messager

• service实现一个 Handler，由其接收来自客户端的每个调用的回调
• Handler 用于创建 Messenger 对象
• Messenger 创建一个 IBinder，service通过 onBind() 使其返回客户端
• 客户端使用 IBinder 将 Messenger（引用service的 Handler）实例化，然后使用后者将 Message 对象发送给服务
• service在其 Handler 中（具体地讲，是在 handleMessage() 方法中）接收每个 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.
     * 第一步
     */
    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.
     * 第二步
     */
    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.
     * 第三步
     */
    @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.
     * 第四步
     */
    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;
        }
    };

    /**
     * Class for interacting with the main interface of the service.
     * 第五步
     */
    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;
        }
    }
}