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)).
   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
 * 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)).
   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:
 * HLS:
   * Allow injection of custom playlist trackers.
   * 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 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. */
   /** Create composite media source without child sources. */
   protected CompositeMediaSource() {
   protected CompositeMediaSource() {
@@ -78,7 +78,7 @@ public abstract class CompositeMediaSource<T> extends BaseMediaSource {
    * @param manifest The manifest of the child source.
    * @param manifest The manifest of the child source.
    */
    */
   protected abstract void onChildSourceInfoRefreshed(
   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.
    * 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 id A unique id to identify the child source preparation. Null is allowed as an id.
    * @param mediaSource The child {@link MediaSource}.
    * @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));
     Assertions.checkArgument(!childSources.containsKey(id));
     SourceInfoRefreshListener sourceListener =
     SourceInfoRefreshListener sourceListener =
         new SourceInfoRefreshListener() {
         new SourceInfoRefreshListener() {
@@ -105,8 +105,9 @@ public abstract class CompositeMediaSource<T> extends BaseMediaSource {
         };
         };
     MediaSourceEventListener eventListener = new ForwardingEventListener(id);
     MediaSourceEventListener eventListener = new ForwardingEventListener(id);
     childSources.put(id, new MediaSourceAndListener(mediaSource, sourceListener, eventListener));
     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.
    * @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.releaseSource(removedChild.listener);
     removedChild.mediaSource.removeEventListener(removedChild.eventListener);
     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.
    * @param windowIndex A window index of the child source.
    * @return The corresponding window index in the composite 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;
     return windowIndex;
   }
   }
 
 
@@ -143,7 +144,7 @@ public abstract class CompositeMediaSource<T> extends BaseMediaSource {
    *     corresponding media period id can be determined.
    *     corresponding media period id can be determined.
    */
    */
   protected @Nullable MediaPeriodId getMediaPeriodIdForChildMediaPeriodId(
   protected @Nullable MediaPeriodId getMediaPeriodIdForChildMediaPeriodId(
-      @Nullable T id, MediaPeriodId mediaPeriodId) {
+      T id, MediaPeriodId mediaPeriodId) {
     return mediaPeriodId;
     return mediaPeriodId;
   }
   }
 
 
@@ -177,10 +178,10 @@ public abstract class CompositeMediaSource<T> extends BaseMediaSource {
 
 
   private final class ForwardingEventListener implements MediaSourceEventListener {
   private final class ForwardingEventListener implements MediaSourceEventListener {
 
 
-    private final @Nullable T id;
+    private final T id;
     private EventDispatcher eventDispatcher;
     private EventDispatcher eventDispatcher;
 
 
-    public ForwardingEventListener(@Nullable T id) {
+    public ForwardingEventListener(T id) {
       this.eventDispatcher = createEventDispatcher(/* mediaPeriodId= */ null);
       this.eventDispatcher = createEventDispatcher(/* mediaPeriodId= */ null);
       this.id = id;
       this.id = id;
     }
     }

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

@@ -63,40 +63,16 @@ public class ConcatenatingMediaSource extends CompositeMediaSource<MediaSourceHo
   private final Map<MediaPeriod, MediaSourceHolder> mediaSourceByMediaPeriod;
   private final Map<MediaPeriod, MediaSourceHolder> mediaSourceByMediaPeriod;
   private final List<EventDispatcher> pendingOnCompletionActions;
   private final List<EventDispatcher> pendingOnCompletionActions;
   private final boolean isAtomic;
   private final boolean isAtomic;
+  private final boolean useLazyPreparation;
   private final Timeline.Window window;
   private final Timeline.Window window;
+  private final Timeline.Period period;
 
 
-  private ExoPlayer player;
+  private @Nullable ExoPlayer player;
   private boolean listenerNotificationScheduled;
   private boolean listenerNotificationScheduled;
   private ShuffleOrder shuffleOrder;
   private ShuffleOrder shuffleOrder;
   private int windowCount;
   private int windowCount;
   private int periodCount;
   private int periodCount;
 
