【Android API指南】App组件(七) - Services(1) - Bound Services

1. 在service中创建一个Binder的实例：
   a. 包含客户端可以调用的公共函数。
   b. 返回当前Service实例，实例包含客户端可以调用的公共函数。
   c. 或者返回service内部类的实例，包含可调用的公共函数。
2. 从onBind()中返回Binder的实例。
3. 在客户端的OnServiceConnected()取得Binder，调用service中的公共函数。

public class LocalService extends Service {
    // Binder given to clients
    private final IBinder mBinder = new LocalBinder();
    // Random number generator
    private final Random mGenerator = new Random();

    /**
     * Class used for the client Binder.  Because we know this service always
     * runs in the same process as its clients, we don't need to deal with IPC.
     */
    public class LocalBinder extends Binder {
        LocalService getService() {
            // Return this instance of LocalService so clients can call public methods
            return LocalService.this;
        }
    }

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

    /** method for clients */
    public int getRandomNumber() {
      return mGenerator.nextInt(100);
    }
}

public class BindingActivity extends Activity {
    LocalService mService;
    boolean mBound = false;

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

    @Override
    protected void onStart() {
        super.onStart();
        // Bind to LocalService
        Intent intent = new Intent(this, LocalService.class);
        bindService(intent, mConnection, Context.BIND_AUTO_CREATE);
    }

    @Override
    protected void onStop() {
        super.onStop();
        // Unbind from the service
        if (mBound) {
            unbindService(mConnection);
            mBound = false;
        }
    }

    /** Called when a button is clicked (the button in the layout file attaches to
      * this method with the android:onClick attribute) */
    public void onButtonClick(View v) {
        if (mBound) {
            // Call a method from the LocalService.
            // However, if this call were something that might hang, then this request should
            // occur in a separate thread to avoid slowing down the activity performance.
            int num = mService.getRandomNumber();
            Toast.makeText(this, "number: " + num, Toast.LENGTH_SHORT).show();
        }
    }

    /** Defines callbacks for service binding, passed to bindService() */
    private ServiceConnection mConnection = new ServiceConnection() {

        @Override
        public void onServiceConnected(ComponentName className,
                IBinder service) {
            // We've bound to LocalService, cast the IBinder and get LocalService instance
            LocalBinder binder = (LocalBinder) service;
            mService = binder.getService();
            mBound = true;
        }

        @Override
        public void onServiceDisconnected(ComponentName arg0) {
            mBound = false;
        }
    };
}

• service实现一个Handler接收客户端的回调。
• Handler被用来创建一个Messenger对象。
• Messenger创建一个IBinder，在onBind()方法中返回给客户端。
• 客户端使用IBinder初始化Messenger，然后使用它发送Message到服务器。
• 服务器接收到Message，在Handler中处理这些Message。

public class MessengerService extends Service {
    /** Command to the service to display a message */
    static final int MSG_SAY_HELLO = 1;

    /**
     * Handler of incoming messages from clients.
     */
    class IncomingHandler extends Handler {
        @Override
        public void handleMessage(Message msg) {
            switch (msg.what) {
                case MSG_SAY_HELLO:
                    Toast.makeText(getApplicationContext(), "hello!", Toast.LENGTH_SHORT).show();
                    break;
                default:
                    super.handleMessage(msg);
            }
        }
    }

    /**
     * Target we publish for clients to send messages to IncomingHandler.
     */
    final Messenger mMessenger = new Messenger(new IncomingHandler());

    /**
     * When binding to the service, we return an interface to our messenger
     * for sending messages to the service.
     */
    @Override
    public IBinder onBind(Intent intent) {
        Toast.makeText(getApplicationContext(), "binding", Toast.LENGTH_SHORT).show();
        return mMessenger.getBinder();
    }
}

public class ActivityMessenger extends Activity {
    /** Messenger for communicating with the service. */
    Messenger mService = null;

    /** Flag indicating whether we have called bind on the service. */
    boolean mBound;

    /**
     * Class for interacting with the main interface of the service.
     */
    private ServiceConnection mConnection = new ServiceConnection() {
        public void onServiceConnected(ComponentName className, IBinder service) {
            // This is called when the connection with the service has been
            // established, giving us the object we can use to
            // interact with the service.  We are communicating with the
            // service using a Messenger, so here we get a client-side
            // representation of that from the raw IBinder object.
            mService = new Messenger(service);
            mBound = true;
        }

        public void onServiceDisconnected(ComponentName className) {
            // This is called when the connection with the service has been
            // unexpectedly disconnected -- that is, its process crashed.
            mService = null;
            mBound = false;
        }
    };

    public void sayHello(View v) {
        if (!mBound) return;
        // Create and send a message to the service, using a supported 'what' value
        Message msg = Message.obtain(null, MessengerService.MSG_SAY_HELLO, 0, 0);
        try {
            mService.send(msg);
        } catch (RemoteException e) {
            e.printStackTrace();
        }
    }

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

    @Override
    protected void onStart() {
        super.onStart();
        // Bind to the service
        bindService(new Intent(this, MessengerService.class), mConnection,
            Context.BIND_AUTO_CREATE);
    }

    @Override
    protected void onStop() {
        super.onStop();
        // Unbind from the service
        if (mBound) {
            unbindService(mConnection);
            mBound = false;
        }
    }
}

1. 实现ServiceConnection。
   包括重写里面的两个方法：
   onServiceConnected()
           系统调用它来取得onBind()返回的IBinder。
   onServiceDisconnected()
           在service被意外丢失时调用，比如service自己崩溃了，或者被系统杀掉了。这个方法在客户端解除绑定时不会被调用。
2. 调用bindService()方法传递ServiceConnection的实现。
3. 当系统调用onServiceConnected方法时，你可以开始使用服务器的方法。
4. 使用unbindService()来断开service连接。
   当客户端被销毁后，会从服务器解除绑定，不过你应该交互完成或者activity暂停时解除绑定，这样能很好的关闭不需要的service。

LocalService mService;
private ServiceConnection mConnection = new ServiceConnection() {
    // Called when the connection with the service is established
    public void onServiceConnected(ComponentName className, IBinder service) {
        // Because we have bound to an explicit
        // service that is running in our own process, we can
        // cast its IBinder to a concrete class and directly access it.
        LocalBinder binder = (LocalBinder) service;
        mService = binder.getService();
        mBound = true;
    }

    // Called when the connection with the service disconnects unexpectedly
    public void onServiceDisconnected(ComponentName className) {
        Log.e(TAG, "onServiceDisconnected");
        mBound = false;
    }
};

Intent intent = new Intent(this, LocalService.class);
bindService(intent, mConnection, Context.BIND_AUTO_CREATE);

• 当连接被破坏时需要抛出一个DeadObjectException异常。这是远程函数唯一会抛出的异常。
• 对象是夸进程引用的。
• 绑定和解除绑定应该是成对出现的。例如：
  a. 在activity中在onStart()绑定，在onStop解除绑定。
  b. 如果你系统activity在被停止后还能接收响应，你可以在onCreate()中绑定，在onDestroy()中解除绑定。
  提示：你不应该再onResume()中绑定，在onPause()中解除绑定，因为这两个方法发生在生命周期的过渡阶段，时间比较短，很难保证进程的启动。同样的，如果有多个activity绑定到相同的service，那么在第二个activity绑定前，当前的activity解除绑定可能会导致service被销毁和重建。