Android ApiDemos示例解析(39)：App-Service-Local Service Binding

本例和下列Local Service Controller 的Activity代码都定义在LocalServiceActivities.Java 中，作为LocalServiceActivities 内部类实现的。调用的Service为LocalService。

LocalService既可以做为“Started” Service，也可以做为”Bound” Service。

一个“Bound” Service 可以通过Client/Service模式提供Service。它运行应用程序组件（比如Activity）与之绑定，然后接受请求并返回响应或者提供进程间通信机制，它的生命周期通常与调用它的组件（如Activity）相同。而对于LocalService即作为“Started” Service又作为“Bound”Service，如果LocalService已作为“Started” Service启动，中即使所有Client都断开与它的绑定，LocalService也不会停止。

如果一个Service需要作为“Bound”Service运行其它组件与之绑定，就必须实现onBind方法。这个方法返回一个IBound对象给Client。Client可以通过这个IBind对象来调用Service的方法。

Client可以通过bindService来实现与“Bound”Service的绑定，Service 与 Client 的绑定是异步实现的，因此Client 需要通过 ServiceConnection 接口来监视与Service之间的连接。 Client 调用bindService 时，bindService 立即返回，之后当Android系统为Client 和Service之间建立起链接后调用 ServiceConnection 的 onServiceConnection 来通知Client 与Service之间的链接建立成功，并给Client返回 Service 提供的IBind对象。

LocalService 只提供给同一个Application的组件使用，不提供进程间通信接口，因此只需要派生一个Binder的子类如果直接的函数调用接口。

// Class for clients to access.  Because we know// this service always runs in the same process as//its clients, we don't need to deal with IPC.
public class LocalBinder extends Binder {LocalService getService() {return LocalService.this;}
}

LocalBinder只提供了一个方法getService ，返回LocalService 对象自身。

LocalService的 onBind方法定义：

@Override
public IBinder onBind(Intent intent) {return mBinder;
}// This is the object that receives interactions from clients.  See
// RemoteService for a more complete example.
private final IBinder mBinder = new LocalBinder();

onBind的返回值为mBinder 为 LocalBinder类对象。它定义了一个方法getService。

再来看看Client类 LocalServiceActivities.Binding 的实现:

private LocalService mBoundService;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.  Because we have bound to a explicit// service that we know is running in our own process, we can// cast its IBinder to a concrete class and directly access it.mBoundService = ((LocalService.LocalBinder)service).getService();// Tell the user about this for our demo.Toast.makeText(Binding.this, R.string.local_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.// Because it is running in our same process, we should never// see this happen.mBoundService = null;Toast.makeText(Binding.this, R.string.local_service_disconnected,Toast.LENGTH_SHORT).show();}
};

mConnection定义了ServiceConnection接口，onServiceConnected ，onServiceDisconnected分别在Service与Client 之间建立链接和断开链接时调用。其中IBinder service 就是 Service 的onBind的返回对象。这里将其通过类型转换为LocalService也就是mBoundService。

Client 通过调用bindService建立与Service之间的绑定：

bindService(new Intent(Binding.this,LocalService.class), mConnection, Context.BIND_AUTO_CREATE);

如果需要断开与Service之间的绑定，则调用unbindService.

unbindService(mConnection);

如果你熟悉Socket编程, Client 绑定”Bound”Service 的方法和使用Socket通信非常类似。

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