Add CompositeMediaSource to handle common tasks for composite media sources.

This removes some boiler-plate code for compostite sources and will also
simplify resuing media source in the future (because this class can keep track
of child listeners).

Issue:#3498

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=183387123

extensions/ima/src/main/java/com/google/android/exoplayer2/ext/ima/ImaAdsMediaSource.java

@@ -20,12 +20,12 @@ import android.support.annotation.Nullable;
 import android.view.ViewGroup;
 import com.google.android.exoplayer2.ExoPlayer;
 import com.google.android.exoplayer2.Timeline;
+import com.google.android.exoplayer2.source.CompositeMediaSource;
 import com.google.android.exoplayer2.source.MediaPeriod;
 import com.google.android.exoplayer2.source.MediaSource;
 import com.google.android.exoplayer2.source.ads.AdsMediaSource;
 import com.google.android.exoplayer2.upstream.Allocator;
 import com.google.android.exoplayer2.upstream.DataSource;
-import java.io.IOException;
 
 /**
  * A {@link MediaSource} that inserts ads linearly with a provided content media source.
@@ -33,9 +33,10 @@ import java.io.IOException;
  * @deprecated Use com.google.android.exoplayer2.source.ads.AdsMediaSource with ImaAdsLoader.
  */
 @Deprecated
-public final class ImaAdsMediaSource implements MediaSource {
+public final class ImaAdsMediaSource extends CompositeMediaSource<Void> {
 
   private final AdsMediaSource adsMediaSource;
+  private Listener listener;
 
   /**
    * Constructs a new source that inserts ads linearly with the content specified by
@@ -74,20 +75,10 @@ public final class ImaAdsMediaSource implements MediaSource {
   }
 
   @Override
-  public void prepareSource(final ExoPlayer player, boolean isTopLevelSource,
-      final Listener listener) {
-    adsMediaSource.prepareSource(player, false, new Listener() {
-      @Override
-      public void onSourceInfoRefreshed(MediaSource source, Timeline timeline,
-          @Nullable Object manifest) {
-        listener.onSourceInfoRefreshed(ImaAdsMediaSource.this, timeline, manifest);
-      }
-    });
-  }
-
-  @Override
-  public void maybeThrowSourceInfoRefreshError() throws IOException {
-    adsMediaSource.maybeThrowSourceInfoRefreshError();
+  public void prepareSource(ExoPlayer player, boolean isTopLevelSource, Listener listener) {
+    super.prepareSource(player, isTopLevelSource, listener);
+    this.listener = listener;
+    prepareChildSource(/* id= */ null, adsMediaSource);
   }
 
   @Override
@@ -101,8 +92,8 @@ public final class ImaAdsMediaSource implements MediaSource {
   }
 
   @Override
-  public void releaseSource() {
-    adsMediaSource.releaseSource();
+  protected void onChildSourceInfoRefreshed(
+      Void id, MediaSource mediaSource, Timeline timeline, @Nullable Object manifest) {
+    listener.onSourceInfoRefreshed(this, timeline, manifest);
   }
-
 }

library/core/src/androidTest/java/com/google/android/exoplayer2/source/ClippingMediaSourceTest.java

@@ -150,7 +150,10 @@ public final class ClippingMediaSourceTest extends InstrumentationTestCase {
     ClippingMediaSource mediaSource = new ClippingMediaSource(fakeMediaSource, startMs, endMs);
     MediaSourceTestRunner testRunner = new MediaSourceTestRunner(mediaSource, null);
     try {
-      return testRunner.prepareSource();
+      Timeline clippedTimeline = testRunner.prepareSource();
+      testRunner.releaseSource();
+      fakeMediaSource.assertReleased();
+      return clippedTimeline;
     } finally {
       testRunner.release();
     }

library/core/src/androidTest/java/com/google/android/exoplayer2/source/ConcatenatingMediaSourceTest.java

@@ -243,7 +243,7 @@ public final class ConcatenatingMediaSourceTest extends TestCase {
    */
   private static Timeline getConcatenatedTimeline(boolean isRepeatOneAtomic,
       Timeline... timelines) throws IOException {
-    MediaSource[] mediaSources = new MediaSource[timelines.length];
+    FakeMediaSource[] mediaSources = new FakeMediaSource[timelines.length];
     for (int i = 0; i < timelines.length; i++) {
       mediaSources[i] = new FakeMediaSource(timelines[i], null);
     }
@@ -251,7 +251,12 @@ public final class ConcatenatingMediaSourceTest extends TestCase {
         new FakeShuffleOrder(mediaSources.length), mediaSources);
     MediaSourceTestRunner testRunner = new MediaSourceTestRunner(mediaSource, null);
     try {
-      return testRunner.prepareSource();
+      Timeline timeline = testRunner.prepareSource();
+      testRunner.releaseSource();
+      for (int i = 0; i < mediaSources.length; i++) {
+        mediaSources[i].assertReleased();
+      }
+      return timeline;
     } finally {
       testRunner.release();
     }

library/core/src/androidTest/java/com/google/android/exoplayer2/source/LoopingMediaSourceTest.java

