安卓无法绑定(bind)服务

Question

android 新手，想搞清楚服务。我正在尝试将服务绑定(bind)到 Activity ，我正在按照文档中的示例进行操作，但我不断在下面标记的行 (appService.playSong(title)) 上收到 NullPointerException。在调试器中检查它会发现 appService 确实为 null。

public class Song extends Activity implements OnClickListener,Runnable {
protected static int currentPosition;
private ProgressBar progress;
private TextView songTitle;
private MPService appService;

private ServiceConnection onService = new ServiceConnection() {
    public void onServiceConnected(ComponentName className,
            IBinder rawBinder) {
        appService = ((MPService.LocalBinder)rawBinder).getService();
    }

    public void onServiceDisconnected(ComponentName classname) {
        appService = null;
    }
};


public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.song);

    Intent bindIntent = new Intent(Song.this,MPService.class);
    bindService(bindIntent,onService,
            Context.BIND_AUTO_CREATE);

    Bundle b = getIntent().getBundleExtra("songdata");
    String title = b.getString("song title");

    // ... 

    appService.playSong(title); // nullpointerexception

    // ...

}

这是服务的相关部分:

package org.example.music;

// imports

public class MPService extends Service {
private MediaPlayer mp;
public static int currentPosition = 0;
public List<String> songs = new ArrayList<String>();
public static String songTitle;
private static final String MEDIA_PATH = new String("/mnt/sdcard/");

@Override
public void onCreate() {
    super.onCreate();

    mp = new MediaPlayer();
    songs = Music.songs;
}

@Override
public int onStartCommand(Intent intent, int flags, int startId) {
    return Service.START_STICKY;
}

public class LocalBinder extends Binder {
    MPService getService() {
        return MPService.this;
    }
}

private final IBinder binder = new LocalBinder();

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

public void playSong(String songPath) {
try {
    mp.reset();
    mp.setDataSource(songPath);
    mp.prepare();
    mp.start();

    mp.setOnCompletionListener(new OnCompletionListener() {
        public void onCompletion(MediaPlayer arg0) {
            nextSong();
        }
    });

    songTitle = songPath.substring(12,songPath.length()-4);

} catch (IOException e) {
    Log.v(getString(R.string.app_name),e.getMessage());
}
}

public void nextSong() {
if (++currentPosition >= songs.size()) {
    currentPosition = 0;
}
String song = MEDIA_PATH+songs.get(currentPosition);
playSong(song);
} 

public void prevSong() {
if (--currentPosition<0) {
    currentPosition=songs.size()-1;
}
String song = Music.MEDIA_PATH+songs.get(currentPosition);
playSong(song);
}

public int getSongPosition() {
return mp.getCurrentPosition();
}

public MediaPlayer getMP() {
return mp;
}
}

我已经在 AndroidManifest.xml 中注册了该服务并设置了 android:enabled="true"。你看到这里有什么明显的错误吗？

Answer 1

您可以进行本地和远程两种绑定(bind)。本地仅供您的应用程序使用，如果供任何实现特定接口(interface)的应用程序使用，则为远程。 您应该从本地绑定(bind)开始。

Local binding tutorial .
Remote binding tutorial .

我没有绑定(bind)的解决方案:

public class MyActivity extends Activity{

  @Override
  public void onCreate(Bundle savedInstanceState){
   ...
   Intent it = new Intent(MyService.ACTIVITY_START_APP);
   it.setClass(getApplicationContext(), MyService.class);
   startService(it);
}

  ...

@Override
    protected void onResume() {
        super.onResume();
        registerBroadcastReceiver();
    }

@Override
    protected void onPause() {
        super.onPause();
        this.unregisterReceiver(this.receiver);
    }

  ...

private BroadcastReceiver receiver = new BroadcastReceiver(){

        @Override
        public void onReceive(Context context, Intent intent) {
            if (intent.getAction().equals(MyService.BROADCAST_INIT)) {
                //do your stuff here after init
            }
        }
    };

private void registerBroadcastReceiver(){
         IntentFilter filter = new IntentFilter();
         filter.addAction(HMyService.BROADCAST_INIT);
         this.registerReceiver(receiver, filter);
    }
}

您的服务:

public class MyService extends Service{

public static final String BROADCAST_INITIAL_DATA = "org.myapp.BROADCAST_INIT";
public static final String ACTIVITY_START_APP = "org.myapp.ACTIVITY_START_APP";

  @Override
  public int onStartCommand (Intent intent, int flags, int startId){
    super.onStartCommand(intent, flags, startId);
    if(intent.getAction().equals(ACTIVITY_START_APP)){
      //do your initialization
      //inform the client/GUI
      Intent i = new Intent();
      i.setAction(BROADCAST_INIT);
      sendBroadcast(i);
    }else{
      //some other stuff like handle buttons
    }              
  }
}

祝你好运。