Make Timeline always non-null from ExoPlayer

Note that we still have null timelines in ExoPlayerImplInternal.
This is deliberate; and is likely necessary to distinguish between
the no-timeline-yet and timeline-is-empty cases (we want to try
and process a pending seek for the latter, but not the former).

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=140200980

demo/src/main/java/com/google/android/exoplayer2/demo/PlayerActivity.java

@@ -360,7 +360,7 @@ public class PlayerActivity extends Activity implements OnClickListener, ExoPlay
       playerWindow = player.getCurrentWindowIndex();
       playerPosition = C.TIME_UNSET;
       Timeline timeline = player.getCurrentTimeline();
-      if (timeline != null && timeline.getWindow(playerWindow, window).isSeekable) {
+      if (!timeline.isEmpty() && timeline.getWindow(playerWindow, window).isSeekable) {
         playerPosition = player.getCurrentPosition();
       }
       player.release();
@@ -417,7 +417,7 @@ public class PlayerActivity extends Activity implements OnClickListener, ExoPlay
 
   @Override
   public void onTimelineChanged(Timeline timeline, Object manifest) {
-    isTimelineStatic = timeline != null && timeline.getWindowCount() > 0
+    isTimelineStatic = !timeline.isEmpty()
         && !timeline.getWindow(timeline.getWindowCount() - 1, window).isDynamic;
   }
 

library/src/main/java/com/google/android/exoplayer2/ExoPlayer.java

@@ -403,12 +403,12 @@ public interface ExoPlayer {
 
   /**
    * Returns the current manifest. The type depends on the {@link MediaSource} passed to
-   * {@link #prepare}.
+   * {@link #prepare}. May be null.
    */
   Object getCurrentManifest();
 
   /**
-   * Returns the current {@link Timeline}, or {@code null} if there is no timeline.
+   * Returns the current {@link Timeline}. Never null, but may be empty.
    */
   Timeline getCurrentTimeline();
 

library/src/main/java/com/google/android/exoplayer2/ExoPlayerImpl.java

@@ -82,6 +82,7 @@ import java.util.concurrent.CopyOnWriteArraySet;
     this.playbackState = STATE_IDLE;
     this.listeners = new CopyOnWriteArraySet<>();
     emptyTrackSelections = new TrackSelectionArray(new TrackSelection[renderers.length]);
+    timeline = Timeline.EMPTY;
     window = new Timeline.Window();
     period = new Timeline.Period();
     trackGroups = TrackGroupArray.EMPTY;
@@ -120,8 +121,8 @@ import java.util.concurrent.CopyOnWriteArraySet;
   @Override
   public void prepare(MediaSource mediaSource, boolean resetPosition, boolean resetState) {
     if (resetState) {
-      if (timeline != null || manifest != null) {
+      if (!timeline.isEmpty() || manifest != null) {
-        timeline = null;
+        timeline = Timeline.EMPTY;
         manifest = null;
         for (EventListener listener : listeners) {
           listener.onTimelineChanged(null, null);
@@ -178,7 +179,7 @@ import java.util.concurrent.CopyOnWriteArraySet;
 
   @Override
   public void seekTo(int windowIndex, long positionMs) {
-    if (windowIndex < 0 || (timeline != null && windowIndex >= timeline.getWindowCount())) {
+    if (windowIndex < 0 || (!timeline.isEmpty() && windowIndex >= timeline.getWindowCount())) {
       throw new IndexOutOfBoundsException();
     }
     pendingSeekAcks++;
@@ -223,7 +224,7 @@ import java.util.concurrent.CopyOnWriteArraySet;
 
   @Override
   public int getCurrentWindowIndex() {
-    if (timeline == null || pendingSeekAcks > 0) {
+    if (timeline.isEmpty() || pendingSeekAcks > 0) {
       return maskingWindowIndex;
     } else {
       return timeline.getPeriod(playbackInfo.periodIndex, period).windowIndex;
@@ -232,7 +233,7 @@ import java.util.concurrent.CopyOnWriteArraySet;
 
