简单跟一下MediaSession源码1

MediaSession mMediaSession = new MediaSession(MusicApp.getContext(), TAG);
mMediaSession.setCallback(mediaSessionCallback);
​
private final MediaSession.Callback mediaSessionCallback = new MediaSession.Callback() {
    //略
}

frameworks\base\media\java\android\media\session\MediaSession.java

public MediaSession(@NonNull Context context, @NonNull String tag) {
    this(context, tag, null);
}

public MediaSession(@NonNull Context context, @NonNull String tag,
        @Nullable Bundle sessionInfo) {
    //略
    //初始化MediaSessionManager
    MediaSessionManager manager = (MediaSessionManager) context
            .getSystemService(Context.MEDIA_SESSION_SERVICE);
    try {
        //创建Binder
        mBinder = manager.createSession(mCbStub, tag, sessionInfo);
        mSessionToken = new Token(Process.myUid(), mBinder.getController());
        //创建MediaController对象
        mController = new MediaController(context, mSessionToken);
    } catch (RemoteException e) {
        throw new RuntimeException("Remote error creating session.", e);
    }
}

@Override
public void onMediaButtonSessionChanged(MediaSessionRecordImpl oldMediaButtonSession,
        MediaSessionRecordImpl newMediaButtonSession) {
    synchronized (mLock) {
        if (oldMediaButtonSession != null) {
            mHandler.postSessionsChanged(oldMediaButtonSession);
        }
        if (newMediaButtonSession != null) {
            //保存MediaButton的接收者
            rememberMediaButtonReceiverLocked(newMediaButtonSession);
            mHandler.postSessionsChanged(newMediaButtonSession);
        }
        pushAddressedPlayerChangedLocked();
    }
}

Media button session is changed to com.biumall.video/VideoApp_MusicService (userId=0)
Media button session is changed to com.biumall.music/MusicApp_MusicService (userId=0)