@@ -115,7 +115,10 @@ public class LoopingMediaSourceTest extends TestCase {
     LoopingMediaSource mediaSource = new LoopingMediaSource(fakeMediaSource, loopCount);
     MediaSourceTestRunner testRunner = new MediaSourceTestRunner(mediaSource, null);
     try {
-      return testRunner.prepareSource();
+      Timeline loopingTimeline = testRunner.prepareSource();
+      testRunner.releaseSource();
+      fakeMediaSource.assertReleased();
+      return loopingTimeline;
     } finally {
       testRunner.release();
     }

library/core/src/androidTest/java/com/google/android/exoplayer2/source/MergingMediaSourceTest.java

@@ -64,7 +64,7 @@ public class MergingMediaSourceTest extends TestCase {
    * forwards the first of the wrapped timelines.
    */
   private static void testMergingMediaSourcePrepare(Timeline... timelines) throws IOException {
-    MediaSource[] mediaSources = new MediaSource[timelines.length];
+    FakeMediaSource[] mediaSources = new FakeMediaSource[timelines.length];
     for (int i = 0; i < timelines.length; i++) {
       mediaSources[i] = new FakeMediaSource(timelines[i], null);
     }
@@ -74,6 +74,10 @@ public class MergingMediaSourceTest extends TestCase {
       Timeline timeline = testRunner.prepareSource();
       // The merged timeline should always be the one from the first child.
       assertThat(timeline).isEqualTo(timelines[0]);
+      testRunner.releaseSource();
+      for (int i = 0; i < mediaSources.length; i++) {
+        mediaSources[i].assertReleased();
+      }
     } finally {
       testRunner.release();
     }

library/core/src/main/java/com/google/android/exoplayer2/source/ClippingMediaSource.java

@@ -16,6 +16,7 @@
 package com.google.android.exoplayer2.source;
 
 import android.support.annotation.IntDef;
+import android.support.annotation.Nullable;
 import com.google.android.exoplayer2.C;
 import com.google.android.exoplayer2.ExoPlayer;
 import com.google.android.exoplayer2.Timeline;
@@ -31,7 +32,7 @@ import java.util.ArrayList;
  * positions. The wrapped source must consist of a single period that starts at the beginning of the
  * corresponding window.
  */
-public final class ClippingMediaSource implements MediaSource, MediaSource.Listener {
+public final class ClippingMediaSource extends CompositeMediaSource<Void> {
 
   /**
    * Thrown when a {@link ClippingMediaSource} cannot clip its wrapped source.
@@ -131,9 +132,10 @@ public final class ClippingMediaSource implements MediaSource, MediaSource.Liste
 
   @Override
   public void prepareSource(ExoPlayer player, boolean isTopLevelSource, Listener listener) {
+    super.prepareSource(player, isTopLevelSource, listener);
     Assertions.checkState(sourceListener == null, MEDIA_SOURCE_REUSED_ERROR_MESSAGE);
     sourceListener = listener;
-    mediaSource.prepareSource(player, false, this);
+    prepareChildSource(/* id= */ null, mediaSource);
   }
 
   @Override
@@ -141,7 +143,7 @@ public final class ClippingMediaSource implements MediaSource, MediaSource.Liste
     if (clippingError != null) {
       throw clippingError;
     }
-    mediaSource.maybeThrowSourceInfoRefreshError();
+    super.maybeThrowSourceInfoRefreshError();
   }
 
   @Override
@@ -160,14 +162,8 @@ public final class ClippingMediaSource implements MediaSource, MediaSource.Liste
   }
 
   @Override
-  public void releaseSource() {
-    mediaSource.releaseSource();
-  }
-
-  // MediaSource.Listener implementation.
-
-  @Override
-  public void onSourceInfoRefreshed(MediaSource source, Timeline timeline, Object manifest) {
+  protected void onChildSourceInfoRefreshed(
+      Void id, MediaSource mediaSource, Timeline timeline, @Nullable Object manifest) {
     if (clippingError != null) {
       return;
     }

library/core/src/main/java/com/google/android/exoplayer2/source/CompositeMediaSource.java