 
-  /** Creates a new concatenating media source. */
-  public ConcatenatingMediaSource() {
-    this(/* isAtomic= */ false, new DefaultShuffleOrder(0));
-  }
-
-  /**
-   * Creates a new concatenating media source.
-   *
-   * @param isAtomic Whether the concatenating media source will be treated as atomic, i.e., treated
-   *     as a single item for repeating and shuffling.
-   */
-  public ConcatenatingMediaSource(boolean isAtomic) {
-    this(isAtomic, new DefaultShuffleOrder(0));
-  }
-
-  /**
-   * Creates a new concatenating media source with a custom shuffle order.
-   *
-   * @param isAtomic Whether the concatenating media source will be treated as atomic, i.e., treated
-   *     as a single item for repeating and shuffling.
-   * @param shuffleOrder The {@link ShuffleOrder} to use when shuffling the child media sources.
-   */
-  public ConcatenatingMediaSource(boolean isAtomic, ShuffleOrder shuffleOrder) {
-    this(isAtomic, shuffleOrder, new MediaSource[0]);
-  }
-
   /**
   /**
    * @param mediaSources The {@link MediaSource}s to concatenate. It is valid for the same
    * @param mediaSources The {@link MediaSource}s to concatenate. It is valid for the same
    *     {@link MediaSource} instance to be present more than once in the array.
    *     {@link MediaSource} instance to be present more than once in the array.
@@ -124,6 +100,25 @@ public class ConcatenatingMediaSource extends CompositeMediaSource<MediaSourceHo
    */
    */
   public ConcatenatingMediaSource(
   public ConcatenatingMediaSource(
       boolean isAtomic, ShuffleOrder shuffleOrder, MediaSource... mediaSources) {
       boolean isAtomic, ShuffleOrder shuffleOrder, MediaSource... mediaSources) {
+    this(isAtomic, /* useLazyPreparation= */ false, shuffleOrder, mediaSources);
+  }
+
+  /**
+   * @param isAtomic Whether the concatenating media source will be treated as atomic, i.e., treated
+   *     as a single item for repeating and shuffling.
+   * @param useLazyPreparation Whether playlist items are prepared lazily. If false, all manifest
+   *     loads and other initial preparation steps happen immediately. If true, these initial
+   *     preparations are triggered only when the player starts buffering the media.
+   * @param shuffleOrder The {@link ShuffleOrder} to use when shuffling the child media sources.
+   * @param mediaSources The {@link MediaSource}s to concatenate. It is valid for the same {@link
+   *     MediaSource} instance to be present more than once in the array.
+   */
+  @SuppressWarnings("initialization")
+  public ConcatenatingMediaSource(
+      boolean isAtomic,
+      boolean useLazyPreparation,
+      ShuffleOrder shuffleOrder,
+      MediaSource... mediaSources) {
     for (MediaSource mediaSource : mediaSources) {
     for (MediaSource mediaSource : mediaSources) {
       Assertions.checkNotNull(mediaSource);
       Assertions.checkNotNull(mediaSource);
     }
     }
@@ -134,7 +129,9 @@ public class ConcatenatingMediaSource extends CompositeMediaSource<MediaSourceHo
     this.pendingOnCompletionActions = new ArrayList<>();
     this.pendingOnCompletionActions = new ArrayList<>();
     this.query = new MediaSourceHolder(/* mediaSource= */ null);
     this.query = new MediaSourceHolder(/* mediaSource= */ null);
     this.isAtomic = isAtomic;
     this.isAtomic = isAtomic;
+    this.useLazyPreparation = useLazyPreparation;
     window = new Timeline.Window();
     window = new Timeline.Window();
+    period = new Timeline.Period();
     addMediaSources(Arrays.asList(mediaSources));
     addMediaSources(Arrays.asList(mediaSources));
   }
   }
 
