Make AdsMediaSource reusable.

Also fixes a bug where deferred media periods were kept in the list for
an unprepared media source although the media period was already released.

Issue:#3498

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=183800889

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

@@ -102,14 +102,11 @@ public final class AdsMediaSource extends CompositeMediaSource<MediaPeriodId> {
   @Nullable private final Handler eventHandler;
   @Nullable private final EventListener eventListener;
   private final Handler mainHandler;
-  private final ComponentListener componentListener;
   private final Map<MediaSource, List<DeferredMediaPeriod>> deferredMediaPeriodByAdMediaSource;
   private final Timeline.Period period;
 
-  private Handler playerHandler;
-  private volatile boolean released;
-
   // Accessed on the player thread.
+  private ComponentListener componentListener;
   private Timeline contentTimeline;
   private Object contentManifest;
   private AdPlaybackState adPlaybackState;
@@ -192,7 +189,6 @@ public final class AdsMediaSource extends CompositeMediaSource<MediaPeriodId> {
     this.eventHandler = eventHandler;
     this.eventListener = eventListener;
     mainHandler = new Handler(Looper.getMainLooper());
-    componentListener = new ComponentListener();
     deferredMediaPeriodByAdMediaSource = new HashMap<>();
     period = new Timeline.Period();
     adGroupMediaSources = new MediaSource[0][];
@@ -204,9 +200,9 @@ public final class AdsMediaSource extends CompositeMediaSource<MediaPeriodId> {
   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);
+    final ComponentListener componentListener = new ComponentListener();
     this.listener = listener;
-    playerHandler = new Handler();
+    this.componentListener = componentListener;
     prepareChildSource(new MediaPeriodId(/* periodIndex= */ 0), contentMediaSource);
     mainHandler.post(new Runnable() {
       @Override
@@ -258,13 +254,27 @@ public final class AdsMediaSource extends CompositeMediaSource<MediaPeriodId> {
 
   @Override
   public void releasePeriod(MediaPeriod mediaPeriod) {
-    ((DeferredMediaPeriod) mediaPeriod).releasePeriod();
+    DeferredMediaPeriod deferredMediaPeriod = (DeferredMediaPeriod) mediaPeriod;
+    List<DeferredMediaPeriod> mediaPeriods =
+        deferredMediaPeriodByAdMediaSource.get(deferredMediaPeriod.mediaSource);
+    if (mediaPeriods != null) {
+      mediaPeriods.remove(deferredMediaPeriod);
+    }
+    deferredMediaPeriod.releasePeriod();
   }
 
   @Override
   public void releaseSource() {
     super.releaseSource();
-    released = true;
+    componentListener.release();
+    componentListener = null;
+    deferredMediaPeriodByAdMediaSource.clear();
+    contentTimeline = null;
+    contentManifest = null;
+    adPlaybackState = null;
+    adGroupMediaSources = new MediaSource[0][];
+    adDurationsUs = new long[0][];
+    listener = null;
     mainHandler.post(new Runnable() {
       @Override
       public void run() {
@@ -301,20 +311,6 @@ public final class AdsMediaSource extends CompositeMediaSource<MediaPeriodId> {
     maybeUpdateSourceInfo();
   }
 
-  private void onLoadError(final IOException error) {
-    Log.w(TAG, "Ad load error", error);
-    if (eventHandler != null && eventListener != null) {
-      eventHandler.post(new Runnable() {
-        @Override
-        public void run() {
-          if (!released) {
-            eventListener.onAdLoadError(error);
-          }
-        }
-      });
-    }
-  }
-
   private void onContentSourceInfoRefreshed(Timeline timeline, Object manifest) {
     contentTimeline = timeline;
     contentManifest = manifest;
@@ -349,6 +345,23 @@ public final class AdsMediaSource extends CompositeMediaSource<MediaPeriodId> {
   /** Listener for component events. All methods are called on the main thread. */
   private final class ComponentListener implements AdsLoader.EventListener {
 
+    private final Handler playerHandler;
+    private volatile boolean released;
+
+    /**
+     * Creates new listener which forwards ad playback states on the creating thread and all other
+     * events on the external event listener thread.
+     */
+    public ComponentListener() {
+      playerHandler = new Handler();
+    }
+
+    /** Releases the component listener. */
+    public void release() {
+      released = true;
+      playerHandler.removeCallbacksAndMessages(null);
+    }
+
     @Override
     public void onAdPlaybackState(final AdPlaybackState adPlaybackState) {
       if (released) {
@@ -367,6 +380,9 @@ public final class AdsMediaSource extends CompositeMediaSource<MediaPeriodId> {
 
     @Override
     public void onAdClicked() {
+      if (released) {
+        return;
+      }
       if (eventHandler != null && eventListener != null) {
         eventHandler.post(new Runnable() {
           @Override
@@ -381,6 +397,9 @@ public final class AdsMediaSource extends CompositeMediaSource<MediaPeriodId> {
 
     @Override
     public void onAdTapped() {
+      if (released) {
+        return;
+      }
       if (eventHandler != null && eventListener != null) {
         eventHandler.post(new Runnable() {
           @Override
@@ -398,15 +417,18 @@ public final class AdsMediaSource extends CompositeMediaSource<MediaPeriodId> {
       if (released) {
         return;
       }
-      playerHandler.post(new Runnable() {
-        @Override
-        public void run() {
-          if (released) {
-            return;
-          }
-          AdsMediaSource.this.onLoadError(error);
-        }
-      });
+      Log.w(TAG, "Ad load error", error);
+      if (eventHandler != null && eventListener != null) {
+        eventHandler.post(
+            new Runnable() {
+              @Override
+              public void run() {
+                if (!released) {
+                  eventListener.onAdLoadError(error);
+                }
+              }
+            });
+      }
     }
 
   }