Implement MappingAudioSource on Android

Inspiration taken from:
- google/ExoPlayer#7087
- google/ExoPlayer#5883
- google/ExoPlayer#7279

just_audio/android/src/main/java/com/ryanheise/just_audio/AudioPlayer.java

@@ -31,6 +31,7 @@
 import com.google.android.exoplayer2.metadata.icy.IcyInfo;
 import com.google.android.exoplayer2.source.ClippingMediaSource;
 import com.google.android.exoplayer2.source.ConcatenatingMediaSource;
+import com.google.android.exoplayer2.source.MaskingMediaSource;
 import com.google.android.exoplayer2.source.MediaSource;
 import com.google.android.exoplayer2.source.ProgressiveMediaSource;
 import com.google.android.exoplayer2.source.ShuffleOrder;
@@ -62,7 +63,7 @@
 import java.util.Map;
 import java.util.Random;
 
-public class AudioPlayer implements MethodCallHandler, Player.Listener, MetadataOutput {
+public class AudioPlayer implements MethodCallHandler, Player.Listener, MetadataOutput, LazyMediaSourceProvider {
 
     static final String TAG = "AudioPlayer";
 
@@ -611,6 +612,17 @@ private MediaSource decodeAudioSource(final Object json) {
                             .setUri(Uri.parse((String)map.get("uri")))
                             .setMimeType(MimeTypes.APPLICATION_M3U8)
                             .build());
+        case "mapping":
+            return new MaskingMediaSource(
+                    new LazyMediaSource(
+                            this,
+                            id,
+                            new MediaItem.Builder()
+                                    .setTag(id)
+                                    .build()
+                    ),
+                    true
+            );
         case "silence":
             return new SilenceMediaSource.Factory()
                     .setDurationUs(getLong(map.get("duration")))
@@ -695,6 +707,29 @@ private DataSource.Factory buildDataSourceFactory() {
         return new DefaultDataSource.Factory(context, httpDataSourceFactory);
     }
 
+    @Override
+    public void createMediaSource(String id, LazyMediaSourceReceiver receiver) {
+        handler.post(() -> {
+            methodChannel.invokeMethod("createMappedAudioSourceSource", Collections.singletonMap("id", id), new Result() {
+                @Override
+                public void success(Object json) {
+                    final MediaSource mediaSource = decodeAudioSource(json);
+                    receiver.onMediaSourceCreated(mediaSource);
+                }
+
+                @Override
+                public void error(String errorCode, String errorMessage, Object errorDetails) {
+                    throw new IllegalStateException("createMappedAudioSourceSource failed. Cannot proceed. (" + errorCode + ", " + errorMessage + ", " + errorDetails + ")");
+                }
+
+                @Override
+                public void notImplemented() {
+                    throw new IllegalArgumentException("createMappedAudioSourceSource is not implemented by the platform.");
+                }
+            });
+        });
+    }
+
     private void load(final MediaSource mediaSource, final long initialPosition, final Integer initialIndex, final Result result) {
         this.initialPos = initialPosition;
         this.initialIndex = initialIndex;

