android Messenger 跨进程通信

源文：http://xwangly.iteye.com/blog/1109424

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:

public class MessengerService extends Service {    /** For showing and hiding our notification. */    NotificationManager mNM;    /** Keeps track of all current registered clients. */    ArrayList mClients = new ArrayList();    /** 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);    }}

Client code:

public class MessengerServiceActivities {    /**     * Example of binding and unbinding to the remote service.     * This demonstrates the implementation of a service which the client will     * bind to, interacting with it through an aidl interface.
     *      * Note that this is implemented as an inner class only keep the sample     * all together; typically this code would appear in some separate class.     */    public static class Binding extends Activity {        /** Messenger for communicating with service. */        Messenger mService = null;        /** Flag indicating whether we have called bind on the service. */        boolean mIsBound;        /** Some text view we are using to show state information. */        TextView mCallbackText;                /**         * Handler of incoming messages from service.         */        class IncomingHandler extends Handler {            @Override            public void handleMessage(Message msg) {                switch (msg.what) {                    case MessengerService.MSG_SET_VALUE:                        mCallbackText.setText("Received from service: " + msg.arg1);                        break;                    default:                        super.handleMessage(msg);                }            }        }                /**         * Target we publish for clients to send messages to IncomingHandler.         */        final Messenger mMessenger = new Messenger(new IncomingHandler());                /**         * 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 service object we can use to                // interact with the service.  We are communicating with our                // service through an IDL interface, so get a client-side                // representation of that from the raw service object.                mService = new Messenger(service);                mCallbackText.setText("Attached.");                // We want to monitor the service for as long as we are                // connected to it.                try {                    Message msg = Message.obtain(null,                            MessengerService.MSG_REGISTER_CLIENT);                    msg.replyTo = mMessenger;                    mService.send(msg);                                        // Give it some value as an example.                    msg = Message.obtain(null,                            MessengerService.MSG_SET_VALUE, this.hashCode(), 0);                    mService.send(msg);                } catch (RemoteException e) {                    // In this case the service has crashed before we could even                    // do anything with it; we can count on soon being                    // disconnected (and then reconnected if it can be restarted)                    // so there is no need to do anything here.                }                                // As part of the sample, tell the user what happened.                Toast.makeText(Binding.this, R.string.remote_service_connected,                        Toast.LENGTH_SHORT).show();            }            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;                mCallbackText.setText("Disconnected.");                // As part of the sample, tell the user what happened.                Toast.makeText(Binding.this, R.string.remote_service_disconnected,                        Toast.LENGTH_SHORT).show();            }        };                void doBindService() {            // Establish a connection with the service.  We use an explicit            // class name because there is no reason to be able to let other            // applications replace our component.            bindService(new Intent(Binding.this,                     MessengerService.class), mConnection, Context.BIND_AUTO_CREATE);            mIsBound = true;            mCallbackText.setText("Binding.");        }                void doUnbindService() {            if (mIsBound) {                // If we have received the service, and hence registered with                // it, then now is the time to unregister.                if (mService != null) {                    try {                        Message msg = Message.obtain(null,                                MessengerService.MSG_UNREGISTER_CLIENT);                        msg.replyTo = mMessenger;                        mService.send(msg);                    } catch (RemoteException e) {                        // There is nothing special we need to do if the service                        // has crashed.                    }                }                                // Detach our existing connection.                unbindService(mConnection);                mIsBound = false;                mCallbackText.setText("Unbinding.");            }        }                /**         * Standard initialization of this activity.  Set up the UI, then wait         * for the user to poke it before doing anything.         */        @Override        protected void onCreate(Bundle savedInstanceState) {            super.onCreate(savedInstanceState);            setContentView(R.layout.messenger_service_binding);            // Watch for button clicks.            Button button = (Button)findViewById(R.id.bind);            button.setOnClickListener(mBindListener);            button = (Button)findViewById(R.id.unbind);            button.setOnClickListener(mUnbindListener);                        mCallbackText = (TextView)findViewById(R.id.callback);            mCallbackText.setText("Not attached.");        }        private OnClickListener mBindListener = new OnClickListener() {            public void onClick(View v) {                doBindService();            }        };        private OnClickListener mUnbindListener = new OnClickListener() {            public void onClick(View v) {                doUnbindService();            }        };    }}

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