深入剖析Android四大组件(六)——相对完美的后台Service实现播放音乐功能

public class MyMusicPlayer extends Service implements
MediaPlayer.OnCompletionListener {
public static final String TAG = "MyMusicPlayer";
public static final String ACTION_ADD_TO_QUEUE = "com.example.musicmedia.ADD_TO_QUEUE";//指定action
private ConcurrentLinkedQueue<Uri> mTrackQueue;//它是一个基于链接节点的无界线程安全队列。该队列的元素遵循先进先出的原则。头是最先加入的，尾是最近加入的。插入元素是追加到尾上。提取一个元素是从头提取。
private MediaPlayer mMediaPlayer;
public IBinder onBind(Intent intent) {
return null;
}
@Override
public void onCreate() {
super.onCreate();
Log.i(TAG, "onCreate");  
mTrackQueue = new ConcurrentLinkedQueue<Uri>();//初始化并发队列
}
@Override
public int onStartCommand(Intent intent, int flags, int startId) {
String action = intent.getAction();
if (ACTION_ADD_TO_QUEUE.equals(action)) {
Uri trackUri = intent.getData();//获取音乐的资源
Log.i("MyMusicPlayer", trackUri.toString());
this.addTrackToQueue(trackUri);//添加资源
}
return START_NOT_STICKY;//返回值确定了该Service不会在系统关闭它后重新启动，执行一次性操作经常用这个。
}
@Override
public void onDestroy() {
super.onDestroy();
Log.i(TAG, "onDestroy");
if (mMediaPlayer != null) {//当service销毁的时候释放mMediaPlayer资源
mMediaPlayer.release();
mMediaPlayer = null;
}
}
//如果已经开始播放就往队尾添加新音乐资源，否则创建MediaPlayer并开始播放
private void addTrackToQueue(Uri trackUri) {
if (mMediaPlayer == null) {
try {
Log.i("MyMusicPlayer", "addTrackQueue:" + trackUri);
mMediaPlayer = MediaPlayer.create(this, trackUri);//初始化mMediaPlayer,因为用MediaPlayer.create初始化所以不用调用prepare()方法准备。该初始化自动完成
mMediaPlayer.setOnCompletionListener(this);//注册一个回调函数,音频播放完成后调用
mMediaPlayer.start();//开始播放
} catch (Exception e) {
stopSelf();//停止service
}
} else {
mTrackQueue.offer(trackUri);//向队列添加元素
}
}
//曲目播放完毕，开始播放下一首或者停止Service
@Override
public void onCompletion(MediaPlayer mediaPlayer) {
mediaPlayer.reset();//重用MediaPlayer对象
Uri nextTrackUri = mTrackQueue.poll();//从队列里面取出元素
if (nextTrackUri != null) {
try {
mMediaPlayer.setDataSource(this, nextTrackUri);//初始化mMediaPlayer
mMediaPlayer.prepare();//准备
mMediaPlayer.start();//播放
} catch (IOException e) {
stopSelf();
}
} else {
stopSelf();//停止service
}
}
}