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###Video Controls implementation with MediaSession 示例实现是在Google最新的Android TV应用程序中完成的。 AOSP示例应用程序实现尚未实现MediaSession（在API 21,22中）。

在上一章中，我解释说，以下视频控件需要被压制。 1.Action的UI更新部分（在上一章完成） 2.视频控制部分（在上一章完成）MediaSession实现，通过MediaController的TransportControls进行视频控制（本章）

3.- 当用户按下电视遥控器的视频控制按钮时，MediaSession可以处理该动作。 它允许其他活动继承视频控制。特别是LeanbackLauncher，家庭显示器，可以在后台播放视频。

4.将MediaMetadata设置为MediaSession（本章） （ “ Now playing card ”将出现在推荐行的顶部。）

在本章中，我们继续使用MediaSession实现视频控件。我们可以通过使用MediaSession将VideoView控件传递给LeanbackLauncher，从而在LeanbackLauncher中实现播放视频背景。

对于3，我们在PlaybackOverlayActivity中创建MediaSession，并从PlaybackOverlayFragment中的MediaController进行控制。

对于4，MediaSession的元数据使用MediaMetadata＆PlaybackState类进行更新，以更新“Now playing card”。

我建议您阅读 Displaying a Now Playing Card，以获得官方解释。

本章的实现与上一章的实现几乎是独立的。在实施MediaSession之前，我将执行requestVisibleBehind方法，以便我们可以在LeanbackLauncher应用程序的背景下播放视频。 ###Implement requestVisibleBehind 这个方法是添加在API 21（Lolipop）中，AOSP的解释解释了

If this call is successful then the activity will remain visible after onPause() is called, and is allowed to continue playing media in the background.

示例实现如下。

@Overridepublic void onPause() {super.onPause();if (!requestVisibleBehind(true)) {// Try to play behind launcher, but if it fails, stop playback.mPlaybackController.playPause(false);}}
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实施后，当您在应用程序中播放视频内容并按“Home”键返回LeanbackLauncher时，该视频将在后台继续播放。。

###本章课堂结构

我们在本章中处理3个课程。

• PlaybackOverlayActivity
  • 管理生命周期，将意图信息传递给PlaybackController
  • MediaSession的生活时间与活动有关
• PlaybackOverlayFragment
  • 处理PlaybackControlsRow的UI
  • MediaController回调功能用于根据当前播放状态更新UI
• PlaybackController
  • 管理视频播放
  • 视频控制功能
  • MediaSessionCallback用于从电视遥控器接收视频控制键

###Create & release MediaSession 到目前为止，我们无法使用遥控器中的视频控制键控制此视频。 让我们实现MediaSession来定义遥控器中每个视频控制键的动作。 首先，我们在PlaybackController的Constractor中创建MediaSession，由PlaybackOverlayActivity调用。

    public PlaybackController(Activity activity) {mActivity = activity;// mVideoView = (VideoView) activity.findViewById(VIDEO_VIEW_RESOURCE_ID);createMediaSession(mActivity);}private void createMediaSession(Activity activity) {if (mSession == null) {mSession = new MediaSession(activity, MEDIA_SESSION_TAG);mMediaSessionCallback = new MediaSessionCallback();mSession.setCallback(mMediaSessionCallback);mSession.setFlags(MediaSession.FLAG_HANDLES_MEDIA_BUTTONS |MediaSession.FLAG_HANDLES_TRANSPORT_CONTROLS);mSession.setActive(true);activity.setMediaController(new MediaController(activity, mSession.getSessionToken()));}}
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我们可以将回调MediaSessionCallback设置为MediaSession。 它定义了将在后面解释的每个视频控制按钮的精确行为。

需要使用参数FLAG_HANDLES_MEDIA_BUTTONS＆FLAG_HANDLES_TRANSPORT_CONTROLS的setFlags方法才能使遥控器键控制视频。

创建后，我们必须在完成后释放MediaSession。

    public void releaseMediaSession() {if(mSession != null) {mSession.release();}}
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###视频控制功能 视频控制功能在MediaSessionCallback类中实现。 顾名思义，每个视频控制动作都在相应的回调函数中实现。 这种回调是从2种方式调用的，即“遥控器媒体键”或“PlaybackControlsRow”中的UI视频控制按钮。

