Android基于Service的音乐播放器

本文开发一个基于Service的音乐播放器，音乐由后台运行的Service负责播放，当后台的播放状态发生变化时，程序将会通过发送广播通知前台Activity更新界面；当点击Activity的界面按钮时，系统将通过发送广播通知后台Service来改变播放状态。

前台Activity界面有两个按钮，分别用于控制播放/暂停、停止，另外还有两个文本框，用于显示正在播放的歌曲名、歌手名。前台Activity的代码如下：

public class MainActivity extends AppCompatActivity implements View.OnClickListener{

  private ImageButton mStart;
  private ImageButton mStop;
  private TextView mMusicName;
  private TextView mSongerName;
  private ActivityReceiver mActivityReceiver;
  public static final String CTL_ACTION = "com.trampcr.action.CTL_ACTION";
  public static final String UPDATE_ACTION = "com.trampcr.action.UPDATE_ACTION";

  //定义音乐播放状态，0x11代表没有播放，0x12代表正在播放，0x13代表暂停
  int status = 0x11;
  String[] musicNames = new String[]{"完美生活", "那一年", "故乡"};
  String[] songerNames = new String[]{"许巍", "许巍", "许巍"};

  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    mStart = (ImageButton) findViewById(R.id.start);
    mStop = (ImageButton) findViewById(R.id.stop);
    mMusicName = (TextView) findViewById(R.id.music_name);
    mSongerName = (TextView) findViewById(R.id.songer_name);

    mStart.setOnClickListener(this);
    mStop.setOnClickListener(this);

    mActivityReceiver = new ActivityReceiver();
    //创建IntentFilter
    IntentFilter filter = new IntentFilter();
    //指定BroadcastReceiver监听的Action
    filter.addAction(UPDATE_ACTION);
    //注册BroadcastReceiver
    registerReceiver(mActivityReceiver, filter);

    Intent intent = new Intent(MainActivity.this, MusicService.class);
    //启动后台Service
    startService(intent);
  }

  public class ActivityReceiver extends BroadcastReceiver {
    @Override
    public void onReceive(Context context, Intent intent) {
      //获取Intent中的update消息，update代表播放状态
      int update = intent.getIntExtra("update", -1);
      //获取Intent中的current消息，current代表当前正在播放的歌曲
      int current = intent.getIntExtra("current", -1);
      if (current >= 0){
        mMusicName.setText(musicNames[current]);
        mSongerName.setText(songerNames[current]);
      }
      switch (update){
        case 0x11:
          mStart.setBackgroundResource(R.drawable.play);
          status = 0x11;
          break;
        //控制系统进入播放状态
        case 0x12:
          //在播放状态下设置使用暂停图标
          mStart.setBackgroundResource(R.drawable.pause);
          status = 0x12;
          break;
        case 0x13:
          //在暂停状态下设置使用播放图标
          mStart.setBackgroundResource(R.drawable.play);
          status = 0x13;
          break;
      }
    }
  }

  @Override
  public void onClick(View v) {
    Intent intent = new Intent(CTL_ACTION);
    switch (v.getId()){
      case R.id.start:
        intent.putExtra("control", 1);
        break;
      case R.id.stop:
        intent.putExtra("control", 2);
        break;
    }
    //发送广播，将被Service中的BroadcastReceiver接收到
    sendBroadcast(intent);
  }
}

ActivityReceiver()用于响应后台Service所发出的广播，该程序将会根据广播Intent里的消息来改变播放状态，并更新程序界面中按钮的图标。

onClick中根据点击的按钮发送广播，发送广播时会把所按下的按钮标识发送出来。

接下来是后台Service，会在播放状态发生改变时对外发送广播。代码如下：

public class MusicService extends Service {

  MyReceiver serviceReceiver;
  AssetManager mAssetManager;
  String[] musics = new String[]{"prefectLife.mp3", "thatYear.mp3", "country.mp3"};
  MediaPlayer mMediaPlayer;
  int status = 0x11;
  int current = 0; // 记录当前正在播放的音乐

  @Nullable
  @Override
  public IBinder onBind(Intent intent) {
    return null;
  }

  @Override
  public void onCreate() {
    super.onCreate();
    mAssetManager = getAssets();
    serviceReceiver = new MyReceiver();
    //创建IntentFilter
    IntentFilter filter = new IntentFilter();
    filter.addAction(MainActivity.CTL_ACTION);
    registerReceiver(serviceReceiver, filter);
    //创建MediaPlayer
    mMediaPlayer = new MediaPlayer();
    //为MediaPlayer播放完成事件绑定监听器
    mMediaPlayer.setOnCompletionListener(new MediaPlayer.OnCompletionListener() {
      @Override
      public void onCompletion(MediaPlayer mp) {
        current++;
        if (current >= 3) {
          current = 0;
        }
        //发送广播通知Activity更改文本框
        Intent sendIntent = new Intent(MainActivity.UPDATE_ACTION);
        sendIntent.putExtra("current", current);
        //发送广播，将被Activity中的BroadcastReceiver接收到
        sendBroadcast(sendIntent);
        //准备并播放音乐
        prepareAndPlay(musics[current]);
      }
    });
  }

  public class MyReceiver extends BroadcastReceiver {
    @Override
    public void onReceive(Context context, Intent intent) {
      int control = intent.getIntExtra("control", -1);
      switch (control){
        case 1: // 播放或暂停
          //原来处于没有播放状态
          if (status ==0x11){
            //准备播放音乐
            prepareAndPlay(musics[current]);
            status = 0x12;
          }
          //原来处于播放状态
          else if (status == 0x12){
            //暂停
            mMediaPlayer.pause();
            status = 0x13; // 改变为暂停状态
          }
          //原来处于暂停状态
          else if (status == 0x13){
            //播放
            mMediaPlayer.start();
            status = 0x12; // 改变状态
          }
          break;
        //停止声音
        case 2:
          //如果原来正在播放或暂停
          if (status == 0x12 || status == 0x13){
            //停止播放
            mMediaPlayer.stop();
            status = 0x11;
          }
      }
      //广播通知Activity更改图标、文本框
      Intent sendIntent = new Intent(MainActivity.UPDATE_ACTION);
      sendIntent.putExtra("update", status);
      sendIntent.putExtra("current", current);
      //发送广播，将被Activity中的BroadcastReceiver接收到
      sendBroadcast(sendIntent);
    }
  }

  private void prepareAndPlay(String music) {
    try {
      //打开指定的音乐文件
      AssetFileDescriptor assetFileDescriptor = mAssetManager.openFd(music);
      mMediaPlayer.reset();
      //使用MediaPlayer加载指定的声音文件
      mMediaPlayer.setDataSource(assetFileDescriptor.getFileDescriptor(), assetFileDescriptor.getStartOffset(), assetFileDescriptor.getLength());
      mMediaPlayer.prepare(); // 准备声音
      mMediaPlayer.start(); // 播放
    } catch (IOException e) {
      e.printStackTrace();
    }
  }
}

MyReceiver用于接收前台Activity所发出的广播，并根据广播的消息内容改变Service的播放状态，当播放状态改变时，该Service对外发送一条广播，广播消息将会被前台Activity接收，前台Activity将会根据广播消息更新界面。

为了让该音乐播放器能按顺序依次播放歌曲，程序为MediaPlayer增加了OnCompletionListener监听器，当MediaPlayer播放完成后将自动播放下一首歌曲。

运行程序，效果图如下：

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