@@ -0,0 +1,113 @@
+/*
+ * Copyright (C) 2018 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.google.android.exoplayer2.source;
+
+import android.support.annotation.CallSuper;
+import android.support.annotation.Nullable;
+import com.google.android.exoplayer2.ExoPlayer;
+import com.google.android.exoplayer2.Timeline;
+import com.google.android.exoplayer2.util.Assertions;
+import java.io.IOException;
+import java.util.HashMap;
+
+/**
+ * Composite {@link MediaSource} consisting of multiple child sources.
+ *
+ * @param <T> The type of the id used to identify prepared child sources.
+ */
+public abstract class CompositeMediaSource<T> implements MediaSource {
+
+  private final HashMap<T, MediaSource> childSources;
+  private ExoPlayer player;
+
+  /** Create composite media source without child sources. */
+  protected CompositeMediaSource() {
+    childSources = new HashMap<>();
+  }
+
+  @Override
+  @CallSuper
+  public void prepareSource(ExoPlayer player, boolean isTopLevelSource, Listener listener) {
+    this.player = player;
+  }
+
+  @Override
+  @CallSuper
+  public void maybeThrowSourceInfoRefreshError() throws IOException {
+    for (MediaSource childSource : childSources.values()) {
+      childSource.maybeThrowSourceInfoRefreshError();
+    }
+  }
+
+  @Override
+  @CallSuper
+  public void releaseSource() {
+    for (MediaSource childSource : childSources.values()) {
+      childSource.releaseSource();
+    }
+    childSources.clear();
+    player = null;
+  }
+
+  /**
+   * Called when the source info of a child source has been refreshed.
+   *
+   * @param id The unique id used to prepare the child source.
+   * @param mediaSource The child source whose source info has been refreshed.
+   * @param timeline The timeline of the child source.
+   * @param manifest The manifest of the child source.
+   */
+  protected abstract void onChildSourceInfoRefreshed(
+      @Nullable T id, MediaSource mediaSource, Timeline timeline, @Nullable Object manifest);
+
+  /**
+   * Prepares a child source.
+   *
+   * <p>{@link #onChildSourceInfoRefreshed(Object, MediaSource, Timeline, Object)} will be called
+   * when the child source updates its timeline and/or manifest with the same {@code id} passed to
+   * this method.
+   *
+   * <p>Any child sources that aren't explicitly released with {@link #releaseChildSource(Object)}
+   * will be released in {@link #releaseSource()}.
+   *
+   * @param id A unique id to identify the child source preparation. Null is allowed as an id.
+   * @param mediaSource The child {@link MediaSource}.
+   */
+  protected void prepareChildSource(@Nullable final T id, final MediaSource mediaSource) {
+    Assertions.checkArgument(!childSources.containsKey(id));
+    childSources.put(id, mediaSource);
+    mediaSource.prepareSource(
+        player,
+        /* isTopLevelSource= */ false,
+        new Listener() {
+          @Override
+          public void onSourceInfoRefreshed(
+              MediaSource source, Timeline timeline, @Nullable Object manifest) {
+            onChildSourceInfoRefreshed(id, mediaSource, timeline, manifest);
+          }
+        });
+  }
+
+  /**
+   * Releases a child source.
+   *
+   * @param id The unique id used to prepare the child source.
+   */
+  protected void releaseChildSource(@Nullable T id) {
+    MediaSource removedChild = childSources.remove(id);
+    removedChild.releaseSource();
+  }
+}

library/core/src/main/java/com/google/android/exoplayer2/source/ConcatenatingMediaSource.java

@@ -15,6 +15,7 @@
  */
 package com.google.android.exoplayer2.source;
 
+import android.support.annotation.Nullable;
 import com.google.android.exoplayer2.C;
 import com.google.android.exoplayer2.ExoPlayer;
 import com.google.android.exoplayer2.Player;
@@ -23,7 +24,6 @@ import com.google.android.exoplayer2.source.ShuffleOrder.DefaultShuffleOrder;
 import com.google.android.exoplayer2.upstream.Allocator;
 import com.google.android.exoplayer2.util.Assertions;
 import com.google.android.exoplayer2.util.Util;
-import java.io.IOException;
 import java.util.HashMap;
 import java.util.IdentityHashMap;
 import java.util.Map;
@@ -32,13 +32,12 @@ import java.util.Map;
  * Concatenates multiple {@link MediaSource}s. It is valid for the same {@link MediaSource} instance
  * to be present more than once in the concatenation.
  */
-public final class ConcatenatingMediaSource implements MediaSource {
+public final class ConcatenatingMediaSource extends CompositeMediaSource<Integer> {
 
   private final MediaSource[] mediaSources;
   private final Timeline[] timelines;
   private final Object[] manifests;
   private final Map<MediaPeriod, Integer> sourceIndexByMediaPeriod;
-  private final boolean[] duplicateFlags;
   private final boolean isAtomic;
   private final ShuffleOrder shuffleOrder;
 