just_audio/android/src/main/java/com/ryanheise/just_audio/LazyMediaSource.java

@@ -0,0 +1,160 @@
+package com.ryanheise.just_audio;
+
+import android.os.Handler;
+
+import com.google.android.exoplayer2.MediaItem;
+import com.google.android.exoplayer2.Timeline;
+import com.google.android.exoplayer2.analytics.PlayerId;
+import com.google.android.exoplayer2.drm.DrmSessionEventListener;
+import com.google.android.exoplayer2.source.MediaPeriod;
+import com.google.android.exoplayer2.source.MediaSource;
+import com.google.android.exoplayer2.source.MediaSourceEventListener;
+import com.google.android.exoplayer2.upstream.Allocator;
+import com.google.android.exoplayer2.upstream.TransferListener;
+
+import java.io.IOException;
+import java.util.HashMap;
+import java.util.Map;
+
+/**
+ * A {@link MediaSource} that lazily defers to another {@link MediaSource} when it is required.
+ * <p>
+ * This {@link MediaSource} must be used with a {@link com.google.android.exoplayer2.source.MaskingMediaSource}.
+ */
+class LazyMediaSource implements MediaSource {
+    private final LazyMediaSourceProvider mediaSourceProvider;
+    public final String id;
+    public final MediaItem placeholderMediaItem;
+
+    private final Map<MediaSourceEventListener, Handler> pendingEventListeners = new HashMap<>();
+    private final Map<DrmSessionEventListener, Handler> pendingDrmEventListeners = new HashMap<>();
+
+    private MediaSource mediaSource;
+
+    LazyMediaSource(LazyMediaSourceProvider mediaSourceProvider, String id, MediaItem placeholderMediaItem) {
+        this.mediaSourceProvider = mediaSourceProvider;
+        this.id = id;
+        this.placeholderMediaItem = placeholderMediaItem;
+    }
+
+    @Override
+    public void addEventListener(Handler handler, MediaSourceEventListener eventListener) {
+        if (mediaSource == null) {
+            pendingEventListeners.put(eventListener, handler);
+        } else {
+            mediaSource.addEventListener(handler, eventListener);
+        }
+    }
+
+    @Override
+    public void removeEventListener(MediaSourceEventListener eventListener) {
+        if (mediaSource == null) {
+            pendingEventListeners.remove(eventListener);
+        } else {
+            mediaSource.removeEventListener(eventListener);
+        }
+    }
+
+    @Override
+    public void addDrmEventListener(Handler handler, DrmSessionEventListener eventListener) {
+        if (mediaSource == null) {
+            pendingDrmEventListeners.put(eventListener, handler);
+        } else {
+            mediaSource.addDrmEventListener(handler, eventListener);
+        }
+    }
+
+    @Override
+    public void removeDrmEventListener(DrmSessionEventListener eventListener) {
+        if (mediaSource == null) {
+            pendingDrmEventListeners.remove(eventListener);
+        } else {
+            mediaSource.removeDrmEventListener(eventListener);
+        }
+    }
+
+    @Override
+    public Timeline getInitialTimeline() {
+        if (mediaSource == null) return null;
+        return mediaSource.getInitialTimeline();
+    }
+
+    @Override
+    public boolean isSingleWindow() {
+        if (mediaSource == null) return false;
+        return mediaSource.isSingleWindow();
+    }
+
+    @Override
+    public MediaItem getMediaItem() {
+        if (mediaSource == null) {
+            return placeholderMediaItem;
+        } else {
+            return mediaSource.getMediaItem();
+        }
+    }
+
+    @Override
+    public void prepareSource(
+            MediaSourceCaller caller,
+            TransferListener mediaTransferListener,
+            PlayerId playerId
+    ) {
+        mediaSourceProvider.createMediaSource(id, (mediaSource) -> {
+            this.mediaSource = mediaSource;
+            for (Map.Entry<MediaSourceEventListener, Handler> entry : pendingEventListeners.entrySet()) {
+                mediaSource.addEventListener(entry.getValue(), entry.getKey());
+            }
+            pendingEventListeners.clear();
+            for (Map.Entry<DrmSessionEventListener, Handler> entry : pendingDrmEventListeners.entrySet()) {
+                mediaSource.addDrmEventListener(entry.getValue(), entry.getKey());
+            }
+            pendingDrmEventListeners.clear();
+            mediaSource.prepareSource(caller, mediaTransferListener, playerId);
+        });
+    }
+
+    @Override
+    public void maybeThrowSourceInfoRefreshError() throws IOException {
+        if (mediaSource == null) return;
+        mediaSource.maybeThrowSourceInfoRefreshError();
+    }
+
+    @Override
+    public void enable(MediaSourceCaller caller) {
+        if (mediaSource == null) throw new IllegalStateException();
+        mediaSource.enable(caller);
+    }
+
+    @Override
+    public MediaPeriod createPeriod(MediaPeriodId id, Allocator allocator, long startPositionUs) {
+        if (mediaSource == null) throw new IllegalStateException();
+        return mediaSource.createPeriod(id, allocator, startPositionUs);
+    }
+
+    @Override
+    public void releasePeriod(MediaPeriod mediaPeriod) {
+        if (mediaSource == null) throw new IllegalStateException();
+        mediaSource.releasePeriod(mediaPeriod);
+    }
+
+    @Override
+    public void disable(MediaSourceCaller caller) {
+        if (mediaSource == null) throw new IllegalStateException();
+        mediaSource.disable(caller);
+    }
+
+    @Override
+    public void releaseSource(MediaSourceCaller caller) {
+        if (mediaSource == null) return;
+        mediaSource.releaseSource(caller);
+    }
+}
+
+interface LazyMediaSourceReceiver {
+    void onMediaSourceCreated(MediaSource mediaSource);
+}
+
+interface LazyMediaSourceProvider {
+    void createMediaSource(String id, LazyMediaSourceReceiver receiver);
+}