Add hook for modifying or cancelling a user invoked seek

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=142007001

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

@@ -330,17 +330,19 @@ public interface ExoPlayer {
   /**
    * Seeks to a position specified in milliseconds in the current window.
    *
-   * @param windowPositionMs The seek position in the current window.
+   * @param positionMs The seek position in the current window, or {@link C#TIME_UNSET} to seek to
+   *     the window's default position.
    */
-  void seekTo(long windowPositionMs);
+  void seekTo(long positionMs);
 
   /**
    * Seeks to a position specified in milliseconds in the specified window.
    *
    * @param windowIndex The index of the window.
-   * @param windowPositionMs The seek position in the specified window.
+   * @param positionMs The seek position in the specified window, or {@link C#TIME_UNSET} to seek to
+   *     the window's default position.
    */
-  void seekTo(int windowIndex, long windowPositionMs);
+  void seekTo(int windowIndex, long positionMs);
 
   /**
    * Stops playback. Use {@code setPlayWhenReady(false)} rather than this method if the intention

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

@@ -148,14 +148,45 @@ public class PlaybackControlView extends FrameLayout {
    * Listener to be notified about changes of the visibility of the UI control.
    */
   public interface VisibilityListener {
+
     /**
      * Called when the visibility changes.
      *
      * @param visibility The new visibility. Either {@link View#VISIBLE} or {@link View#GONE}.
      */
     void onVisibilityChange(int visibility);
+
   }
 
+  /**
+   * Dispatches seek operations to the player.
+   */
+  public interface SeekDispatcher {
+
+    /**
+     * @param player The player to seek.
+     * @param windowIndex The index of the window.
+     * @param positionMs The seek position in the specified window, or {@link C#TIME_UNSET} to seek
+     *     to the window's default position.
+     * @return True if the seek was dispatched. False otherwise.
+     */
+    boolean dispatchSeek(ExoPlayer player, int windowIndex, long positionMs);
+
+  }
+
+  /**
+   * Default {@link SeekDispatcher} that dispatches seeks to the player without modification.
+   */
+  public static final SeekDispatcher DEFAULT_SEEK_DISPATCHER = new SeekDispatcher() {
+
+    @Override
+    public boolean dispatchSeek(ExoPlayer player, int windowIndex, long positionMs) {
+      player.seekTo(windowIndex, positionMs);
+      return true;
+    }
+
+  };
+
   public static final int DEFAULT_FAST_FORWARD_MS = 15000;
   public static final int DEFAULT_REWIND_MS = 5000;
   public static final int DEFAULT_SHOW_TIMEOUT_MS = 5000;
@@ -178,6 +209,7 @@ public class PlaybackControlView extends FrameLayout {
   private final Timeline.Window currentWindow;
 
   private ExoPlayer player;
+  private SeekDispatcher seekDispatcher;
   private VisibilityListener visibilityListener;
 
   private boolean isAttachedToWindow;
@@ -234,6 +266,7 @@ public class PlaybackControlView extends FrameLayout {
     formatBuilder = new StringBuilder();
     formatter = new Formatter(formatBuilder, Locale.getDefault());
     componentListener = new ComponentListener();
+    seekDispatcher = DEFAULT_SEEK_DISPATCHER;
 
     LayoutInflater.from(context).inflate(controllerLayoutId, this);
     setDescendantFocusability(FOCUS_AFTER_DESCENDANTS);
@@ -306,6 +339,16 @@ public class PlaybackControlView extends FrameLayout {
     this.visibilityListener = listener;
   }
 