@@ -84,40 +83,25 @@ public final class ConcatenatingMediaSource implements MediaSource {
     timelines = new Timeline[mediaSources.length];
     manifests = new Object[mediaSources.length];
     sourceIndexByMediaPeriod = new HashMap<>();
-    duplicateFlags = buildDuplicateFlags(mediaSources);
   }
 
   @Override
   public void prepareSource(ExoPlayer player, boolean isTopLevelSource, Listener listener) {
+    super.prepareSource(player, isTopLevelSource, listener);
     Assertions.checkState(this.listener == null, MEDIA_SOURCE_REUSED_ERROR_MESSAGE);
     this.listener = listener;
+    boolean[] duplicateFlags = buildDuplicateFlags(mediaSources);
     if (mediaSources.length == 0) {
       listener.onSourceInfoRefreshed(this, Timeline.EMPTY, null);
     } else {
       for (int i = 0; i < mediaSources.length; i++) {
         if (!duplicateFlags[i]) {
-          final int index = i;
-          mediaSources[i].prepareSource(player, false, new Listener() {
-            @Override
-            public void onSourceInfoRefreshed(MediaSource source, Timeline timeline,
-                Object manifest) {
-              handleSourceInfoRefreshed(index, timeline, manifest);
-            }
-          });
+          prepareChildSource(i, mediaSources[i]);
         }
       }
     }
   }
 
-  @Override
-  public void maybeThrowSourceInfoRefreshError() throws IOException {
-    for (int i = 0; i < mediaSources.length; i++) {
-      if (!duplicateFlags[i]) {
-        mediaSources[i].maybeThrowSourceInfoRefreshError();
-      }
-    }
-  }
-
   @Override
   public MediaPeriod createPeriod(MediaPeriodId id, Allocator allocator) {
     int sourceIndex = timeline.getChildIndexByPeriodIndex(id.periodIndex);
@@ -136,22 +120,17 @@ public final class ConcatenatingMediaSource implements MediaSource {
   }
 
   @Override
-  public void releaseSource() {
-    for (int i = 0; i < mediaSources.length; i++) {
-      if (!duplicateFlags[i]) {
-        mediaSources[i].releaseSource();
-      }
-    }
-  }
-
-  private void handleSourceInfoRefreshed(int sourceFirstIndex, Timeline sourceTimeline,
-      Object sourceManifest) {
+  protected void onChildSourceInfoRefreshed(
+      Integer sourceFirstIndex,
+      MediaSource mediaSource,
+      Timeline sourceTimeline,
+      @Nullable Object sourceManifest) {
     // Set the timeline and manifest.
     timelines[sourceFirstIndex] = sourceTimeline;
     manifests[sourceFirstIndex] = sourceManifest;
     // Also set the timeline and manifest for any duplicate entries of the same source.
     for (int i = sourceFirstIndex + 1; i < mediaSources.length; i++) {
-      if (mediaSources[i] == mediaSources[sourceFirstIndex]) {
+      if (mediaSources[i] == mediaSource) {
         timelines[i] = sourceTimeline;
         manifests[i] = sourceManifest;
       }

library/core/src/main/java/com/google/android/exoplayer2/source/DynamicConcatenatingMediaSource.java

@@ -25,11 +25,11 @@ import com.google.android.exoplayer2.ExoPlaybackException;
 import com.google.android.exoplayer2.ExoPlayer;
 import com.google.android.exoplayer2.PlayerMessage;
 import com.google.android.exoplayer2.Timeline;
+import com.google.android.exoplayer2.source.DynamicConcatenatingMediaSource.MediaSourceHolder;
 import com.google.android.exoplayer2.source.ShuffleOrder.DefaultShuffleOrder;
 import com.google.android.exoplayer2.upstream.Allocator;
 import com.google.android.exoplayer2.util.Assertions;
 import com.google.android.exoplayer2.util.Util;
-import java.io.IOException;
 import java.util.ArrayList;
 import java.util.Collection;
 import java.util.Collections;
@@ -41,7 +41,8 @@ import java.util.Map;
  * Concatenates multiple {@link MediaSource}s. The list of {@link MediaSource}s can be modified
  * during playback. Access to this class is thread-safe.
  */
-public final class DynamicConcatenatingMediaSource implements MediaSource, PlayerMessage.Target {
+public final class DynamicConcatenatingMediaSource extends CompositeMediaSource<MediaSourceHolder>
+    implements PlayerMessage.Target {
 
