Add MappingAudioSource with Web implementation

just_audio/lib/just_audio.dart

@@ -2144,6 +2144,9 @@ abstract class IndexedAudioSource extends AudioSource {
   @override
   void _shuffle({int? initialIndex}) {}
 
+  @override
+  IndexedAudioSourceMessage _toMessage();
+
   @override
   List<IndexedAudioSource> get sequence => [this];
 
@@ -2245,7 +2248,7 @@ class ProgressiveAudioSource extends UriAudioSource {
       : super(uri, headers: headers, tag: tag, duration: duration);
 
   @override
-  AudioSourceMessage _toMessage() => ProgressiveAudioSourceMessage(
+  IndexedAudioSourceMessage _toMessage() => ProgressiveAudioSourceMessage(
       id: _id, uri: _effectiveUri.toString(), headers: headers, tag: tag);
 }
 
@@ -2269,7 +2272,7 @@ class DashAudioSource extends UriAudioSource {
       : super(uri, headers: headers, tag: tag, duration: duration);
 
   @override
-  AudioSourceMessage _toMessage() => DashAudioSourceMessage(
+  IndexedAudioSourceMessage _toMessage() => DashAudioSourceMessage(
       id: _id, uri: _effectiveUri.toString(), headers: headers, tag: tag);
 }
 
@@ -2292,7 +2295,7 @@ class HlsAudioSource extends UriAudioSource {
       : super(uri, headers: headers, tag: tag, duration: duration);
 
   @override
-  AudioSourceMessage _toMessage() => HlsAudioSourceMessage(
+  IndexedAudioSourceMessage _toMessage() => HlsAudioSourceMessage(
       id: _id, uri: _effectiveUri.toString(), headers: headers, tag: tag);
 }
 
@@ -2312,10 +2315,29 @@ class SilenceAudioSource extends IndexedAudioSource {
   }) : super(tag: tag, duration: duration);
 
   @override
-  AudioSourceMessage _toMessage() =>
+  IndexedAudioSourceMessage _toMessage() =>
       SilenceAudioSourceMessage(id: _id, duration: duration);
 }
 
+class MappingAudioSource<T> extends IndexedAudioSource {
+  final T identifier;
+  final Future<IndexedAudioSource> Function(T identifier) createAudioSource;
+
+  MappingAudioSource(
+    this.identifier,
+    this.createAudioSource, {
+    dynamic tag,
+    Duration? duration,
+  }) : super(tag: tag, duration: duration);
+
+  @override
+  IndexedAudioSourceMessage _toMessage() => MappingAudioSourceMessage<T>(
+        id: _id,
+        createAudioSourceMessage: () => createAudioSource(identifier)
+            .then((audioSource) => audioSource._toMessage()),
+      );
+}
+
 /// An [AudioSource] representing a concatenation of multiple audio sources to
 /// be played in succession. This can be used to create playlists. Playback
 /// between items will be gapless on Android, iOS and macOS, while there will
@@ -2562,7 +2584,7 @@ class ClippingAudioSource extends IndexedAudioSource {
   }
 
   @override
-  AudioSourceMessage _toMessage() => ClippingAudioSourceMessage(
+  IndexedAudioSourceMessage _toMessage() => ClippingAudioSourceMessage(
       id: _id,
       child: child._toMessage() as UriAudioSourceMessage,
       start: start,
@@ -2634,7 +2656,7 @@ abstract class StreamAudioSource extends IndexedAudioSource {
   Future<StreamAudioResponse> request([int? start, int? end]);
 
   @override
-  AudioSourceMessage _toMessage() => ProgressiveAudioSourceMessage(
+  IndexedAudioSourceMessage _toMessage() => ProgressiveAudioSourceMessage(
       id: _id, uri: _uri.toString(), headers: null, tag: tag);
 }
 

just_audio_platform_interface/lib/just_audio_platform_interface.dart

@@ -1116,6 +1116,26 @@ class SilenceAudioSourceMessage extends IndexedAudioSourceMessage {
       };
 }
 
+/// Information about a mapping audio source to be communicated with the
+/// platform implementation.
+class MappingAudioSourceMessage<T> extends IndexedAudioSourceMessage {
+  /// The closure can only be used by platform implementations that pass by
+  /// reference.
+  final Future<IndexedAudioSourceMessage> Function() createAudioSourceMessage;
+
+  MappingAudioSourceMessage({
+    required String id,
+    required this.createAudioSourceMessage,
+    dynamic tag,
+  }) : super(id: id, tag: tag);
+
+  @override
+  Map<dynamic, dynamic> toMap() => <dynamic, dynamic>{
+        'type': 'mapping',
+        'id': id,
+      };
+}
+
 /// Information about a concatenating audio source to be communicated with the
 /// platform implementation.
 class ConcatenatingAudioSourceMessage extends AudioSourceMessage {

just_audio_web/lib/just_audio_web.dart

@@ -498,6 +498,16 @@ class Html5AudioPlayer extends JustAudioPlayer {
     } else if (audioSourceMessage is HlsAudioSourceMessage) {
       return HlsAudioSourcePlayer(this, audioSourceMessage.id,
           Uri.parse(audioSourceMessage.uri), audioSourceMessage.headers);
+    } else if (audioSourceMessage is MappingAudioSourceMessage) {
+      return MappingAudioSourcePlayer(
+        this,
+        audioSourceMessage.id,
+        () async {
+          final innerMessage =
+              await audioSourceMessage.createAudioSourceMessage();
+          return decodeAudioSource(innerMessage) as IndexedAudioSourcePlayer;
+        },
+      );
     } else if (audioSourceMessage is ConcatenatingAudioSourceMessage) {
       return ConcatenatingAudioSourcePlayer(
           this,
@@ -698,6 +708,51 @@ class HlsAudioSourcePlayer extends UriAudioSourcePlayer {
       : super(html5AudioPlayer, id, uri, headers);
 }
 
+class MappingAudioSourcePlayer extends IndexedAudioSourcePlayer {
+  final Future<IndexedAudioSourcePlayer> Function() _generateInnerPlayer;
+  IndexedAudioSourcePlayer? _innerPlayer;
+
+  MappingAudioSourcePlayer(
+      Html5AudioPlayer html5AudioPlayer, String id, this._generateInnerPlayer)
+      : super(html5AudioPlayer, id);
+
+  @override
+  Future<Duration?> load([int? initialPosition]) async =>
+      (_innerPlayer = await _generateInnerPlayer()).load(initialPosition);
+
+  @override
+  List<IndexedAudioSourcePlayer> get sequence => [this];
+
+  @override
+  List<int> get shuffleIndices => [0];
+
+  @override
+  Future<void> play() => _innerPlayer!.play();
+
+  @override
+  Future<void> pause() => _innerPlayer!.pause();
+
+  @override
+  Future<void> seek(int position) => _innerPlayer!.seek(position);
+
+  @override
+  Future<void> complete() => _innerPlayer!.complete();
+
+  @override
+  Future<void> timeUpdated(double seconds) async =>
+      _innerPlayer?.timeUpdated(seconds);
+
+  @override
+  Duration? get duration => _innerPlayer?.duration;
+
+  @override
+  Duration get position => _innerPlayer?.position ?? Duration.zero;
+
+  @override
+  Duration get bufferedPosition =>
+      _innerPlayer?.bufferedPosition ?? Duration.zero;
+}
+
 /// A player for a [ConcatenatingAudioSourceMessage].
 class ConcatenatingAudioSourcePlayer extends AudioSourcePlayer {
   /// The players for each child audio source.