public void playPause(boolean doPlay) {if (mCurrentPlaybackState == PlaybackState.STATE_NONE) {/* Callbacks for mVideoView */setupCallbacks();}//if (doPlay && mCurrentPlaybackState != PlaybackState.STATE_PLAYING) {if (doPlay) { // PlayLog.d(TAG, "playPause: play");if(mCurrentPlaybackState == PlaybackState.STATE_PLAYING) {/* if current state is already playing, do nothing */return;} else {mCurrentPlaybackState = PlaybackState.STATE_PLAYING;mVideoView.start();mStartTimeMillis = System.currentTimeMillis();}} else { // PauseLog.d(TAG, "playPause: pause");if(mCurrentPlaybackState == PlaybackState.STATE_PAUSED) {/* if current state is already paused, do nothing */return;} else {mCurrentPlaybackState = PlaybackState.STATE_PAUSED;}setPosition(mVideoView.getCurrentPosition());mVideoView.pause();}updatePlaybackState();}public void fastForward() {if (mDuration != -1) {// Fast forward 10 seconds.setPosition(getCurrentPosition() + (10 * 1000));mVideoView.seekTo(mPosition);}}public void rewind() {// rewind 10 secondssetPosition(getCurrentPosition() - (10 * 1000));mVideoView.seekTo(mPosition);}private class MediaSessionCallback extends MediaSession.Callback {@Overridepublic void onPlay() {playPause(true);}@Overridepublic void onPause() {playPause(false);}@Overridepublic void onSkipToNext() {if (++mCurrentItem >= mItems.size()) { // Current Item is set to next heremCurrentItem = 0;}Movie movie = mItems.get(mCurrentItem);//Movie movie = VideoProvider.getMovieById(mediaId);if (movie != null) {setVideoPath(movie.getVideoUrl());//mCurrentPlaybackState = PlaybackState.STATE_PAUSED;//updateMetadata(movie);updateMetadata();playPause(mCurrentPlaybackState == PlaybackState.STATE_PLAYING);} else {Log.e(TAG, "onSkipToNext movie is null!");}}@Overridepublic void onSkipToPrevious() {if (--mCurrentItem < 0) { // Current Item is set to previous heremCurrentItem = mItems.size()-1;}Movie movie = mItems.get(mCurrentItem);//Movie movie = VideoProvider.getMovieById(mediaId);if (movie != null) {setVideoPath(movie.getVideoUrl());//mCurrentPlaybackState = PlaybackState.STATE_PAUSED;updateMetadata();playPause(mCurrentPlaybackState == PlaybackState.STATE_PLAYING);} else {Log.e(TAG, "onSkipToPrevious movie is null!");}}@Overridepublic void onPlayFromMediaId(String mediaId, Bundle extras) {mCurrentItem = Integer.parseInt(mediaId);Movie movie = mItems.get(mCurrentItem);//Movie movie = VideoProvider.getMovieById(mediaId);if (movie != null) {setVideoPath(movie.getVideoUrl());// mCurrentPlaybackState = PlaybackState.STATE_PAUSED;// updateMetadata(movie);updateMetadata();playPause(mCurrentPlaybackState == PlaybackState.STATE_PLAYING);}}@Overridepublic void onSeekTo(long pos) {setPosition((int) pos);mVideoView.seekTo(mPosition);updatePlaybackState();}@Overridepublic void onFastForward() {fastForward();}@Overridepublic void onRewind() {rewind();}}
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###视频控制按遥控器键

    private void updatePlaybackState() {PlaybackState.Builder stateBuilder = new PlaybackState.Builder().setActions(getAvailableActions());int state = PlaybackState.STATE_PLAYING;if (mCurrentPlaybackState == PlaybackState.STATE_PAUSED || mCurrentPlaybackState == PlaybackState.STATE_NONE) {state = PlaybackState.STATE_PAUSED;}stateBuilder.setState(state, getCurrentPosition(), 1.0f);mSession.setPlaybackState(stateBuilder.build());}private long getAvailableActions() {long actions = PlaybackState.ACTION_PLAY |PlaybackState.ACTION_PAUSE |PlaybackState.ACTION_PLAY_PAUSE |PlaybackState.ACTION_REWIND |PlaybackState.ACTION_FAST_FORWARD |PlaybackState.ACTION_SKIP_TO_PREVIOUS |PlaybackState.ACTION_SKIP_TO_NEXT |PlaybackState.ACTION_PLAY_FROM_MEDIA_ID |PlaybackState.ACTION_PLAY_FROM_SEARCH;return actions;}
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在这个例子中，可以通过getAvailableActions方法来确定可用的操作，通过使用逻辑分离来添加操作。 ###从UI控制视频- MediaController.getTransportControls

要通过VideoDetailsFragment中的PlaybackControlsRow控制MediaSession，我们使用MediaController。 MediaController是在PlaybackController的构造函数中创建的，它拥有MediaSession的标记。