 
@@ -293,7 +290,7 @@ public class ConcatenatingMediaSource extends CompositeMediaSource<MediaSourceHo
       player
       player
           .createMessage(this)
           .createMessage(this)
           .setType(MSG_REMOVE)
           .setType(MSG_REMOVE)
-          .setPayload(new MessageData<>(index, null, actionOnCompletion))
+          .setPayload(new MessageData<Void>(index, null, actionOnCompletion))
           .send();
           .send();
     } else if (actionOnCompletion != null) {
     } else if (actionOnCompletion != null) {
       actionOnCompletion.run();
       actionOnCompletion.run();
@@ -402,7 +399,10 @@ public class ConcatenatingMediaSource extends CompositeMediaSource<MediaSourceHo
         new DeferredMediaPeriod(holder.mediaSource, idInSource, allocator);
         new DeferredMediaPeriod(holder.mediaSource, idInSource, allocator);
     mediaSourceByMediaPeriod.put(mediaPeriod, holder);
     mediaSourceByMediaPeriod.put(mediaPeriod, holder);
     holder.activeMediaPeriods.add(mediaPeriod);
     holder.activeMediaPeriods.add(mediaPeriod);
-    if (holder.isPrepared) {
+    if (!holder.hasStartedPreparing) {
+      holder.hasStartedPreparing = true;
+      prepareChildSource(holder, holder.mediaSource);
+    } else if (holder.isPrepared) {
       mediaPeriod.createPeriod();
       mediaPeriod.createPeriod();
     }
     }
     return mediaPeriod;
     return mediaPeriod;
@@ -410,7 +410,8 @@ public class ConcatenatingMediaSource extends CompositeMediaSource<MediaSourceHo
 
 
   @Override
   @Override
   public final void releasePeriod(MediaPeriod mediaPeriod) {
   public final void releasePeriod(MediaPeriod mediaPeriod) {
-    MediaSourceHolder holder = mediaSourceByMediaPeriod.remove(mediaPeriod);
+    MediaSourceHolder holder =
+        Assertions.checkNotNull(mediaSourceByMediaPeriod.remove(mediaPeriod));
     ((DeferredMediaPeriod) mediaPeriod).releasePeriod();
     ((DeferredMediaPeriod) mediaPeriod).releasePeriod();
     holder.activeMediaPeriods.remove(mediaPeriod);
     holder.activeMediaPeriods.remove(mediaPeriod);
     if (holder.activeMediaPeriods.isEmpty() && holder.isRemoved) {
     if (holder.activeMediaPeriods.isEmpty() && holder.isRemoved) {
@@ -510,7 +511,7 @@ public class ConcatenatingMediaSource extends CompositeMediaSource<MediaSourceHo
 
 
   private void scheduleListenerNotification(@Nullable EventDispatcher actionOnCompletion) {
   private void scheduleListenerNotification(@Nullable EventDispatcher actionOnCompletion) {
     if (!listenerNotificationScheduled) {
     if (!listenerNotificationScheduled) {
-      player.createMessage(this).setType(MSG_NOTIFY_LISTENER).send();
+      Assertions.checkNotNull(player).createMessage(this).setType(MSG_NOTIFY_LISTENER).send();
       listenerNotificationScheduled = true;
       listenerNotificationScheduled = true;
     }
     }
     if (actionOnCompletion != null) {
     if (actionOnCompletion != null) {
@@ -530,7 +531,11 @@ public class ConcatenatingMediaSource extends CompositeMediaSource<MediaSourceHo
             mediaSourceHolders, windowCount, periodCount, shuffleOrder, isAtomic),
             mediaSourceHolders, windowCount, periodCount, shuffleOrder, isAtomic),
         /* manifest= */ null);
         /* manifest= */ null);
     if (!actionsOnCompletion.isEmpty()) {
     if (!actionsOnCompletion.isEmpty()) {
-      player.createMessage(this).setType(MSG_ON_COMPLETION).setPayload(actionsOnCompletion).send();
+      Assertions.checkNotNull(player)
+          .createMessage(this)
+          .setType(MSG_ON_COMPLETION)
+          .setPayload(actionsOnCompletion)
+          .send();
     }
     }
   }
   }
 
