Add clear method to ConcatenatingMediaSource.

This simplifies clearing a playlist without having to call removeMediaSource
repeatedly. It will also update the timeline only once.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=187863857

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

@@ -50,8 +50,9 @@ public class ConcatenatingMediaSource extends CompositeMediaSource<MediaSourceHo
   private static final int MSG_ADD_MULTIPLE = 1;
   private static final int MSG_REMOVE = 2;
   private static final int MSG_MOVE = 3;
-  private static final int MSG_NOTIFY_LISTENER = 4;
-  private static final int MSG_ON_COMPLETION = 5;
+  private static final int MSG_CLEAR = 4;
+  private static final int MSG_NOTIFY_LISTENER = 5;
+  private static final int MSG_ON_COMPLETION = 6;
 
   // Accessed on the app thread.
   private final List<MediaSource> mediaSourcesPublic;
@@ -333,6 +334,30 @@ public class ConcatenatingMediaSource extends CompositeMediaSource<MediaSourceHo
     }
   }
 
+  /** Clears the playlist. */
+  public final synchronized void clear() {
+    clear(/* actionOnCompletion= */ null);
+  }
+
+  /**
+   * Clears the playlist and executes a custom action on completion.
+   *
+   * @param actionOnCompletion A {@link Runnable} which is executed immediately after the playlist
+   *     has been cleared.
+   */
+  public final synchronized void clear(@Nullable Runnable actionOnCompletion) {
+    mediaSourcesPublic.clear();
+    if (player != null) {
+      player
+          .createMessage(this)
+          .setType(MSG_CLEAR)
+          .setPayload(actionOnCompletion != null ? new EventDispatcher(actionOnCompletion) : null)
+          .send();
+    } else if (actionOnCompletion != null) {
+      actionOnCompletion.run();
+    }
+  }
+
   /** Returns the number of media sources in the playlist. */
   public final synchronized int getSize() {
     return mediaSourcesPublic.size();
@@ -445,6 +470,10 @@ public class ConcatenatingMediaSource extends CompositeMediaSource<MediaSourceHo
         moveMediaSourceInternal(moveMessage.index, moveMessage.customData);
         scheduleListenerNotification(moveMessage.actionOnCompletion);
         break;
+      case MSG_CLEAR:
+        clearInternal();
+        scheduleListenerNotification((EventDispatcher) message);
+        break;
       case MSG_NOTIFY_LISTENER:
         notifyListener();
         break;
@@ -545,9 +574,14 @@ public class ConcatenatingMediaSource extends CompositeMediaSource<MediaSourceHo
     scheduleListenerNotification(/* actionOnCompletion= */ null);
   }
 
+  private void clearInternal() {
+    for (int index = mediaSourceHolders.size() - 1; index >= 0; index--) {
+      removeMediaSourceInternal(index);
+    }
+  }
+
   private void removeMediaSourceInternal(int index) {
-    MediaSourceHolder holder = mediaSourceHolders.get(index);
-    mediaSourceHolders.remove(index);
+    MediaSourceHolder holder = mediaSourceHolders.remove(index);
     Timeline oldTimeline = holder.timeline;
     correctOffsets(
         index,

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

@@ -820,6 +820,37 @@ public final class ConcatenatingMediaSourceTest {
     childSource.assertReleased();
   }
 
+  @Test
+  public void testClear() throws IOException {
+    DummyMainThread dummyMainThread = new DummyMainThread();
+    final FakeMediaSource preparedChildSource = createFakeMediaSource();
+    final FakeMediaSource unpreparedChildSource =
+        new FakeMediaSource(/* timeline= */ null, /* manifest= */ null);
+    dummyMainThread.runOnMainThread(
+        new Runnable() {
+          @Override
+          public void run() {
+            mediaSource.addMediaSource(preparedChildSource);
+            mediaSource.addMediaSource(unpreparedChildSource);
+          }
+        });
+    testRunner.prepareSource();
+    final TimelineGrabber timelineGrabber = new TimelineGrabber(testRunner);
+
+    dummyMainThread.runOnMainThread(
+        new Runnable() {
+          @Override
+          public void run() {
+            mediaSource.clear(timelineGrabber);
+          }
+        });
+
+    Timeline timeline = timelineGrabber.assertTimelineChangeBlocking();
+    assertThat(timeline.isEmpty()).isTrue();
+    preparedChildSource.assertReleased();
+    unpreparedChildSource.assertReleased();
+  }
+
   private static FakeMediaSource[] createMediaSources(int count) {
     FakeMediaSource[] sources = new FakeMediaSource[count];
     for (int i = 0; i < count; i++) {