   @Override
   public long getDuration() {
-    if (timeline == null) {
+    if (timeline.isEmpty()) {
       return C.TIME_UNSET;
     }
     return timeline.getWindow(getCurrentWindowIndex(), window).getDurationMs();
@@ -240,7 +241,7 @@ import java.util.concurrent.CopyOnWriteArraySet;
 
   @Override
   public long getCurrentPosition() {
-    if (timeline == null || pendingSeekAcks > 0) {
+    if (timeline.isEmpty() || pendingSeekAcks > 0) {
       return maskingWindowPositionMs;
     } else {
       timeline.getPeriod(playbackInfo.periodIndex, period);
@@ -251,7 +252,7 @@ import java.util.concurrent.CopyOnWriteArraySet;
   @Override
   public long getBufferedPosition() {
     // TODO - Implement this properly.
-    if (timeline == null || pendingSeekAcks > 0) {
+    if (timeline.isEmpty() || pendingSeekAcks > 0) {
       return maskingWindowPositionMs;
     } else {
       timeline.getPeriod(playbackInfo.periodIndex, period);
@@ -261,7 +262,7 @@ import java.util.concurrent.CopyOnWriteArraySet;
 
   @Override
   public int getBufferedPercentage() {
-    if (timeline == null) {
+    if (timeline.isEmpty()) {
       return 0;
     }
     long bufferedPosition = getBufferedPosition();

library/src/main/java/com/google/android/exoplayer2/Timeline.java

@@ -91,6 +91,46 @@ package com.google.android.exoplayer2;
  */
 public abstract class Timeline {
 
+  /**
+   * An empty timeline.
+   */
+  public static final Timeline EMPTY = new Timeline() {
+
+    @Override
+    public int getWindowCount() {
+      return 0;
+    }
+
+    @Override
+    public Window getWindow(int windowIndex, Window window, boolean setIds,
+        long defaultPositionProjectionUs) {
+      throw new IndexOutOfBoundsException();
+    }
+
+    @Override
+    public int getPeriodCount() {
+      return 0;
+    }
+
+    @Override
+    public Period getPeriod(int periodIndex, Period period, boolean setIds) {
+      throw new IndexOutOfBoundsException();
+    }
+
+    @Override
+    public int getIndexOfPeriod(Object uid) {
+      return C.INDEX_UNSET;
+    }
+
+  };
+
+  /**
+   * Returns whether the timeline is empty.
+   */
+  public final boolean isEmpty() {
+    return getWindowCount() == 0;
+  }
+
   /**
    * Returns the number of windows in the timeline.
    */

library/src/main/java/com/google/android/exoplayer2/ui/PlaybackControlView.java

@@ -422,11 +422,11 @@ public class PlaybackControlView extends FrameLayout {
       return;
     }
     Timeline currentTimeline = player != null ? player.getCurrentTimeline() : null;
-    boolean haveTimeline = currentTimeline != null;
+    boolean haveNonEmptyTimeline = currentTimeline != null && !currentTimeline.isEmpty();
     boolean isSeekable = false;
     boolean enablePrevious = false;
     boolean enableNext = false;
-    if (haveTimeline) {
+    if (haveNonEmptyTimeline) {
       int currentWindowIndex = player.getCurrentWindowIndex();
       currentTimeline.getWindow(currentWindowIndex, currentWindow);
       isSeekable = currentWindow.isSeekable;
@@ -525,7 +525,7 @@ public class PlaybackControlView extends FrameLayout {
 
   private void previous() {
     Timeline currentTimeline = player.getCurrentTimeline();
-    if (currentTimeline == null) {
+    if (currentTimeline.isEmpty()) {
       return;
     }
     int currentWindowIndex = player.getCurrentWindowIndex();
@@ -540,7 +540,7 @@ public class PlaybackControlView extends FrameLayout {
 
   private void next() {
     Timeline currentTimeline = player.getCurrentTimeline();
-    if (currentTimeline == null) {
+    if (currentTimeline.isEmpty()) {
       return;
     }
     int currentWindowIndex = player.getCurrentWindowIndex();