Messenger的使用

在Android编程中，要想实现IPC，有如下几种方法：

1，使用AIDL，在需要实现多线程和并发处理时使用

2，继承Binder类来实现

3，使用Messenger

这一节我们研究Messenger的使用。以后有机会研究前两种。

首先，通过查看Messenger的源代码，我们可以看到它有两个构造函数：

/**
* Create a new Messenger pointing to the given Handler. Any Message
* objects sent through this Messenger will appear in the Handler as if
* {@link Handler#sendMessage(Message) Handler.sendMessage(Message)} had
* been called directly.
*
* @param target The Handler that will receive sent messages.
*/
public Messenger(Handler target) {
mTarget = target.getIMessenger();
}

和
/**
* Create a Messenger from a raw IBinder, which had previously been
* retrieved with {@link #getBinder}.
*
* @param target The IBinder this Messenger should communicate with.
*/
public Messenger(IBinder target) {
mTarget = IMessenger.Stub.asInterface(target);
}

前者用来给服务端调用创建Messenger，后者给客户端调用。

使用方法

首先，服务端实现一个

Handler

 ,用来接收客户端的信息
使用上述Handler创建一个

 
[code]Messenger

[/code] 对象（它是Handler类中的一个引用，Handler类的成员变量mMessenger指向这个新创建的对象）。
上述Messenger对象创建一个

IBinder

 对象,
服务端的onBind()返回该对象给客户端
客户端使用上述IBinder对象初始化Messenger对象(该对象引用服务端的Handler),然后服务端就可以使用它来发生 

Message

 对象给服务端。
服务端在它的Handler对象中收到上述Message对象，使用 

handleMessage()

 方法来处理。

使用这种方式，不存在客户端调用服务端的“方法”，而是客户端发送“信息”（即Message对象），服务端在它的Handler中接收并处理。
如下是使用Messager的服务端的代码：

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();
    }
}

上述 

handleMessage()

 方法就是服务端接收消息并决定如何处理的地方。
客户端只需要使用服务端返回的IBinder创建Messenger对象，然后使用该对象的send()方法发生消息即可。代码如下（先绑定服务，然后发送

MSG_SAY_HELLO

 消息给服务端：

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;
        }
    }
}

上述示例没有显示服务端如何响应客户端。如果需要服务端响应的话，需要将客户端创建的Messenger对象赋值给的Message的replyTo参数，如下：
服务端：public class MessengerService extends Service {

public static final int MSG_TAG_REMOTE = 0x110;
public static final int MSG_TAG_CLIENT = 0x111;

private int mCounter = 0;

class ServiceHandler extends Handler{

@Override
public void handleMessage(Message msg) {
switch (msg.what) {
case MSG_TAG_REMOTE:
Messenger client = msg.replyTo;
try {
client.send(Message.obtain(null, MSG_TAG_CLIENT, ++mCounter,0));
} catch (RemoteException e) {
e.printStackTrace();
}

}

super.handleMessage(msg);
}

}

final Messenger mMessenger = new Messenger(new ServiceHandler());

/* (non-Javadoc)
* @see android.app.Service#onBind(android.content.Intent)
*/
@Override
public IBinder onBind(Intent intent) {
return (mMessenger == null) ? null : mMessenger.getBinder();
}

}
客户端：

public class MessengerServceActivity extends Activity {

private TextView mTextView;
private Messenger mServiceMessenger;
final Messenger mClientMessenger = new Messenger(new ClientHandler());

ServiceConnection mServiceConn = new ServiceConnection(){

@Override
public void onServiceConnected(ComponentName name, IBinder service) {
mServiceMessenger = new Messenger(service);
Message msg = Message.obtain(null, MessengerService.MSG_TAG_REMOTE);
msg.replyTo = mClientMessenger;
try {
mServiceMessenger.send(msg);
} catch (RemoteException e) {
e.printStackTrace();
}

}

@Override
public void onServiceDisconnected(ComponentName name) {
mServiceMessenger = null;
}

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

mTextView = (TextView) findViewById(R.id.content_show);
bindService(new Intent(this, MessengerService.class), mServiceConn, BIND_AUTO_CREATE);
}

class ClientHandler extends Handler{

@Override
public void handleMessage(Message msg) {
switch (msg.what) {
case MessengerService.MSG_TAG_CLIENT:
if (mTextView != null) {
mTextView.setText(msg.arg1+"");
}
break;
default:
super.handleMessage(msg);
break;
}
}
}
}