content: /*
   * Copyright (C) 2016 The Android Open Source Project
   *
   * Licensed under the Apache License, Version 2.0 (the "License");
   * you may not use this file except in compliance with the License.
   * You may obtain a copy of the License at
   *
   *      http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
   */
  package com.google.android.exoplayer2.trackselection;
  
  import com.google.android.exoplayer2.ExoPlaybackException;
  import com.google.android.exoplayer2.ExoPlayer;
  import com.google.android.exoplayer2.Renderer;
  import com.google.android.exoplayer2.RendererCapabilities;
  import com.google.android.exoplayer2.RendererConfiguration;
  import com.google.android.exoplayer2.source.TrackGroupArray;
  
  /**
   * The component of an {@link ExoPlayer} responsible for selecting tracks to be consumed by each of
   * the player's {@link Renderer}s. The {@link DefaultTrackSelector} implementation should be
   * suitable for most use cases.
   *
   * <h3>Interactions with the player</h3>
   * The following interactions occur between the player and its track selector during playback.
   * <p>
   * <ul>
   *     <li>When the player is created it will initialize the track selector by calling
   *     {@link #init(InvalidationListener)}.</li>
   *     <li>When the player needs to make a track selection it will call
   *     {@link #selectTracks(RendererCapabilities[], TrackGroupArray)}. This typically occurs at the
   *     start of playback, when the player starts to buffer a new period of the media being played,
   *     and when the track selector invalidates its previous selections.</li>
   *     <li>The player may perform a track selection well in advance of the selected tracks becoming
   *     active, where active is defined to mean that the renderers are actually consuming media
   *     corresponding to the selection that was made. For example when playing media containing
   *     multiple periods, the track selection for a period is made when the player starts to buffer
   *     that period. Hence if the player's buffering policy is to maintain a 30 second buffer, the
   *     selection will occur approximately 30 seconds in advance of it becoming active. In fact the
   *     selection may never become active, for example if the user seeks to some other period of the
   *     media during the 30 second gap. The player indicates to the track selector when a selection
   *     it has previously made becomes active by calling {@link #onSelectionActivated(Object)}.</li>
   *     <li>If the track selector wishes to indicate to the player that selections it has previously
   *     made are invalid, it can do so by calling
   *     {@link InvalidationListener#onTrackSelectionsInvalidated()} on the
   *     {@link InvalidationListener} that was passed to {@link #init(InvalidationListener)}. A
   *     track selector may wish to do this if its configuration has changed, for example if it now
   *     wishes to prefer audio tracks in a particular language. This will trigger the player to make
   *     new track selections. Note that the player will have to re-buffer in the case that the new
   *     track selection for the currently playing period differs from the one that was invalidated.
   *     </li>
   * </ul>
   *
   * <h3>Renderer configuration</h3>
   * The {@link TrackSelectorResult} returned by
   * {@link #selectTracks(RendererCapabilities[], TrackGroupArray)} contains not only
   * {@link TrackSelection}s for each renderer, but also {@link RendererConfiguration}s defining
   * configuration parameters that the renderers should apply when consuming the corresponding media.
   * Whilst it may seem counter-intuitive for a track selector to also specify renderer configuration
   * information, in practice the two are tightly bound together. It may only be possible to play a
   * certain combination tracks if the renderers are configured in a particular way. Equally, it may
   * only be possible to configure renderers in a particular way if certain tracks are selected. Hence
   * it makes sense to determined the track selection and corresponding renderer configurations in a
   * single step.
   *
   * <h3>Threading model</h3>
   * All calls made by the player into the track selector are on the player's internal playback
   * thread. The track selector may call {@link InvalidationListener#onTrackSelectionsInvalidated()}
   * from any thread.
   */
  public abstract class TrackSelector {
  
    /**
     * Notified when selections previously made by a {@link TrackSelector} are no longer valid.
     */
    public interface InvalidationListener {
  
      /**
       * Called by a {@link TrackSelector} to indicate that selections it has previously made are no
       * longer valid. May be called from any thread.
       */
      void onTrackSelectionsInvalidated();
  
    }
  
    private InvalidationListener listener;
  
    /**
     * Called by the player to initialize the selector.
     *
     * @param listener An invalidation listener that the selector can call to indicate that selections
     *     it has previously made are no longer valid.
     */
    public final void init(InvalidationListener listener) {
      this.listener = listener;
    }
  
    /**
     * Called by the player to perform a track selection.
     *
     * @param rendererCapabilities The {@link RendererCapabilities} of the renderers for which tracks
     *     are to be selected.
     * @param trackGroups The available track groups.
     * @return A {@link TrackSelectorResult} describing the track selections.
     * @throws ExoPlaybackException If an error occurs selecting tracks.
     */
    public abstract TrackSelectorResult selectTracks(RendererCapabilities[] rendererCapabilities,
        TrackGroupArray trackGroups) throws ExoPlaybackException;
  
    /**
     * Called by the player when a {@link TrackSelectorResult} previously generated by
     * {@link #selectTracks(RendererCapabilities[], TrackGroupArray)} is activated.
     *
     * @param info The value of {@link TrackSelectorResult#info} in the activated selection.
     */
    public abstract void onSelectionActivated(Object info);
  
    /**
     * Calls {@link InvalidationListener#onTrackSelectionsInvalidated()} to invalidate all previously
     * generated track selections.
     */
    protected final void invalidate() {
      if (listener != null) {
        listener.onTrackSelectionsInvalidated();
      }
    }
  
  }
  