Allow lazy preparation of child source in a concatenation.

This adds an optional parameter to ConcatenatingMediaSource to prepare
child sources only lazily when are needed. This is helpful for long playlists
of media sources with manifests to prevent a lot of simultaneous manifest
loads.

Issue:#3972

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=198855676

RELEASENOTES.md

@@ -10,6 +10,9 @@
   changes ([#4276](https://github.com/google/ExoPlayer/issues/4276)).
 * IMA: Don't advertise support for video/mpeg ad media, as we don't have an
   extractor for this ([#4297](https://github.com/google/ExoPlayer/issues/4297)).
+* Add support for lazy preparation of playlist media sources in
+  `ConcatenatingMediaSource`
+  ([#3972](https://github.com/google/ExoPlayer/issues/3972)).
 * HLS:
   * Allow injection of custom playlist trackers.
 

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

@@ -35,8 +35,8 @@ public abstract class CompositeMediaSource<T> extends BaseMediaSource {
 
   private final HashMap<T, MediaSourceAndListener> childSources;
 
-  private ExoPlayer player;
-  private Handler eventHandler;
+  private @Nullable ExoPlayer player;
+  private @Nullable Handler eventHandler;
 
   /** Create composite media source without child sources. */
   protected CompositeMediaSource() {
@@ -78,7 +78,7 @@ public abstract class CompositeMediaSource<T> extends BaseMediaSource {
    * @param manifest The manifest of the child source.
    */
   protected abstract void onChildSourceInfoRefreshed(
-      @Nullable T id, MediaSource mediaSource, Timeline timeline, @Nullable Object manifest);
+      T id, MediaSource mediaSource, Timeline timeline, @Nullable Object manifest);
 
   /**
    * Prepares a child source.
@@ -93,7 +93,7 @@ public abstract class CompositeMediaSource<T> extends BaseMediaSource {
    * @param id A unique id to identify the child source preparation. Null is allowed as an id.
    * @param mediaSource The child {@link MediaSource}.
    */
-  protected final void prepareChildSource(@Nullable final T id, MediaSource mediaSource) {
+  protected final void prepareChildSource(final T id, MediaSource mediaSource) {
     Assertions.checkArgument(!childSources.containsKey(id));
     SourceInfoRefreshListener sourceListener =
         new SourceInfoRefreshListener() {
@@ -105,8 +105,9 @@ public abstract class CompositeMediaSource<T> extends BaseMediaSource {
         };
     MediaSourceEventListener eventListener = new ForwardingEventListener(id);
     childSources.put(id, new MediaSourceAndListener(mediaSource, sourceListener, eventListener));
-    mediaSource.addEventListener(eventHandler, eventListener);
-    mediaSource.prepareSource(player, /* isTopLevelSource= */ false, sourceListener);
+    mediaSource.addEventListener(Assertions.checkNotNull(eventHandler), eventListener);
+    mediaSource.prepareSource(
+        Assertions.checkNotNull(player), /* isTopLevelSource= */ false, sourceListener);
   }
 
   /**
@@ -114,8 +115,8 @@ public abstract class CompositeMediaSource<T> extends BaseMediaSource {
    *
    * @param id The unique id used to prepare the child source.
    */
-  protected final void releaseChildSource(@Nullable T id) {
-    MediaSourceAndListener removedChild = childSources.remove(id);
+  protected final void releaseChildSource(T id) {
+    MediaSourceAndListener removedChild = Assertions.checkNotNull(childSources.remove(id));
     removedChild.mediaSource.releaseSource(removedChild.listener);
     removedChild.mediaSource.removeEventListener(removedChild.eventListener);
   }
@@ -128,7 +129,7 @@ public abstract class CompositeMediaSource<T> extends BaseMediaSource {
    * @param windowIndex A window index of the child source.
    * @return The corresponding window index in the composite source.
    */
-  protected int getWindowIndexForChildWindowIndex(@Nullable T id, int windowIndex) {
+  protected int getWindowIndexForChildWindowIndex(T id, int windowIndex) {
     return windowIndex;
   }
 
@@ -143,7 +144,7 @@ public abstract class CompositeMediaSource<T> extends BaseMediaSource {
    *     corresponding media period id can be determined.
    */
   protected @Nullable MediaPeriodId getMediaPeriodIdForChildMediaPeriodId(
-      @Nullable T id, MediaPeriodId mediaPeriodId) {
+      T id, MediaPeriodId mediaPeriodId) {
     return mediaPeriodId;
   }
 
