Android ApiDemos示范解析(41)：App->Service->Messenger Service

Android ApiDemos示例解析(41)：App->Service->Messenger Service

前面LocalService 主要是提供同一Application中组件来使用，如果希望支持不同应用或进程使用Service。可以通过Messenger。使用Messgener可以用来支持进程间通信而无需使用AIDL。

下面步骤说明里Messenger的使用方法：

在Service中定义一个Handler来处理来自Client的请求。使用这个Handler创建一个Messenger (含有对Handler的引用).Messenger创建一个IBinder对象返回给Client( onBind方法）。Client 使用从Service返回的IBinder重新构造一个Messenger 对象，提供这个Messenger对象可以给Service 发送消息。Service提供Handler接受来自Client的消息Message. 提供handleMessage来处理消息。

在这种方式下，Service没有定义可以供Client直接调用的方法。而是通过”Message”来传递信息。

本例Messenger Service 涉及到两个类 MessengerServiceActivities 和 MessengerService.

首先看看Service的定义，在MessengerService定义了一个IncomingHandler ，用于处理来自Client的消息。

/** * 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); } }}

然后使用这个IncomingHandler定义一个Messenger。

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

应为这种方法采用的“Bound” Service模式，onBind 需要返回一个IBind对象, 可以通过mMessenger.getBinder()返回与这个Messenger关联的IBinder对象，Client可以通过这个IBinder对象重新构造一个Messenger对象，从而建立起与Service之间的通信链路。

@Overridepublic IBinder onBind(Intent intent) { return mMessenger.getBinder();}

再看看Client 的代码MessengerServiceActivities 在 ServiceConnection的 onServiceConnected的定义，这个方法返回MessengerService 的onBind 定义的IBinder对象：

/** Messenger for communicating with service. */ Messenger mService = null; ....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. } ....

本例实现了Client与Service 之间的双向通信，因此在Client也定义了一个Messenger对象mMessenger，用于处理来自Service的消息。

有了 mService对象，就可以使用send向Service发送消息，如过需要Service 返回信息，可以定义message.replyTo 对象。

 

 

 

 

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