Extend main Player Javadoc

The main interface documentation hasn't been updated substantially
since 2017 and is missing notes for many of its current features and
requirements.

Also change the recommendation for implementors from BasePlayer to
SimpleBasePlayer to ensure new classes are more likely to cover all
of the interface requirements.

#minor-release

PiperOrigin-RevId: 534027117
(cherry picked from commit d75e24a)

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

@@ -51,26 +51,125 @@
 import java.util.List;
 
 /**
- * A media player interface defining traditional high-level functionality, such as the ability to
- * play, pause, seek and query properties of the currently playing media.
+ * A media player interface defining high-level functionality, such as the ability to play, pause,
+ * seek and query properties of the currently playing media.
  *
- * <p>All methods must be called from a single {@linkplain #getApplicationLooper() application
- * thread} unless indicated otherwise. Callbacks in registered listeners are called on the same
- * thread.
- *
- * <p>This interface includes some convenience methods that can be implemented by calling other
- * methods in the interface. {@link BasePlayer} implements these convenience methods so inheriting
- * {@link BasePlayer} is recommended when implementing the interface so that only the minimal set of
- * required methods can be implemented.
+ * <h2>Player features and usage</h2>
  *
  * <p>Some important properties of media players that implement this interface are:
  *
  * <ul>
- *   <li>They can provide a {@link Timeline} representing the structure of the media being played,
- *       which can be obtained by calling {@link #getCurrentTimeline()}.
- *   <li>They can provide a {@link Tracks} defining the currently available tracks and which are
- *       selected to be rendered, which can be obtained by calling {@link #getCurrentTracks()}.
+ *   <li>All methods must be called from a single {@linkplain #getApplicationLooper() application
+ *       thread} unless indicated otherwise. Callbacks in registered listeners are called on the
+ *       same thread.
+ *   <li>The available functionality can be limited. Player instances provide a set of {@link
+ *       #getAvailableCommands() availabe commands} to signal feature support and users of the
+ *       interface must only call methods if the corresponding {@link Command} is available.
+ *   <li>Users can register {@link Player.Listener} callbacks that get informed about state changes.
+ *   <li>Player instances need to update the visible state immediately after each method call, even
+ *       if the actual changes are handled on background threads or even other devices. This
+ *       simplifies the usage for callers of methods as no asynchronous handling needs to be
+ *       considered.
+ *   <li>Player instances can provide playlist operations, like 'set', 'add', 'remove', 'move' or
+ *       'replace' of {@link MediaItem} instances. The player can also support {@linkplain
+ *       RepeatMode repeat modes} and shuffling within this playlist. The player provides a {@link
+ *       Timeline} representing the structure of the playlist and all its items, which can be
+ *       obtained by calling {@link #getCurrentTimeline()}
+ *   <li>Player instances can provide seeking within the currently playing item and to other items,
+ *       using the various {@code seek...} methods.
+ *   <li>Player instances can provide {@link Tracks} defining the currently available and selected
+ *       tracks, which can be obtained by calling {@link #getCurrentTracks()}. Users can also modify
+ *       track selection behavior by setting {@link TrackSelectionParameters} with {@link
+ *       #setTrackSelectionParameters}.
+ *   <li>Player instances can provide {@link MediaMetadata} about the currently playing item, which
+ *       can be obtained by calling {@link #getMediaMetadata()}.
+ *   <li>Player instances can provide information about ads in its media structure, for example via
+ *       {@link #isPlayingAd()}.
+ *   <li>Player instances can accept different types of video outputs, like {@link
+ *       #setVideoSurfaceView SurfaceView} or {@link #setVideoTextureView TextureView} for video
+ *       rendering.
+ *   <li>Player instances can handle {@linkplain #setPlaybackSpeed playback speed}, {@linkplain
+ *       #getAudioAttributes audio attributes}, and {@linkplain #setVolume audio volume}.
+ *   <li>Player instances can provide information about the {@linkplain #getDeviceInfo playback
+ *       device}, which may be remote, and allow to change the device's volume.