   private static final int MSG_ADD = 0;
   private static final int MSG_ADD_MULTIPLE = 1;
@@ -331,6 +332,7 @@ public final class DynamicConcatenatingMediaSource implements MediaSource, Playe
   @Override
   public synchronized void prepareSource(ExoPlayer player, boolean isTopLevelSource,
       Listener listener) {
+    super.prepareSource(player, isTopLevelSource, listener);
     Assertions.checkState(this.listener == null, MEDIA_SOURCE_REUSED_ERROR_MESSAGE);
     this.player = player;
     this.listener = listener;
@@ -341,13 +343,6 @@ public final class DynamicConcatenatingMediaSource implements MediaSource, Playe
     maybeNotifyListener(null);
   }
 
-  @Override
-  public void maybeThrowSourceInfoRefreshError() throws IOException {
-    for (int i = 0; i < mediaSourceHolders.size(); i++) {
-      mediaSourceHolders.get(i).mediaSource.maybeThrowSourceInfoRefreshError();
-    }
-  }
-
   @Override
   public MediaPeriod createPeriod(MediaPeriodId id, Allocator allocator) {
     int mediaSourceHolderIndex = findMediaSourceHolderByPeriodIndex(id.periodIndex);
@@ -378,10 +373,12 @@ public final class DynamicConcatenatingMediaSource implements MediaSource, Playe
   }
 
   @Override
-  public void releaseSource() {
-    for (int i = 0; i < mediaSourceHolders.size(); i++) {
-      mediaSourceHolders.get(i).mediaSource.releaseSource();
-    }
+  protected void onChildSourceInfoRefreshed(
+      MediaSourceHolder mediaSourceHolder,
+      MediaSource mediaSource,
+      Timeline timeline,
+      @Nullable Object manifest) {
+    updateMediaSourceInternal(mediaSourceHolder, timeline);
   }
 
   @Override
@@ -459,12 +456,7 @@ public final class DynamicConcatenatingMediaSource implements MediaSource, Playe
     }
     correctOffsets(newIndex, newTimeline.getWindowCount(), newTimeline.getPeriodCount());
     mediaSourceHolders.add(newIndex, newMediaSourceHolder);
-    newMediaSourceHolder.mediaSource.prepareSource(player, false, new Listener() {
-      @Override
-      public void onSourceInfoRefreshed(MediaSource source, Timeline newTimeline, Object manifest) {
-        updateMediaSourceInternal(newMediaSourceHolder, newTimeline);
-      }
-    });
+    prepareChildSource(newMediaSourceHolder, newMediaSourceHolder.mediaSource);
   }
 
   private void addMediaSourcesInternal(int index, Collection<MediaSource> mediaSources) {
@@ -505,7 +497,7 @@ public final class DynamicConcatenatingMediaSource implements MediaSource, Playe
     mediaSourceHolders.remove(index);
     Timeline oldTimeline = holder.timeline;
     correctOffsets(index, -oldTimeline.getWindowCount(), -oldTimeline.getPeriodCount());
-    holder.mediaSource.releaseSource();
+    releaseChildSource(holder);
   }
 
   private void moveMediaSourceInternal(int currentIndex, int newIndex) {
@@ -545,10 +537,8 @@ public final class DynamicConcatenatingMediaSource implements MediaSource, Playe
     return index;
   }
 
-  /**
-   * Data class to hold playlist media sources together with meta data needed to process them.
-   */
-  private static final class MediaSourceHolder implements Comparable<MediaSourceHolder> {
+  /** Data class to hold playlist media sources together with meta data needed to process them. */
+  /* package */ static final class MediaSourceHolder implements Comparable<MediaSourceHolder> {
 
     public final MediaSource mediaSource;
     public final Object uid;
@@ -593,16 +583,14 @@ public final class DynamicConcatenatingMediaSource implements MediaSource, Playe
 
   }
 
-  /**
-   * Message used to post actions from app thread to playback thread.
-   */
-  private static final class MessageData<CustomType> {
+  /** Message used to post actions from app thread to playback thread. */
+  private static final class MessageData<T> {
 
     public final int index;
-    public final CustomType customData;
+    public final T customData;
     public final @Nullable EventDispatcher actionOnCompletion;
 