+  /**
+   * Sets the {@link SeekDispatcher}.
+   *
+   * @param seekDispatcher The {@link SeekDispatcher}, or null to use
+   *     {@link #DEFAULT_SEEK_DISPATCHER}.
+   */
+  public void setSeekDispatcher(SeekDispatcher seekDispatcher) {
+    this.seekDispatcher = seekDispatcher == null ? DEFAULT_SEEK_DISPATCHER : seekDispatcher;
+  }
+
   /**
    * Sets the rewind increment in milliseconds.
    *
@@ -550,9 +593,9 @@ public class PlaybackControlView extends FrameLayout {
     currentTimeline.getWindow(currentWindowIndex, currentWindow);
     if (currentWindowIndex > 0 && (player.getCurrentPosition() <= MAX_POSITION_FOR_SEEK_TO_PREVIOUS
         || (currentWindow.isDynamic && !currentWindow.isSeekable))) {
-      player.seekToDefaultPosition(currentWindowIndex - 1);
+      seekTo(currentWindowIndex - 1, C.TIME_UNSET);
     } else {
-      player.seekTo(0);
+      seekTo(0);
     }
   }
 
@@ -563,9 +606,9 @@ public class PlaybackControlView extends FrameLayout {
     }
     int currentWindowIndex = player.getCurrentWindowIndex();
     if (currentWindowIndex < currentTimeline.getWindowCount() - 1) {
-      player.seekToDefaultPosition(currentWindowIndex + 1);
+      seekTo(currentWindowIndex + 1, C.TIME_UNSET);
     } else if (currentTimeline.getWindow(currentWindowIndex, currentWindow, false).isDynamic) {
-      player.seekToDefaultPosition();
+      seekTo(currentWindowIndex, C.TIME_UNSET);
     }
   }
 
@@ -573,14 +616,27 @@ public class PlaybackControlView extends FrameLayout {
     if (rewindMs <= 0) {
       return;
     }
-    player.seekTo(Math.max(player.getCurrentPosition() - rewindMs, 0));
+    seekTo(Math.max(player.getCurrentPosition() - rewindMs, 0));
   }
 
   private void fastForward() {
     if (fastForwardMs <= 0) {
       return;
     }
-    player.seekTo(Math.min(player.getCurrentPosition() + fastForwardMs, player.getDuration()));
+    seekTo(Math.min(player.getCurrentPosition() + fastForwardMs, player.getDuration()));
+  }
+
+  private void seekTo(long positionMs) {
+    seekTo(player.getCurrentWindowIndex(), positionMs);
+  }
+
+  private void seekTo(int windowIndex, long positionMs) {
+    boolean dispatched = seekDispatcher.dispatchSeek(player, windowIndex, positionMs);
+    if (!dispatched) {
+      // The seek wasn't dispatched. If the progress bar was dragged by the user to perform the
+      // seek then it'll now be in the wrong position. Trigger a progress update to snap it back.
+      updateProgress();
+    }
   }
 
   @Override
@@ -688,7 +744,7 @@ public class PlaybackControlView extends FrameLayout {
     public void onStopTrackingTouch(SeekBar seekBar) {
       dragging = false;
       if (player != null) {
-        player.seekTo(positionValue(seekBar.getProgress()));
+        seekTo(positionValue(seekBar.getProgress()));
       }
       hideAfterTimeout();
     }

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

@@ -44,6 +44,7 @@ import com.google.android.exoplayer2.text.TextRenderer;
 import com.google.android.exoplayer2.trackselection.TrackSelection;
 import com.google.android.exoplayer2.trackselection.TrackSelectionArray;
 import com.google.android.exoplayer2.ui.AspectRatioFrameLayout.ResizeMode;
+import com.google.android.exoplayer2.ui.PlaybackControlView.SeekDispatcher;
 import com.google.android.exoplayer2.util.Assertions;
 import java.util.List;
 
@@ -440,6 +441,17 @@ public final class SimpleExoPlayerView extends FrameLayout {
     controller.setVisibilityListener(listener);
   }
 
+  /**
+   * Sets the {@link SeekDispatcher}.
+   *
+   * @param seekDispatcher The {@link SeekDispatcher}, or null to use
+   *     {@link PlaybackControlView#DEFAULT_SEEK_DISPATCHER}.
+   */
+  public void setSeekDispatcher(SeekDispatcher seekDispatcher) {
+    Assertions.checkState(controller != null);
+    controller.setSeekDispatcher(seekDispatcher);
+  }
+
   /**
    * Sets the rewind increment in milliseconds.
    *