Android Messenger 跨進程通信

Messenger:信使

官方文檔解釋：它引用了一個Handler對象，以便others能夠向它發送消息(使用mMessenger.send(Message msg)方法)。該類允許跨進程間基於Message的通信(即兩個進程間可以通過Message進行通信)，在服務端使用Handler創建一個Messenger，客戶端持有這個Messenger就可以與服務端通信了。

以前我們使用Handler+Message的方式進行通信，都是在同一個進程中，從線程持有一個主線程的Handler對象，並向主線程發送消息。

而Android既然可以使用bindler機制進行跨進行通信，所以我們當然可以將Handler與bindler結合起來進行跨進程發送消息。

查看API就可以發現，Messenger就是這種方式的實現。

一般使用方法如下：

１。遠程通過

創建一個信使對象

２。客戶端使用bindlerService請求連接遠程

３。遠程onBind方法返回一個bindler

4.客戶端使用遠程返回的bindler得到一個信使（即得到遠程信使）

public void onServiceConnected(ComponentName name, IBinder service) {

rMessenger = new Messenger(service); 　　　　

......

}

這裡雖然是new了一個Messenger，但我們查看它的實現

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

發現它的mTarget是通過Aidl得到的，實際上就是遠程創建的那個。

５。客戶端可以使用這個遠程信使對象向遠程發送消息：rMessenger.send(msg);

這樣遠程服務端的Handler對象就能收到消息了，然後可以在其handlerMessage(Message msg)方法中進行處理。（該Handler對象就是第一步服務端創建Messenger時使用的參數mHandler）.

經過這５個步驟貌似只有客戶端向服務端發送消息，這樣的消息傳遞是單向的，那麼如何實現雙向傳遞呢？

首先需要在第５步稍加修改，在send(msg)前通過msm.replyTo = mMessenger將自己的信使設置到消息中，這樣服務端接收到消息時同時也得到了客戶端的信使對象了，然後服務端可以通過/得到客戶端的信使對象，並向它發送消息 cMessenger = msg.replyTo; cMessenger.send(message);

即完成了從服務端向客戶端發送消息的功能，這樣客服端可以在自己的Handler對象的handlerMessage方法中接收服務端發送來的message進行處理。

雙向通信宣告完成。

以下代碼來自ApiDemo

Service code:

1. public class MessengerService extends Service {
2. /** For showing and hiding our notification. */
3. NotificationManager mNM;
4. /** Keeps track of all current registered clients. */
5. ArrayList<Messenger> mClients = new ArrayList<Messenger>();
6. /** Holds last value set by a client. */
7. int mValue = 0;
9. /**
10. * Command to the service to register a client, receiving callbacks
11. * from the service. The Message's replyTo field must be a Messenger of
12. * the client where callbacks should be sent.
13. */
14. static final int MSG_REGISTER_CLIENT = 1;
16. /**
17. * Command to the service to unregister a client, ot stop receiving callbacks
18. * from the service. The Message's replyTo field must be a Messenger of
19. * the client as previously given with MSG_REGISTER_CLIENT.
20. */
21. static final int MSG_UNREGISTER_CLIENT = 2;
23. /**
24. * Command to service to set a new value. This can be sent to the
25. * service to supply a new value, and will be sent by the service to
26. * any registered clients with the new value.
27. */
28. static final int MSG_SET_VALUE = 3;
30. /**
31. * Handler of incoming messages from clients.
32. */
33. class IncomingHandler extends Handler {
34. @Override
35. public void handleMessage(Message msg) {
36. switch (msg.what) {
37. case MSG_REGISTER_CLIENT:
38. mClients.add(msg.replyTo);
39. break;
40. case MSG_UNREGISTER_CLIENT:
41. mClients.remove(msg.replyTo);
42. break;
43. case MSG_SET_VALUE:
44. mValue = msg.arg1;
45. for (int i = mClients.size() - 1; i >= 0; i --) {
46. try {
47. mClients.get(i).send(Message.obtain(null,
48. MSG_SET_VALUE, mValue, 0));
49. } catch (RemoteException e) {
50. // The client is dead. Remove it from the list;
51. // we are going through the list from back to front
52. // so this is safe to do inside the loop.
53. mClients.remove(i);
54. }
55. }
56. break;
57. default:
58. super.handleMessage(msg);
59. }
60. }
61. }
63. /**
64. * Target we publish for clients to send messages to IncomingHandler.
65. */
66. final Messenger mMessenger = new Messenger(new IncomingHandler());
68. @Override
69. public void onCreate() {
70. mNM = (NotificationManager)getSystemService(NOTIFICATION_SERVICE);
72. // Display a notification about us starting.
73. showNotification();
74. }
76. @Override
77. public void onDestroy() {
78. // Cancel the persistent notification.
79. mNM.cancel(R.string.remote_service_started);
81. // Tell the user we stopped.
82. Toast.makeText(this, R.string.remote_service_stopped, Toast.LENGTH_SHORT).show();
83. }
85. /**
86. * When binding to the service, we return an interface to our messenger
87. * for sending messages to the service.
88. */
89. @Override
90. public IBinder onBind(Intent intent) {
91. return mMessenger.getBinder();
92. }
94. /**
95. * Show a notification while this service is running.
96. */
97. private void showNotification() {
98. // In this sample, we'll use the same text for the ticker and the expanded notification
99. CharSequence text = getText(R.string.remote_service_started);
101. // Set the icon, scrolling text and timestamp
102. Notification notification = new Notification(R.drawable.stat_sample, text,
103. System.currentTimeMillis());
105. // The PendingIntent to launch our activity if the user selects this notification
106. PendingIntent contentIntent = PendingIntent.getActivity(this, 0,
107. new Intent(this, Controller.class), 0);
109. // Set the info for the views that show in the notification panel.
110. notification.setLatestEventInfo(this, getText(R.string.remote_service_label),
111. text, contentIntent);
113. // Send the notification.
114. // We use a string id because it is a unique number. We use it later to cancel.
115. mNM.notify(R.string.remote_service_started, notification);
116. }
117. }