-    public MessageData(int index, CustomType customData, @Nullable Runnable actionOnCompletion) {
+    public MessageData(int index, T customData, @Nullable Runnable actionOnCompletion) {
       this.index = index;
       this.actionOnCompletion = actionOnCompletion != null
           ? new EventDispatcher(actionOnCompletion) : null;

library/core/src/main/java/com/google/android/exoplayer2/source/LoopingMediaSource.java

@@ -15,6 +15,7 @@
  */
 package com.google.android.exoplayer2.source;
 
+import android.support.annotation.Nullable;
 import com.google.android.exoplayer2.C;
 import com.google.android.exoplayer2.ExoPlayer;
 import com.google.android.exoplayer2.Player;
@@ -22,21 +23,20 @@ import com.google.android.exoplayer2.Timeline;
 import com.google.android.exoplayer2.source.ShuffleOrder.UnshuffledShuffleOrder;
 import com.google.android.exoplayer2.upstream.Allocator;
 import com.google.android.exoplayer2.util.Assertions;
-import java.io.IOException;
 
 /**
  * Loops a {@link MediaSource} a specified number of times.
- * <p>
- * Note: To loop a {@link MediaSource} indefinitely, it is usually better to use
- * {@link ExoPlayer#setRepeatMode(int)}.
+ *
+ * <p>Note: To loop a {@link MediaSource} indefinitely, it is usually better to use {@link
+ * ExoPlayer#setRepeatMode(int)}.
  */
-public final class LoopingMediaSource implements MediaSource {
+public final class LoopingMediaSource extends CompositeMediaSource<Void> {
 
   private final MediaSource childSource;
   private final int loopCount;
 
   private int childPeriodCount;
-  private boolean wasPrepareSourceCalled;
+  private Listener listener;
 
   /**
    * Loops the provided source indefinitely. Note that it is usually better to use
@@ -62,22 +62,10 @@ public final class LoopingMediaSource implements MediaSource {
 
   @Override
   public void prepareSource(ExoPlayer player, boolean isTopLevelSource, final Listener listener) {
-    Assertions.checkState(!wasPrepareSourceCalled, MEDIA_SOURCE_REUSED_ERROR_MESSAGE);
-    wasPrepareSourceCalled = true;
-    childSource.prepareSource(player, false, new Listener() {
-      @Override
-      public void onSourceInfoRefreshed(MediaSource source, Timeline timeline, Object manifest) {
-        childPeriodCount = timeline.getPeriodCount();
-        Timeline loopingTimeline = loopCount != Integer.MAX_VALUE
-            ? new LoopingTimeline(timeline, loopCount) : new InfinitelyLoopingTimeline(timeline);
-        listener.onSourceInfoRefreshed(LoopingMediaSource.this, loopingTimeline, manifest);
-      }
-    });
-  }
-
-  @Override
-  public void maybeThrowSourceInfoRefreshError() throws IOException {
-    childSource.maybeThrowSourceInfoRefreshError();
+    super.prepareSource(player, isTopLevelSource, listener);
+    Assertions.checkState(this.listener == null, MEDIA_SOURCE_REUSED_ERROR_MESSAGE);
+    this.listener = listener;
+    prepareChildSource(/* id= */ null, childSource);
   }
 
   @Override
@@ -94,8 +82,14 @@ public final class LoopingMediaSource implements MediaSource {
   }
 
   @Override
-  public void releaseSource() {
-    childSource.releaseSource();
+  protected void onChildSourceInfoRefreshed(
+      Void id, MediaSource mediaSource, Timeline timeline, @Nullable Object manifest) {
+    childPeriodCount = timeline.getPeriodCount();
+    Timeline loopingTimeline =
+        loopCount != Integer.MAX_VALUE
+            ? new LoopingTimeline(timeline, loopCount)
+            : new InfinitelyLoopingTimeline(timeline);
+    listener.onSourceInfoRefreshed(this, loopingTimeline, manifest);
   }
 
   private static final class LoopingTimeline extends AbstractConcatenatedTimeline {

library/core/src/main/java/com/google/android/exoplayer2/source/MergingMediaSource.java

@@ -16,6 +16,7 @@
 package com.google.android.exoplayer2.source;
 
 import android.support.annotation.IntDef;
+import android.support.annotation.Nullable;
 import com.google.android.exoplayer2.ExoPlayer;
 import com.google.android.exoplayer2.Timeline;
 import com.google.android.exoplayer2.upstream.Allocator;
@@ -28,10 +29,10 @@ import java.util.Arrays;
 