@@ -177,10 +178,10 @@ public abstract class CompositeMediaSource<T> extends BaseMediaSource {
 
   private final class ForwardingEventListener implements MediaSourceEventListener {
 
-    private final @Nullable T id;
+    private final T id;
     private EventDispatcher eventDispatcher;
 
-    public ForwardingEventListener(@Nullable T id) {
+    public ForwardingEventListener(T id) {
       this.eventDispatcher = createEventDispatcher(/* mediaPeriodId= */ null);
       this.id = id;
     }

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

@@ -25,6 +25,7 @@ import com.google.android.exoplayer2.Player;
 import com.google.android.exoplayer2.Timeline;
 import com.google.android.exoplayer2.Timeline.Period;
 import com.google.android.exoplayer2.source.MediaSource.MediaPeriodId;
+import com.google.android.exoplayer2.source.ShuffleOrder.DefaultShuffleOrder;
 import com.google.android.exoplayer2.testutil.DummyMainThread;
 import com.google.android.exoplayer2.testutil.FakeMediaSource;
 import com.google.android.exoplayer2.testutil.FakeShuffleOrder;
@@ -913,6 +914,58 @@ public final class ConcatenatingMediaSourceTest {
         new MediaPeriodId(/* periodIndex= */ 0, /* windowSequenceNumber= */ 0));
   }
 
+  @Test
+  public void testChildSourceIsNotPreparedWithLazyPreparation() throws IOException {
+    FakeMediaSource[] childSources = createMediaSources(/* count= */ 2);
+    mediaSource =
+        new ConcatenatingMediaSource(
+            /* isAtomic= */ false,
+            /* useLazyPreparation= */ true,
+            new DefaultShuffleOrder(0),
+            childSources);
+    testRunner = new MediaSourceTestRunner(mediaSource, /* allocator= */ null);
+    testRunner.prepareSource();
+
+    assertThat(childSources[0].isPrepared()).isFalse();
+    assertThat(childSources[1].isPrepared()).isFalse();
+  }
+
+  @Test
+  public void testChildSourceIsPreparedWithLazyPreparationAfterPeriodCreation() throws IOException {
+    FakeMediaSource[] childSources = createMediaSources(/* count= */ 2);
+    mediaSource =
+        new ConcatenatingMediaSource(
+            /* isAtomic= */ false,
+            /* useLazyPreparation= */ true,
+            new DefaultShuffleOrder(0),
+            childSources);
+    testRunner = new MediaSourceTestRunner(mediaSource, /* allocator= */ null);
+    testRunner.prepareSource();
+    testRunner.createPeriod(new MediaPeriodId(/* periodIndex= */ 0, /* windowSequenceNumber= */ 0));
+
+    assertThat(childSources[0].isPrepared()).isTrue();
+    assertThat(childSources[1].isPrepared()).isFalse();
+  }
+
+  @Test
+  public void testChildSourceWithLazyPreparationOnlyPreparesSourceOnce() throws IOException {
+    FakeMediaSource[] childSources = createMediaSources(/* count= */ 2);
+    mediaSource =
+        new ConcatenatingMediaSource(
+            /* isAtomic= */ false,
+            /* useLazyPreparation= */ true,
+            new DefaultShuffleOrder(0),
+            childSources);
+    testRunner = new MediaSourceTestRunner(mediaSource, /* allocator= */ null);
+    testRunner.prepareSource();
+
+    // The lazy preparation must only be triggered once, even if we create multiple periods from
+    // the media source. FakeMediaSource.prepareSource asserts that it's not called twice, so
+    // creating two periods shouldn't throw.
+    testRunner.createPeriod(new MediaPeriodId(/* periodIndex= */ 0, /* windowSequenceNumber= */ 0));
+    testRunner.createPeriod(new MediaPeriodId(/* periodIndex= */ 0, /* windowSequenceNumber= */ 0));
+  }
+
   private void assertCompletedAllMediaPeriodLoads(Timeline timeline) {
     Timeline.Period period = new Timeline.Period();
     Timeline.Window window = new Timeline.Window();

testutils/src/main/java/com/google/android/exoplayer2/testutil/FakeMediaSource.java

@@ -159,9 +159,9 @@ public class FakeMediaSource extends BaseMediaSource {
     }
   }
 
-  /** Asserts that the source has been prepared. */
-  public void assertPrepared() {
-    assertThat(preparedSource).isTrue();
+  /** Returns whether the source is currently prepared. */
+  public boolean isPrepared() {
+    return preparedSource;
   }
 
   /**