Android Messenger 跨进程通信

Messenger:信使

官方文档解释：它引用了一个Handler对象，以便others能够向它发送消息(使用mMessenger.send(Message msg)方法)。该类允许跨进程间基于Message的通信(即两个进程间可以通过Message进行通信)，在服务端使用Handler创建一个Messenger，客户端持有这个Messenger就可以与服务端通信了。

以前我们使用Handler+Message的方式进行通信，都是在同一个进程中，从线程持有一个主线程的Handler对象，并向主线程发送消息。

而Android既然可以使用bindler机制进行跨进行通信，所以我们当然可以将Handler与bindler结合起来进行跨进程发送消息。

查看API就可以发现，Messenger就是这种方式的实现。

一般使用方法如下：

１。远程通过

mMessenger = new Messenger(mHandler)

创建一个信使对象

２。客户端使用bindlerService请求连接远程

３。远程onBind方法返回一个bindler

return mMessenger.getBinder();

4.客户端使用远程返回的bindler得到一个信使（即得到远程信使）

public void onServiceConnected(ComponentName name, IBinder service) {

rMessenger = new Messenger(service); 　　　　

......

}

这里虽然是new了一个Messenger，但我们查看它的实现

public Messenger(IBinder target) { mTarget = IMessenger.Stub.asInterface(target); }

发现它的mTarget是通过Aidl得到的，实际上就是远程创建的那个。

５。客户端可以使用这个远程信使对象向远程发送消息：rMessenger.send(msg);

这样远程服务端的Handler对象就能收到消息了，然后可以在其handlerMessage(Message msg)方法中进行处理。（该Handler对象就是第一步服务端创建Messenger时使用的参数mHandler）.

经过这５个步骤貌似只有客户端向服务端发送消息，这样的消息传递是单向的，那么如何实现双向传递呢？

首先需要在第５步稍加修改，在send(msg)前通过msm.replyTo = mMessenger将自己的信使设置到消息中，这样服务端接收到消息时同时也得到了客户端的信使对象了，然后服务端可以通过/得到客户端的信使对象，并向它发送消息 cMessenger = msg.replyTo; cMessenger.send(message);

即完成了从服务端向客户端发送消息的功能，这样客服端可以在自己的Handler对象的handlerMessage方法中接收服务端发送来的message进行处理。

双向通信宣告完成。

以下代码来自ApiDemo

Service code:

[java]

public class MessengerService extends Service {
/** For showing and hiding our notification. */
NotificationManager mNM;
/** Keeps track of all current registered clients. */
ArrayList<Messenger> mClients = new ArrayList<Messenger>();
/** Holds last value set by a client. */
int mValue = 0;

/**
* Command to the service to register a client, receiving callbacks
* from the service. The Message's replyTo field must be a Messenger of
* the client where callbacks should be sent.
*/
static final int MSG_REGISTER_CLIENT = 1;

/**
* Command to the service to unregister a client, ot stop receiving callbacks
* from the service. The Message's replyTo field must be a Messenger of
* the client as previously given with MSG_REGISTER_CLIENT.
*/
static final int MSG_UNREGISTER_CLIENT = 2;

/**
* Command to service to set a new value. This can be sent to the
* service to supply a new value, and will be sent by the service to
* any registered clients with the new value.
*/
static final int MSG_SET_VALUE = 3;

/**
* Handler of incoming messages from clients.
*/
class IncomingHandler extends Handler {
@Override
public void handleMessage(Message msg) {
switch (msg.what) {
case MSG_REGISTER_CLIENT:
mClients.add(msg.replyTo);
break;
case MSG_UNREGISTER_CLIENT:
mClients.remove(msg.replyTo);
break;
case MSG_SET_VALUE:
mValue = msg.arg1;
for (int i = mClients.size() - 1; i >= 0; i --) {
try {
mClients.get(i).send(Message.obtain(null,
MSG_SET_VALUE, mValue, 0));
} catch (RemoteException e) {
// The client is dead. Remove it from the list;
// we are going through the list from back to front
// so this is safe to do inside the loop.
mClients.remove(i);
}
}
break;
default:
super.handleMessage(msg);
}
}
}

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

@Override
public void onCreate() {
mNM = (NotificationManager)getSystemService(NOTIFICATION_SERVICE);

// Display a notification about us starting.
showNotification();
}

@Override
public void onDestroy() {
// Cancel the persistent notification.
mNM.cancel(R.string.remote_service_started);

// Tell the user we stopped.
Toast.makeText(this, R.string.remote_service_stopped, Toast.LENGTH_SHORT).show();
}

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

/**
* Show a notification while this service is running.
*/
private void showNotification() {
// In this sample, we'll use the same text for the ticker and the expanded notification
CharSequence text = getText(R.string.remote_service_started);

// Set the icon, scrolling text and timestamp
Notification notification = new Notification(R.drawable.stat_sample, text,
System.currentTimeMillis());

// The PendingIntent to launch our activity if the user selects this notification
PendingIntent contentIntent = PendingIntent.getActivity(this, 0,
new Intent(this, Controller.class), 0);

// Set the info for the views that show in the notification panel.
notification.setLatestEventInfo(this, getText(R.string.remote_service_label),
text, contentIntent);

// Send the notification.
// We use a string id because it is a unique number. We use it later to cancel.
mNM.notify(R.string.remote_service_started, notification);
}
}