Android -- Messager与Service

如果你需要你的service和其他进程通信，那么你可以使用一个Messenger来提供这个接口。

这种方法允许你在不使用 AIDL的情况下，进行跨进程通信IPC。

实现步骤                                                                              

下面是一个如何使用 Messenger的小总结：

　　1. service实现一个 Handler 接收客户端每一次调用的回调。

　　2. Handler 用来创建一个Messenger对象，它是一个Handler的引用。

　　3. Messenger创建一个 IBinder，service从 onBind()中把它返回给客户端。

　　4. 客户端使用这个IBinder来实例化Messenger （service的Handler的引用），客户端使用它来向service发送Message对象。

　　5. service在它的Handler中接收每一个Message对象，在它的 handleMessage()方法中。

Code                                                                                   

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();
    }
}

　　注意 Handler中的 handleMessage() 方法是service接收到来的 Message并且决定做什么的地方。

　　客户端需要做的仅仅是创建一个基于service所返回的 IBinder的 Messenger，然后用 send()方法发送信息。

　　比如，这里有一个简单的activity和service绑定并且发送信息给service：

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;
        }
    }
}

　　注意这个例子并没有展示service如何响应客户端，如果你想要service响应，你需要在客户端中创建一个 Messenger。

　　然后当客户端接收到onServiceConnected()回调方法时，它会发送一个 Message到service，在它的send() 方法的replyTo参数中包含了客户端的Messenger。