 /**
  * Merges multiple {@link MediaSource}s.
- * <p>
- * The {@link Timeline}s of the sources being merged must have the same number of periods.
+ *
+ * <p>The {@link Timeline}s of the sources being merged must have the same number of periods.
  */
-public final class MergingMediaSource implements MediaSource {
+public final class MergingMediaSource extends CompositeMediaSource<Integer> {
 
   /**
    * Thrown when a {@link MergingMediaSource} cannot merge its sources.
@@ -98,16 +99,11 @@ public final class MergingMediaSource implements MediaSource {
 
   @Override
   public void prepareSource(ExoPlayer player, boolean isTopLevelSource, Listener listener) {
+    super.prepareSource(player, isTopLevelSource, listener);
     Assertions.checkState(this.listener == null, MEDIA_SOURCE_REUSED_ERROR_MESSAGE);
     this.listener = listener;
     for (int i = 0; i < mediaSources.length; i++) {
-      final int sourceIndex = i;
-      mediaSources[sourceIndex].prepareSource(player, false, new Listener() {
-        @Override
-        public void onSourceInfoRefreshed(MediaSource source, Timeline timeline, Object manifest) {
-          handleSourceInfoRefreshed(sourceIndex, timeline, manifest);
-        }
-      });
+      prepareChildSource(i, mediaSources[i]);
     }
   }
 
@@ -116,9 +112,7 @@ public final class MergingMediaSource implements MediaSource {
     if (mergeError != null) {
       throw mergeError;
     }
-    for (MediaSource mediaSource : mediaSources) {
-      mediaSource.maybeThrowSourceInfoRefreshError();
-    }
+    super.maybeThrowSourceInfoRefreshError();
   }
 
   @Override
@@ -139,21 +133,16 @@ public final class MergingMediaSource implements MediaSource {
   }
 
   @Override
-  public void releaseSource() {
-    for (MediaSource mediaSource : mediaSources) {
-      mediaSource.releaseSource();
-    }
-  }
-
-  private void handleSourceInfoRefreshed(int sourceIndex, Timeline timeline, Object manifest) {
+  protected void onChildSourceInfoRefreshed(
+      Integer id, MediaSource mediaSource, Timeline timeline, @Nullable Object manifest) {
     if (mergeError == null) {
       mergeError = checkTimelineMerges(timeline);
     }
     if (mergeError != null) {
       return;
     }
-    pendingTimelineSources.remove(mediaSources[sourceIndex]);
-    if (sourceIndex == 0) {
+    pendingTimelineSources.remove(mediaSource);
+    if (mediaSource == mediaSources[0]) {
       primaryTimeline = timeline;
       primaryManifest = manifest;
     }

library/core/src/main/java/com/google/android/exoplayer2/source/ads/AdsMediaSource.java

@@ -24,10 +24,12 @@ import android.view.ViewGroup;
 import com.google.android.exoplayer2.C;
 import com.google.android.exoplayer2.ExoPlayer;
 import com.google.android.exoplayer2.Timeline;
+import com.google.android.exoplayer2.source.CompositeMediaSource;
 import com.google.android.exoplayer2.source.DeferredMediaPeriod;
 import com.google.android.exoplayer2.source.ExtractorMediaSource;
 import com.google.android.exoplayer2.source.MediaPeriod;
 import com.google.android.exoplayer2.source.MediaSource;
+import com.google.android.exoplayer2.source.MediaSource.MediaPeriodId;
 import com.google.android.exoplayer2.source.MediaSourceEventListener;
 import com.google.android.exoplayer2.upstream.Allocator;
 import com.google.android.exoplayer2.upstream.DataSource;
@@ -39,10 +41,8 @@ import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
 
-/**
- * A {@link MediaSource} that inserts ads linearly with a provided content media source.
- */
-public final class AdsMediaSource implements MediaSource {
+/** A {@link MediaSource} that inserts ads linearly with a provided content media source. */
+public final class AdsMediaSource extends CompositeMediaSource<MediaPeriodId> {
 
   /** Factory for creating {@link MediaSource}s to play ad media. */
   public interface MediaSourceFactory {
@@ -107,7 +107,6 @@ public final class AdsMediaSource implements MediaSource {
   private final Timeline.Period period;
 
   private Handler playerHandler;
-  private ExoPlayer player;
   private volatile boolean released;
 
   // Accessed on the player thread.
@@ -203,17 +202,12 @@ public final class AdsMediaSource implements MediaSource {
 
