Post onPrepared so it runs after createPeriod has finished.

Issue: #1853

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=134409897

library/src/main/java/com/google/android/exoplayer2/source/MediaSource.java

@@ -55,9 +55,9 @@ public interface MediaSource {
   /**
    * Returns a {@link MediaPeriod} corresponding to the period at the specified index.
    * <p>
-   * {@link Callback#onPrepared(MediaPeriod)} is called when the new period is prepared. If
-   * preparation fails, {@link MediaPeriod#maybeThrowPrepareError()} will throw an
-   * {@link IOException} if called on the returned instance.
+   * {@link Callback#onPrepared(MediaPeriod)} is called after this method has returned, when the new
+   * period is prepared. If preparation fails, {@link MediaPeriod#maybeThrowPrepareError()} will
+   * throw an {@link IOException} if called on the returned instance.
    *
    * @param index The index of the period.
    * @param callback A callback to receive updates from the period.

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

@@ -19,7 +19,6 @@ import android.support.annotation.IntDef;
 import com.google.android.exoplayer2.Timeline;
 import com.google.android.exoplayer2.source.MediaPeriod.Callback;
 import com.google.android.exoplayer2.upstream.Allocator;
-import com.google.android.exoplayer2.util.Assertions;
 import java.io.IOException;
 import java.lang.annotation.Retention;
 import java.lang.annotation.RetentionPolicy;
@@ -125,7 +124,6 @@ public final class MergingMediaSource implements MediaSource {
     MergingMediaPeriod mergingPeriod = new MergingMediaPeriod(callback, periods);
     for (int i = 0; i < periods.length; i++) {
       periods[i] = mediaSources[i].createPeriod(index, mergingPeriod, allocator, positionUs);
-      Assertions.checkState(periods[i] != null, "Child source must not return null period");
     }
     return mergingPeriod;
   }

library/src/main/java/com/google/android/exoplayer2/source/SingleSampleMediaPeriod.java

@@ -51,6 +51,7 @@ import java.util.Arrays;
   private final int eventSourceId;
   private final TrackGroupArray tracks;
   private final ArrayList<SampleStreamImpl> sampleStreams;
+  private final Handler handler;
   /* package */ final Loader loader;
   /* package */ final Format format;
 
@@ -60,7 +61,7 @@ import java.util.Arrays;
 
   public SingleSampleMediaPeriod(Uri uri, DataSource.Factory dataSourceFactory, Format format,
       int minLoadableRetryCount, Handler eventHandler, EventListener eventListener,
-      int eventSourceId) {
+      int eventSourceId, final Callback callback) {
     this.uri = uri;
     this.dataSourceFactory = dataSourceFactory;
     this.format = format;
@@ -70,12 +71,20 @@ import java.util.Arrays;
     this.eventSourceId = eventSourceId;
     tracks = new TrackGroupArray(new TrackGroup(format));
     sampleStreams = new ArrayList<>();
+    handler = new Handler();
     loader = new Loader("Loader:SingleSampleMediaPeriod");
     sampleData = new byte[INITIAL_SAMPLE_SIZE];
+    handler.post(new Runnable() {
+      @Override
+      public void run() {
+        callback.onPrepared(SingleSampleMediaPeriod.this);
+      }
+    });
   }
 
   public void release() {
     loader.release();
+    handler.removeCallbacksAndMessages(null);
   }
 
   @Override

library/src/main/java/com/google/android/exoplayer2/source/SingleSampleMediaSource.java

@@ -98,10 +98,8 @@ public final class SingleSampleMediaSource implements MediaSource {
   public MediaPeriod createPeriod(int index, Callback callback, Allocator allocator,
       long positionUs) {
     Assertions.checkArgument(index == 0);
-    MediaPeriod mediaPeriod = new SingleSampleMediaPeriod(uri, dataSourceFactory, format,
-        minLoadableRetryCount, eventHandler, eventListener, eventSourceId);
-    callback.onPrepared(mediaPeriod);
-    return mediaPeriod;
+    return new SingleSampleMediaPeriod(uri, dataSourceFactory, format, minLoadableRetryCount,
+        eventHandler, eventListener, eventSourceId, callback);
   }
 
   @Override

library/src/main/java/com/google/android/exoplayer2/source/dash/DashMediaPeriod.java

@@ -15,6 +15,7 @@
  */
 package com.google.android.exoplayer2.source.dash;
 
+import android.os.Handler;
 import com.google.android.exoplayer2.C;
 import com.google.android.exoplayer2.Format;
 import com.google.android.exoplayer2.source.AdaptiveMediaSourceEventListener.EventDispatcher;
@@ -51,6 +52,7 @@ import java.util.List;
   private final Callback callback;
   private final Allocator allocator;
   private final TrackGroupArray trackGroups;
+  private final Handler handler;
 
   private ChunkSampleStream<DashChunkSource>[] sampleStreams;
   private CompositeSequenceableLoader sequenceableLoader;
@@ -76,7 +78,13 @@ import java.util.List;
     sequenceableLoader = new CompositeSequenceableLoader(sampleStreams);
     period = manifest.getPeriod(index);
     trackGroups = buildTrackGroups(period);
-    callback.onPrepared(this);
+    handler = new Handler();
+    handler.post(new Runnable() {
+      @Override
+      public void run() {
+        DashMediaPeriod.this.callback.onPrepared(DashMediaPeriod.this);
+      }
+    });
   }
 
   public void updateManifest(DashManifest manifest, int index) {
@@ -95,6 +103,7 @@ import java.util.List;
     for (ChunkSampleStream<DashChunkSource> sampleStream : sampleStreams) {
       sampleStream.release();
     }
+    handler.removeCallbacksAndMessages(null);
   }
 
   @Override

library/src/main/java/com/google/android/exoplayer2/source/smoothstreaming/SsMediaPeriod.java

@@ -15,6 +15,7 @@
  */
 package com.google.android.exoplayer2.source.smoothstreaming;
 
+import android.os.Handler;
 import android.util.Base64;
 import com.google.android.exoplayer2.C;
 import com.google.android.exoplayer2.extractor.mp4.TrackEncryptionBox;
@@ -50,6 +51,7 @@ import java.util.ArrayList;
   private final Allocator allocator;
   private final TrackGroupArray trackGroups;
   private final TrackEncryptionBox[] trackEncryptionBoxes;
+  private final Handler handler;
 
   private SsManifest manifest;
   private ChunkSampleStream<SsChunkSource>[] sampleStreams;
@@ -74,10 +76,16 @@ import java.util.ArrayList;
     } else {
       trackEncryptionBoxes = null;
     }
+    handler = new Handler();
     this.manifest = manifest;
     sampleStreams = newSampleStreamArray(0);
     sequenceableLoader = new CompositeSequenceableLoader(sampleStreams);
-    callback.onPrepared(this);
+    handler.post(new Runnable() {
+      @Override
+      public void run() {
+        SsMediaPeriod.this.callback.onPrepared(SsMediaPeriod.this);
+      }
+    });
   }
 
   public void updateManifest(SsManifest manifest) {
@@ -92,6 +100,7 @@ import java.util.ArrayList;
     for (ChunkSampleStream<SsChunkSource> sampleStream : sampleStreams) {
       sampleStream.release();
     }
+    handler.removeCallbacksAndMessages(null);
   }
 
   @Override