UniversalMusicPlayer 学习笔记(二)

上一篇简要的看了看MusicPlayerActivity持有的MediaBrowser和MidiaController两个对象。
接下来，跟踪MusicPlayerActivity的生命周期，来了解程序的运行过程。（因为MusicPlayerActivity继承BaseActivity，在MusicPlayerActivity的生命周期函数中，默认执行了BaseActivity的生命周期函数）。
BaseActivity的onCreate()函数中的部分代码:

mMediaBrowser = new MediaBrowserCompat(this,
            new ComponentName(this, MusicService.class), mConnectionCallback, null);

public MediaBrowserCompat(Context context, ComponentName serviceComponent,ConnectionCallback callback, Bundle rootHints)
创建了在前面的提到的MediaBrowserCompat对象。
BaseActivity的onStart()函数中的部分代码:

 mMediaBrowser.connect();

点进去看它的实现：

 try {
     bound = mContext.bindService(intent,mServiceConnection, Context.BIND_AUTO_CREATE);
     } catch (Exception ex) {
       Log.e(TAG, "Failed binding to service " + mServiceComponent);
            }

到这儿，我们发现，原来 mMediaBrowser.connect()主要是实现了捆绑service，这个service从mMediaBrowser的构造函数里可以知道是MusicService.（连接service有两种方式：Started和 Bound.两者的主要区别就是Bound可以实现service和启动他的组件进行交互—通过IBinder.详细的介绍参考官方的开发文档）

## MusicService##

/**
 * This class provides a MediaBrowser through a service. It exposes the media library to a browsing
 * client, through the onGetRoot and onLoadChildren methods. It also creates a MediaSession and
 * exposes it through its MediaSession.Token, which allows the client to create a MediaController
 * that connects to and send control commands to the MediaSession remotely. This is useful for
 * user interfaces that need to interact with your media session, like Android Auto. You can
 * (should) also use the same service from your app's UI, which gives a seamless playback
 * experience to the user.
 *
 * To implement a MediaBrowserService, you need to:
 *
 * 

 *
 * 
 Extend {@link android.service.media.MediaBrowserService}, implementing the media browsing
 *      related methods {@link android.service.media.MediaBrowserService#onGetRoot} and
 *      {@link android.service.media.MediaBrowserService#onLoadChildren};
 * 
 In onCreate, start a new {@link android.media.session.MediaSession} and notify its parent
 *      with the session's token {@link android.service.media.MediaBrowserService#setSessionToken};
 *
 * 
 Set a callback on the
 *      {@link android.media.session.MediaSession#setCallback(android.media.session.MediaSession.Callback)}.
 *      The callback will receive all the user's actions, like play, pause, etc;
 *
 * 
 Handle all the actual music playing using any method your app prefers (for example,
 *      {@link android.media.MediaPlayer})
 *
 * 
 Update playbackState, "now playing" metadata and queue, using MediaSession proper methods
 *      {@link android.media.session.MediaSession#setPlaybackState(android.media.session.PlaybackState)}
 *      {@link android.media.session.MediaSession#setMetadata(android.media.MediaMetadata)} and
 *      {@link android.media.session.MediaSession#setQueue(java.util.List)})
 *
 * 
 Declare and export the service in AndroidManifest with an intent receiver for the action
 *      android.media.browse.MediaBrowserService
 *
 * 
 *
 * To make your app compatible with Android Auto, you also need to:
 *
 * 

慢慢的来看这段介绍:
① 它通过onGetRoot和onLoadChildren 方法把媒体资源提供给客户端。
② 创建MediaSession，并且把MediaSession.Token暴露出来，MediaSession.Token用来创建MediaController（这个前面已经提到过）。
③MusicService继承于MediaBrowserServiceCompat，它需要完成以下要求：
a：实现onGetRoot和onLoadChildren 方法。
b：在onCreate()函数中创建一个MediaSession对象并且通知它的父类（MediaBrowserServiceCompat）。

 mSession = new MediaSessionCompat(this, "MusicService");
        setSessionToken(mSession.getSessionToken());

c：通过MediaSession#setCallback设置callback对象，这个对象用来接收用户的操作，例如播放，暂停。

  mSession.setCallback(mPlaybackManager.getMediaSessionCallback());

到此，一张图总结一下MediaBrowserCompat、MusicService(MediaServiceCompat)和MediaControllerCompat的关系。

MediaControllerCompat 和 MediaSession的详细关系（先有个大题印象）：