当用户点击视频控制按钮时，它将使用MediaController.getTransportControls（）方法调用MediaSessionCallback方法。

/* onClick */playbackControlsRowPresenter.setOnActionClickedListener(new OnActionClickedListener() {public void onActionClicked(Action action) {if (action.getId() == mPlayPauseAction.getId()) {/* PlayPause action */if (mPlayPauseAction.getIndex() == PlaybackControlsRow.PlayPauseAction.PLAY) {mMediaController.getTransportControls().play();} else if (mPlayPauseAction.getIndex() == PlaybackControlsRow.PlayPauseAction.PAUSE) {mMediaController.getTransportControls().pause();}} else if (action.getId() == mSkipNextAction.getId()) {/* SkipNext action */mMediaController.getTransportControls().skipToNext();} else if (action.getId() == mSkipPreviousAction.getId()) {/* SkipPrevious action */mMediaController.getTransportControls().skipToPrevious();} else if (action.getId() == mFastForwardAction.getId()) {/* FastForward action  */mMediaController.getTransportControls().fastForward();} else if (action.getId() == mRewindAction.getId()) {/* Rewind action */mMediaController.getTransportControls().rewind();}
}
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视频控制部分完成。 但是，我们需要根据视频控制 更新PlayControlsRow的UI。 ###更新VideoDetailsFragment的UI 当执行视频控制动作时，需要更改UI，视频播放状态已更改。 我们可以使用MediaController的回调函数获取此事件。 以下介绍2种回调方法。

• onPlaybackStateChanged
• onMetadataChanged

要使用这些回调方法，您可以使扩展MediaController.Callback类的子类，并覆盖这些方法。 要使用这个类，我们可以调用MediaController的registerCallback / unregisterCallback方法来获取MediaController的事件。

    @Overridepublic void onAttach(Activity activity) {super.onAttach(activity);mMediaController = getActivity().getMediaController();Log.d(TAG, "register callback of mediaController");if(mMediaController == null){Log.e(TAG, "mMediaController is null");}mMediaController.registerCallback(mMediaControllerCallback);}@Overridepublic void onDetach() {if (mMediaController != null) {Log.d(TAG, "unregister callback of mediaController");mMediaController.unregisterCallback(mMediaControllerCallback);}super.onDetach();}private class MediaControllerCallback extends MediaController.Callback {@Overridepublic void onPlaybackStateChanged(final PlaybackState state) {Log.d(TAG, "playback state changed: " + state.toString());}@Overridepublic void onMetadataChanged(final MediaMetadata metadata) {Log.d(TAG, "received update of media metadata");}}
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###更新Playback StateChanged中的视频控制图标和更新PlaybackState PlaybackState在播放控制器中更新。

private void updatePlaybackState() {PlaybackState.Builder stateBuilder = new PlaybackState.Builder().setActions(getAvailableActions());int state = PlaybackState.STATE_PLAYING;if (mCurrentPlaybackState == PlaybackState.STATE_PAUSED || mCurrentPlaybackState == PlaybackState.STATE_NONE) {state = PlaybackState.STATE_PAUSED;}// stateBuilder.setState(state, mPosition, 1.0f);stateBuilder.setState(state, getCurrentPosition(), 1.0f);mSession.setPlaybackState(stateBuilder.build());}
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例如，它将在playPause方法中调用。 当用户开始播放视频状态将从STATE_PLAYING变为STATE_PAUSED，反之亦然。 PlaybackState更新被设置（通知）到MediaSession。 ###Callback

当PlaybackState由上面的setPlaybackState更改时，可以使用onPlaybackStateChanged回调接收此事件。 我们可以在PlaybackControlsRow中更新播放/暂停图标。

    private class MediaControllerCallback extends MediaController.Callback {@Overridepublic void onPlaybackStateChanged(final PlaybackState state) {Log.d(TAG, "playback state changed: " + state.toString());mHandler.post(new Runnable() {@Overridepublic void run() {if (state.getState() == PlaybackState.STATE_PLAYING) {mPlaybackController.setCurrentPlaybackState(PlaybackState.STATE_PLAYING);startProgressAutomation();// setFadingEnabled(false);mPlayPauseAction.setIndex(PlaybackControlsRow.PlayPauseAction.PAUSE);mPlayPauseAction.setIcon(mPlayPauseAction.getDrawable(PlaybackControlsRow.PlayPauseAction.PAUSE));notifyChanged(mPlayPauseAction);} else if (state.getState() == PlaybackState.STATE_PAUSED) {mPlaybackController.setCurrentPlaybackState(PlaybackState.STATE_PAUSED);// setFadingEnabled(false);mPlayPauseAction.setIndex(PlaybackControlsRow.PlayPauseAction.PLAY);mPlayPauseAction.setIcon(mPlayPauseAction.getDrawable(PlaybackControlsRow.PlayPauseAction.PLAY));notifyChanged(mPlayPauseAction);}int currentTime = (int) state.getPosition();mPlaybackControlsRow.setCurrentTime(currentTime);// mPlaybackControlsRow.setBufferedProgress(currentTime + SIMULATED_BUFFERED_TIME);mPlaybackControlsRow.setBufferedProgress(mPlaybackController.calcBufferedTime(currentTime));}});}...}
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###在onMetadataChanged上更新媒体信息 更新MediaMetadata