 
@@ -551,7 +556,10 @@ public class ConcatenatingMediaSource extends CompositeMediaSource<MediaSourceHo
         newMediaSourceHolder.timeline.getWindowCount(),
         newMediaSourceHolder.timeline.getWindowCount(),
         newMediaSourceHolder.timeline.getPeriodCount());
         newMediaSourceHolder.timeline.getPeriodCount());
     mediaSourceHolders.add(newIndex, newMediaSourceHolder);
     mediaSourceHolders.add(newIndex, newMediaSourceHolder);
-    prepareChildSource(newMediaSourceHolder, newMediaSourceHolder.mediaSource);
+    if (!useLazyPreparation) {
+      newMediaSourceHolder.hasStartedPreparing = true;
+      prepareChildSource(newMediaSourceHolder, newMediaSourceHolder.mediaSource);
+    }
   }
   }
 
 
   private void addMediaSourcesInternal(
   private void addMediaSourcesInternal(
@@ -578,7 +586,7 @@ public class ConcatenatingMediaSource extends CompositeMediaSource<MediaSourceHo
           windowOffsetUpdate,
           windowOffsetUpdate,
           periodOffsetUpdate);
           periodOffsetUpdate);
     }
     }
-    mediaSourceHolder.timeline = deferredTimeline.cloneWithNewTimeline(timeline);
+    mediaSourceHolder.timeline = deferredTimeline.cloneWithNewTimeline(timeline, period);
     if (!mediaSourceHolder.isPrepared && !timeline.isEmpty()) {
     if (!mediaSourceHolder.isPrepared && !timeline.isEmpty()) {
       timeline.getWindow(/* windowIndex= */ 0, window);
       timeline.getWindow(/* windowIndex= */ 0, window);
       long defaultPeriodPositionUs =
       long defaultPeriodPositionUs =
@@ -662,21 +670,23 @@ public class ConcatenatingMediaSource extends CompositeMediaSource<MediaSourceHo
     public int childIndex;
     public int childIndex;
     public int firstWindowIndexInChild;
     public int firstWindowIndexInChild;
     public int firstPeriodIndexInChild;
     public int firstPeriodIndexInChild;
+    public boolean hasStartedPreparing;
     public boolean isPrepared;
     public boolean isPrepared;
     public boolean isRemoved;
     public boolean isRemoved;
     public List<DeferredMediaPeriod> activeMediaPeriods;
     public List<DeferredMediaPeriod> activeMediaPeriods;
 
 
     public MediaSourceHolder(MediaSource mediaSource) {
     public MediaSourceHolder(MediaSource mediaSource) {
       this.mediaSource = mediaSource;
       this.mediaSource = mediaSource;
-      this.uid = System.identityHashCode(this);
       this.timeline = new DeferredTimeline();
       this.timeline = new DeferredTimeline();
       this.activeMediaPeriods = new ArrayList<>();
       this.activeMediaPeriods = new ArrayList<>();
+      this.uid = System.identityHashCode(this);
     }
     }
 
 
     public void reset(int childIndex, int firstWindowIndexInChild, int firstPeriodIndexInChild) {
     public void reset(int childIndex, int firstWindowIndexInChild, int firstPeriodIndexInChild) {
       this.childIndex = childIndex;
       this.childIndex = childIndex;
       this.firstWindowIndexInChild = firstWindowIndexInChild;
       this.firstWindowIndexInChild = firstWindowIndexInChild;
       this.firstPeriodIndexInChild = firstPeriodIndexInChild;
       this.firstPeriodIndexInChild = firstPeriodIndexInChild;
+      this.hasStartedPreparing = false;
       this.isPrepared = false;
       this.isPrepared = false;
       this.isRemoved = false;
       this.isRemoved = false;
       this.activeMediaPeriods.clear();
       this.activeMediaPeriods.clear();
@@ -814,13 +824,12 @@ public class ConcatenatingMediaSource extends CompositeMediaSource<MediaSourceHo
   private static final class DeferredTimeline extends ForwardingTimeline {
   private static final class DeferredTimeline extends ForwardingTimeline {
 
 
     private static final Object DUMMY_ID = new Object();
     private static final Object DUMMY_ID = new Object();
-    private static final Period period = new Period();
     private static final DummyTimeline dummyTimeline = new DummyTimeline();
     private static final DummyTimeline dummyTimeline = new DummyTimeline();
 
 
     private final Object replacedId;
     private final Object replacedId;
 
 
     public DeferredTimeline() {
     public DeferredTimeline() {
-      this(dummyTimeline, /* replacedId= */ null);
+      this(dummyTimeline, DUMMY_ID);
     }
     }
 
 
     private DeferredTimeline(Timeline timeline, Object replacedId) {
     private DeferredTimeline(Timeline timeline, Object replacedId) {
@@ -828,10 +837,10 @@ public class ConcatenatingMediaSource extends CompositeMediaSource<MediaSourceHo
       this.replacedId = replacedId;
       this.replacedId = replacedId;
     }
     }
 
