice. Use with caution.
-@available(iOS 10.0, *)
 public final class _AudioSession: Loggable {
 
     public struct Configuration {
         let category: AVAudioSession.Category
         let mode: AVAudioSession.Mode
+        let routeSharingPolicy: AVAudioSession.RouteSharingPolicy
         let options: AVAudioSession.CategoryOptions
 
-        public init(category: AVAudioSession.Category = .playback, mode: AVAudioSession.Mode = .default, options: AVAudioSession.CategoryOptions = []) {
+        public init(
+            category: AVAudioSession.Category = .playback,
+            mode: AVAudioSession.Mode = .default,
+            routeSharingPolicy: AVAudioSession.RouteSharingPolicy = .default,
+            options: AVAudioSession.CategoryOptions = []
+        ) {
             self.category = category
             self.mode = mode
+            self.routeSharingPolicy = routeSharingPolicy
             self.options = options
         }
     }
@@ -74,7 +76,7 @@ public final class _AudioSession: Loggable {
         do {
             let config = user.audioConfiguration
             if #available(iOS 11.0, *) {
-                try audioSession.setCategory(config.category, mode: config.mode, policy: .longForm, options: config.options)
+                try audioSession.setCategory(config.category, mode: config.mode, policy: config.routeSharingPolicy, options: config.options)
             } else {
                 try audioSession.setCategory(config.category, mode: config.mode, options: config.options)
             }
@@ -154,5 +156,4 @@ public final class _AudioSession: Loggable {
             break
         }
     }
-    
 }