+ * </ul>
+ *
+ * <h2>API stability guarantees</h2>
+ *
+ * <p>The majority of the Player interface and its related classes are part of the stable API that
+ * guarantees backwards-compatibility for users of the API. Only more advances use cases may need to
+ * rely on {@link UnstableApi} classes and methods that are subject to incompatible changes or even
+ * removal in a future release. Implementors of the Player interface are not covered by these API
+ * stability guarantees.
+ *
+ * <h2>Player state</h2>
+ *
+ * <p>Users can listen to state changes by adding a {@link Player.Listener} with {@link
+ * #addListener}.
+ *
+ * <p>The main elements of the overall player state are:
+ *
+ * <ul>
+ *   <li>Playlist
+ *       <ul>
+ *         <li>{@link MediaItem} instances can be added with methods like {@link #setMediaItem} to
+ *             define what the player will be playing.
+ *         <li>The current playlist can be obtained via {@link #getCurrentTimeline} and convenience
+ *             methods like {@link #getMediaItemCount} or {@link #getCurrentMediaItem}.
+ *         <li>With an empty playlist, the player can only be in {@link #STATE_IDLE} or {@link
+ *             #STATE_ENDED}.
+ *       </ul>
+ *   <li>Playback state
+ *       <ul>
+ *         <li>{@link #STATE_IDLE}: This is the initial state, the state when the player is
+ *             {@linkplain #stop stopped}, and when playback {@linkplain #getPlayerError failed}.
+ *             The player will hold only limited resources in this state. {@link #prepare} must be
+ *             called to transition away from this state.
+ *         <li>{@link #STATE_BUFFERING}: The player is not able to immediately play from its current
+ *             position. This mostly happens because more data needs to be loaded.
+ *         <li>{@link #STATE_READY}: The player is able to immediately play from its current
+ *             position.
+ *         <li>{@link #STATE_ENDED}: The player finished playing all media, or there is no media to
+ *             play.
+ *       </ul>
+ *   <li>Play/Pause, playback suppression and isPlaying
+ *       <ul>
+ *         <li>{@linkplain #getPlayWhenReady() playWhenReady}: Indicates the user intention to play.
+ *             It can be set with {@link #play} or {@link #pause}.
+ *         <li>{@linkplain #getPlaybackSuppressionReason() playback suppression}: Defines a reason
+ *             for which playback will be suppressed even if {@linkplain #getPlayWhenReady()
+ *             playWhenReady} is {@code true}.
+ *         <li>{@link #isPlaying()}: Whether the player is playing (that is, its position is
+ *             advancing and media is being presented). This will only be {@code true} if playback
+ *             state is {@link #STATE_READY}, {@linkplain #getPlayWhenReady() playWhenReady} is
+ *             {@code true}, and playback is not suppressed.
+ *       </ul>
+ *   <li>Playback position
+ *       <ul>
+ *         <li>{@linkplain #getCurrentMediaItemIndex() media item index}: The index in the playlist.
+ *         <li>{@linkplain #isPlayingAd() ad insertion}: Whether an inserted ad is playing and which
+ *             {@linkplain #getCurrentAdGroupIndex() ad group index} and {@linkplain
+ *             #getCurrentAdIndexInAdGroup() ad index in the group} it belongs to
+ *         <li>{@linkplain #getCurrentPosition() current position}: The current position of the
+ *             playback. This is the same as the {@linkplain #getContentPosition() content position}
+ *             unless an ad is playing, where this indicates the position in the inserted ad.
+ *       </ul>
  * </ul>
+ *
+ * <p>Note that there are no callbacks for normal playback progression, only for {@linkplain
+ * Listener#onMediaItemTransition transitions between media items} and other {@linkplain
+ * Listener#onPositionDiscontinuity position discontinuities}. Code that needs to monitor playback
+ * progress (for example, an UI progress bar) should query the current position in appropriate
+ * intervals.
+ *
+ * <h2>Implementing the Player interface</h2>
+ *
+ * <p>Implementing the Player interface is complex, as the interface includes many convenience
+ * methods that need to provide a consistent state and behavior, requires correct handling of
+ * listeners and available commands, and expects immediate state changes even if methods are
+ * internally handled asynchronously. For this reason, implementations are advised to inherit {@link
+ * SimpleBasePlayer} that handles all of these complexities and provides a simpler integration point
+ * for implementors of the interface.
  */
 public interface Player {