 
-    public DeferredTimeline cloneWithNewTimeline(Timeline timeline) {
+    public DeferredTimeline cloneWithNewTimeline(Timeline timeline, Period period) {
       return new DeferredTimeline(
       return new DeferredTimeline(
           timeline,
           timeline,
-          replacedId == null && timeline.getPeriodCount() > 0
+          replacedId == DUMMY_ID && timeline.getPeriodCount() > 0
               ? timeline.getPeriod(0, period, true).uid
               ? timeline.getPeriod(0, period, true).uid
               : replacedId);
               : replacedId);
     }
     }
@@ -873,8 +882,7 @@ public class ConcatenatingMediaSource extends CompositeMediaSource<MediaSourceHo
           /* isSeekable= */ false,
           /* isSeekable= */ false,
           // Dynamic window to indicate pending timeline updates.
           // Dynamic window to indicate pending timeline updates.
           /* isDynamic= */ true,
           /* isDynamic= */ true,
-          // Position can't be projected yet as the default position is still unknown.
-          /* defaultPositionUs= */ defaultPositionProjectionUs > 0 ? C.TIME_UNSET : 0,
+          /* defaultPositionUs= */ 0,
           /* durationUs= */ C.TIME_UNSET,
           /* durationUs= */ C.TIME_UNSET,
           /* firstPeriodIndex= */ 0,
           /* firstPeriodIndex= */ 0,
           /* lastPeriodIndex= */ 0,
           /* lastPeriodIndex= */ 0,
@@ -889,8 +897,8 @@ public class ConcatenatingMediaSource extends CompositeMediaSource<MediaSourceHo
     @Override
     @Override
     public Period getPeriod(int periodIndex, Period period, boolean setIds) {
     public Period getPeriod(int periodIndex, Period period, boolean setIds) {
       return period.set(
       return period.set(
-          /* id= */ null,
-          /* uid= */ null,
+          /* id= */ 0,
+          /* uid= */ DeferredTimeline.DUMMY_ID,
           /* windowIndex= */ 0,
           /* windowIndex= */ 0,
           /* durationUs = */ C.TIME_UNSET,
           /* durationUs = */ C.TIME_UNSET,
           /* positionInWindowUs= */ 0);
           /* positionInWindowUs= */ 0);
@@ -898,7 +906,7 @@ public class ConcatenatingMediaSource extends CompositeMediaSource<MediaSourceHo
 
 
     @Override
     @Override
     public int getIndexOfPeriod(Object uid) {
     public int getIndexOfPeriod(Object uid) {
-      return uid == null ? 0 : C.INDEX_UNSET;
+      return uid == DeferredTimeline.DUMMY_ID ? 0 : C.INDEX_UNSET;
     }
     }
   }
   }
 }
 }

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;
 import com.google.android.exoplayer2.Timeline.Period;
 import com.google.android.exoplayer2.Timeline.Period;
 import com.google.android.exoplayer2.source.MediaSource.MediaPeriodId;
 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.DummyMainThread;
 import com.google.android.exoplayer2.testutil.FakeMediaSource;
 import com.google.android.exoplayer2.testutil.FakeMediaSource;
 import com.google.android.exoplayer2.testutil.FakeShuffleOrder;
 import com.google.android.exoplayer2.testutil.FakeShuffleOrder;
@@ -913,6 +914,58 @@ public final class ConcatenatingMediaSourceTest {
         new MediaPeriodId(/* periodIndex= */ 0, /* windowSequenceNumber= */ 0));
         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) {
   private void assertCompletedAllMediaPeriodLoads(Timeline timeline) {
     Timeline.Period period = new Timeline.Period();
     Timeline.Period period = new Timeline.Period();
     Timeline.Window window = new Timeline.Window();
     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;
   }
   }
 
 
   /**
   /**