MediaMetadata类用于设置视频的元数据信息。 我们可以通过MediaMetadata.Builder中的put方法设置元数据的每个属性。 MediaMetadata更新被设置（通知）到MediaSession。

    public void updateMetadata(Movie movie) {final MediaMetadata.Builder metadataBuilder = new MediaMetadata.Builder();String title = movie.getTitle().replace("_", " -");metadataBuilder.putString(MediaMetadata.METADATA_KEY_MEDIA_ID, Long.toString(movie.getId()));metadataBuilder.putString(MediaMetadata.METADATA_KEY_DISPLAY_TITLE, title);metadataBuilder.putString(MediaMetadata.METADATA_KEY_DISPLAY_SUBTITLE, movie.getStudio());metadataBuilder.putString(MediaMetadata.METADATA_KEY_DISPLAY_DESCRIPTION, movie.getDescription());metadataBuilder.putString(MediaMetadata.METADATA_KEY_DISPLAY_ICON_URI, movie.getCardImageUrl());metadataBuilder.putLong(MediaMetadata.METADATA_KEY_DURATION, mDuration);// And at minimum the title and artist for legacy supportmetadataBuilder.putString(MediaMetadata.METADATA_KEY_TITLE, title);metadataBuilder.putString(MediaMetadata.METADATA_KEY_ARTIST, movie.getStudio());Glide.with(mActivity).load(Uri.parse(movie.getCardImageUrl())).asBitmap().into(new SimpleTarget<Bitmap>(500, 500) {@Overridepublic void onResourceReady(Bitmap bitmap, GlideAnimation anim) {metadataBuilder.putBitmap(MediaMetadata.METADATA_KEY_ART, bitmap);mSession.setMetadata(metadataBuilder.build());}});}
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通过将MediaMetadata设置为MediaSession，Android TV将在LeanbackLauncher上显示Now Playing Card ，稍后再说明。 ###CallBack

当MediaMetadata已经通过setMetadata以上改变时，该事件可以与onMetadataChanged回调被接收。我们可以更新PlaybackControlsRow的项目值。

    private class MediaControllerCallback extends MediaController.Callback {...@Overridepublic void onMetadataChanged(final MediaMetadata metadata) {Log.d(TAG, "received update of media metadata");updateMovieView(metadata.getString(MediaMetadata.METADATA_KEY_DISPLAY_TITLE),metadata.getString(MediaMetadata.METADATA_KEY_DISPLAY_SUBTITLE),metadata.getString(MediaMetadata.METADATA_KEY_DISPLAY_ICON_URI),metadata.getLong(MediaMetadata.METADATA_KEY_DURATION));}}private void updateMovieView(String title, String studio, String cardImageUrl, long duration) {Log.d(TAG, "updateMovieView");if (mPlaybackControlsRow.getItem() != null) {Movie item = (Movie) mPlaybackControlsRow.getItem();item.setTitle(title);item.setStudio(studio);} else {Log.e(TAG, "mPlaybackControlsRow.getItem is null!");}mPlaybackControlsRow.setTotalTime((int) duration);mPlaybackControlsRow.setCurrentTime(0);mPlaybackControlsRow.setBufferedProgress(0);mRowsAdapter.notifyArrayItemRangeChanged(0, mRowsAdapter.size());// Show the video card image if there is enough room in the UI for it.// If you have many primary actions, you may not have enough room.if (SHOW_IMAGE) {mPlaybackControlsRowTarget = new PicassoPlaybackControlsRowTarget(mPlaybackControlsRow);updateVideoImage(cardImageUrl);}}
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###Now Playing Card

如果MediaMetadata正确设置为MediaSession，则现在播放卡将显示在LeanbackLauncher（主屏幕）中。 它向用户通知当前播放媒体的信息。 此外，现在播放卡使用户可以回到您的应用程序来控制视频（暂停/转到下一个视频等）。 源码在github上. 关注微信公众号，定期为你推荐移动开发相关文章。