MediaControllerCompat的播放状态

我在本指南中使用MediaSessionCompat构建后台音频应用程序时遇到了一个问题。

在我的Activity中，我将UI连接到媒体控制器，如下所示

private MediaBrowserCompat.ConnectionCallback mediaBrowserConnectionCallbacks = new MediaBrowserCompat.ConnectionCallback() {
@Override
public void onConnected() {
super.onConnected();
try {
mediaControllerCompat = new MediaControllerCompat(StreamPlayerActivity.this, mediaBrowserCompat.getSessionToken());
mediaControllerCompat.registerCallback(mediaControllerCallbacks);
mediaControllerCompat.setMediaController(StreamPlayerActivity.this, mediaControllerCompat);
mediaControllerCompat.getTransportControls().playFromMediaId(String.valueOf(R.raw.hann), null);
Log.d(TAG, "connectedt" + currentState + "t" + mediaControllerCompat.getPlaybackState());
} catch( RemoteException e ) {
e.printStackTrace();
Log.d(TAG, "connection failed");
}
}
};
private MediaControllerCompat.Callback mediaControllerCallbacks = new MediaControllerCompat.Callback() {
@Override
public void onPlaybackStateChanged(PlaybackStateCompat state) {
super.onPlaybackStateChanged(state);
if( state == null ) {
return;
}
switch( state.getState() ) {
case PlaybackStateCompat.STATE_PLAYING: {
currentState = STATE_PLAYING;
break;
}
case PlaybackStateCompat.STATE_PAUSED: {
currentState = STATE_PAUSED;
break;
}
}
}
};

在我的Service中，我按照以下构建会话

private MediaSessionCompat.Callback mediaSessionCallbacks = new MediaSessionCompat.Callback() {
@Override
public void onPlay() {
super.onPlay();
if( !successfullyRetrievedAudioFocus() ) {
return;
}
mediaSessionCompat.setActive(true);
setMediaPlaybackState(PlaybackStateCompat.STATE_PLAYING);
mediaPlayer.start();
Log.d(TAG, "mp started");
}
@Override
public void onPause() {
super.onPause();
if( mediaPlayer.isPlaying() ) {
mediaPlayer.pause();
setMediaPlaybackState(PlaybackStateCompat.STATE_PAUSED);
}
}
@Override
public void onPlayFromMediaId(String mediaId, Bundle extras) {
super.onPlayFromMediaId(mediaId, extras);
try {
AssetFileDescriptor afd = getResources().openRawResourceFd(Integer.valueOf(mediaId));
if (afd == null) {
return;
}
try {
mediaPlayer.setDataSource(afd.getFileDescriptor(), afd.getStartOffset(), afd.getLength());
Log.d(TAG, "datasource set");
} catch (IllegalStateException e) {
mediaPlayer.release();
initMediaPlayer();
mediaPlayer.setDataSource(afd.getFileDescriptor(), afd.getStartOffset(), afd.getLength());
}
afd.close();
initMediaSessionMetadata();
} catch (IOException e) {
return;
}
try {
mediaPlayer.prepare();
Log.d(TAG, "media prepared");
} catch (IOException e) { Log.d(TAG, "media preparation failed");
}
}
};

基本上，我一行一行地遵循了网站的指南，只是我忽略了显示当前音频信息的Notification。在我的设备上测试时，我得到了一个NullPointerException错误。

logcat发现这是在尝试mediaControllerCompat.getPlaybackState().getState()时发生的。我想我的MediaControllerCompat.getPlaybackState()就是null。即使是MediaControllerCompat的Android Devleopers网站似乎也不能解决我的问题。也许只有我一个人，但在尝试检索其值之前，我没有看到任何定义MediaControllerCompat的PlaybackState的代码。

如何解决此问题？

提前感谢您的帮助。