   @Override
   public void prepareSource(final ExoPlayer player, boolean isTopLevelSource, Listener listener) {
+    super.prepareSource(player, isTopLevelSource, listener);
     Assertions.checkArgument(isTopLevelSource);
     Assertions.checkState(this.listener == null, MEDIA_SOURCE_REUSED_ERROR_MESSAGE);
     this.listener = listener;
-    this.player = player;
     playerHandler = new Handler();
-    contentMediaSource.prepareSource(player, false, new Listener() {
-      @Override
-      public void onSourceInfoRefreshed(MediaSource source, Timeline timeline, Object manifest) {
-        AdsMediaSource.this.onContentSourceInfoRefreshed(timeline, manifest);
-      }
-    });
+    prepareChildSource(new MediaPeriodId(/* periodIndex= */ 0), contentMediaSource);
     mainHandler.post(new Runnable() {
       @Override
       public void run() {
@@ -222,26 +216,14 @@ public final class AdsMediaSource implements MediaSource {
     });
   }
 
-  @Override
-  public void maybeThrowSourceInfoRefreshError() throws IOException {
-    contentMediaSource.maybeThrowSourceInfoRefreshError();
-    for (MediaSource[] mediaSources : adGroupMediaSources) {
-      for (MediaSource mediaSource : mediaSources) {
-        if (mediaSource != null) {
-          mediaSource.maybeThrowSourceInfoRefreshError();
-        }
-      }
-    }
-  }
-
   @Override
   public MediaPeriod createPeriod(MediaPeriodId id, Allocator allocator) {
     if (adPlaybackState.adGroupCount > 0 && id.isAd()) {
-      final int adGroupIndex = id.adGroupIndex;
-      final int adIndexInAdGroup = id.adIndexInAdGroup;
+      int adGroupIndex = id.adGroupIndex;
+      int adIndexInAdGroup = id.adIndexInAdGroup;
       if (adGroupMediaSources[adGroupIndex].length <= adIndexInAdGroup) {
         Uri adUri = adPlaybackState.adUris[id.adGroupIndex][id.adIndexInAdGroup];
-        final MediaSource adMediaSource =
+        MediaSource adMediaSource =
             adMediaSourceFactory.createMediaSource(adUri, eventHandler, eventListener);
         int oldAdCount = adGroupMediaSources[id.adGroupIndex].length;
         if (adIndexInAdGroup >= oldAdCount) {
@@ -253,13 +235,7 @@ public final class AdsMediaSource implements MediaSource {
         }
         adGroupMediaSources[adGroupIndex][adIndexInAdGroup] = adMediaSource;
         deferredMediaPeriodByAdMediaSource.put(adMediaSource, new ArrayList<DeferredMediaPeriod>());
-        adMediaSource.prepareSource(player, false, new MediaSource.Listener() {
-          @Override
-          public void onSourceInfoRefreshed(MediaSource source, Timeline timeline,
-              @Nullable Object manifest) {
-            onAdSourceInfoRefreshed(adMediaSource, adGroupIndex, adIndexInAdGroup, timeline);
-          }
-        });
+        prepareChildSource(id, adMediaSource);
       }
       MediaSource mediaSource = adGroupMediaSources[adGroupIndex][adIndexInAdGroup];
       DeferredMediaPeriod deferredMediaPeriod =
@@ -287,15 +263,8 @@ public final class AdsMediaSource implements MediaSource {
 
   @Override
   public void releaseSource() {
+    super.releaseSource();
     released = true;
-    contentMediaSource.releaseSource();
-    for (MediaSource[] mediaSources : adGroupMediaSources) {
-      for (MediaSource mediaSource : mediaSources) {
-        if (mediaSource != null) {
-          mediaSource.releaseSource();
-        }
-      }
-    }
     mainHandler.post(new Runnable() {
       @Override
       public void run() {
@@ -304,6 +273,21 @@ public final class AdsMediaSource implements MediaSource {
     });
   }
 
+  @Override
+  protected void onChildSourceInfoRefreshed(
+      MediaPeriodId mediaPeriodId,
+      MediaSource mediaSource,
+      Timeline timeline,
+      @Nullable Object manifest) {
+    if (mediaPeriodId.isAd()) {
+      int adGroupIndex = mediaPeriodId.adGroupIndex;
+      int adIndexInAdGroup = mediaPeriodId.adIndexInAdGroup;
+      onAdSourceInfoRefreshed(mediaSource, adGroupIndex, adIndexInAdGroup, timeline);
+    } else {
+      onContentSourceInfoRefreshed(timeline, manifest);
+    }
+  }
+
   // Internal methods.
 
   private void onAdPlaybackState(AdPlaybackState adPlaybackState) {