Add convenience methods player.next() and player.previous()

This simplifies code skipping items in a playlist programatically.

Issue:#4863

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=214580742

RELEASENOTES.md

@@ -8,6 +8,9 @@
 * Fix an issue where audio and video would desynchronize when playing
   concatenations of gapless content
   ([#4559](https://github.com/google/ExoPlayer/issues/4559)).
+* Add convenience methods `Player.next`, `Player.previous`, `Player.hasNext`
+  and `Player.hasPrevious`
+  ([#4863](https://github.com/google/ExoPlayer/issues/4863)).
 
 ### 2.9.0 ###
 

extensions/cast/src/main/java/com/google/android/exoplayer2/ext/cast/CastPlayer.java

@@ -382,6 +382,32 @@ public final class CastPlayer implements Player {
     }
   }
 
+  @Override
+  public boolean hasPrevious() {
+    return getPreviousWindowIndex() != C.INDEX_UNSET;
+  }
+
+  @Override
+  public void previous() {
+    int previousWindowIndex = getPreviousWindowIndex();
+    if (previousWindowIndex != C.INDEX_UNSET) {
+      seekToDefaultPosition(previousWindowIndex);
+    }
+  }
+
+  @Override
+  public boolean hasNext() {
+    return getNextWindowIndex() != C.INDEX_UNSET;
+  }
+
+  @Override
+  public void next() {
+    int nextWindowIndex = getPreviousWindowIndex();
+    if (nextWindowIndex != C.INDEX_UNSET) {
+      seekToDefaultPosition(nextWindowIndex);
+    }
+  }
+
   @Override
   public void setPlaybackParameters(@Nullable PlaybackParameters playbackParameters) {
     // Unsupported by the RemoteMediaClient API. Do nothing.

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

@@ -348,6 +348,32 @@ import java.util.concurrent.CopyOnWriteArraySet;
     }
   }
 
+  @Override
+  public boolean hasPrevious() {
+    return getPreviousWindowIndex() != C.INDEX_UNSET;
+  }
+
+  @Override
+  public void previous() {
+    int previousWindowIndex = getPreviousWindowIndex();
+    if (previousWindowIndex != C.INDEX_UNSET) {
+      seekToDefaultPosition(previousWindowIndex);
+    }
+  }
+
+  @Override
+  public boolean hasNext() {
+    return getNextWindowIndex() != C.INDEX_UNSET;
+  }
+
+  @Override
+  public void next() {
+    int nextWindowIndex = getPreviousWindowIndex();
+    if (nextWindowIndex != C.INDEX_UNSET) {
+      seekToDefaultPosition(nextWindowIndex);
+    }
+  }
+
   @Override
   public void setPlaybackParameters(@Nullable PlaybackParameters playbackParameters) {
     if (playbackParameters == null) {

library/core/src/main/java/com/google/android/exoplayer2/Player.java

@@ -655,6 +655,32 @@ public interface Player {
    */
   void seekTo(int windowIndex, long positionMs);
 
+  /**
+   * Returns whether a previous window exists, which may depend on the current repeat mode and
+   * whether shuffle mode is enabled.
+   */
+  boolean hasPrevious();
+
+  /**
+   * Seeks to the default position of the previous window in the timeline, which may depend on the
+   * current repeat mode and whether shuffle mode is enabled. Does nothing if {@link #hasPrevious()}
+   * is {@code false}.
+   */
+  void previous();
+
+  /**
+   * Returns whether a next window exists, which may depend on the current repeat mode and whether
+   * shuffle mode is enabled.
+   */
+  boolean hasNext();
+
+  /**
+   * Seeks to the default position of the next window in the timeline, which may depend on the
+   * current repeat mode and whether shuffle mode is enabled. Does nothing if {@link #hasNext()} is
+   * {@code false}.
+   */
+  void next();
+
   /**
    * Attempts to set the playback parameters. Passing {@code null} sets the parameters to the
    * default, {@link PlaybackParameters#DEFAULT}, which means there is no speed or pitch adjustment.

library/core/src/main/java/com/google/android/exoplayer2/SimpleExoPlayer.java

@@ -955,6 +955,36 @@ public class SimpleExoPlayer
     player.seekTo(windowIndex, positionMs);
   }
 
+  @Override
+  public boolean hasPrevious() {
+    verifyApplicationThread();
+    return getPreviousWindowIndex() != C.INDEX_UNSET;
+  }
+
+  @Override
+  public void previous() {
+    verifyApplicationThread();
+    if (hasPrevious()) {
+      analyticsCollector.notifySeekStarted();
+      player.previous();
+    }
+  }
+
+  @Override
+  public boolean hasNext() {
+    verifyApplicationThread();
+    return getNextWindowIndex() != C.INDEX_UNSET;
+  }
+
+  @Override
+  public void next() {
+    verifyApplicationThread();
+    if (hasNext()) {
+      analyticsCollector.notifySeekStarted();
+      player.next();
+    }
+  }
+
   @Override
   public void setPlaybackParameters(@Nullable PlaybackParameters playbackParameters) {
     verifyApplicationThread();

testutils_robolectric/src/main/java/com/google/android/exoplayer2/testutil/StubExoPlayer.java

@@ -149,6 +149,26 @@ public abstract class StubExoPlayer implements ExoPlayer {
     throw new UnsupportedOperationException();
   }
 
+  @Override
+  public boolean hasPrevious() {
+    throw new UnsupportedOperationException();
+  }
+
+  @Override
+  public void previous() {
+    throw new UnsupportedOperationException();
+  }
+
+  @Override
+  public boolean hasNext() {
+    throw new UnsupportedOperationException();
+  }
+
+  @Override
+  public void next() {
+    throw new UnsupportedOperationException();
+  }
+
   @Override
   public void setPlaybackParameters(PlaybackParameters playbackParameters) {
     throw new UnsupportedOperationException();