Android -- Messager与Service
如果你需要你的service和其他进程通信，那么你可以使用一个Messenger来提供这个接口。
这种方法允许你在不使用 AIDL的情况下，进行跨进程通信IPC。
实现步骤
下面是一个如何使用 Messenger的小总结：
1. service实现一个 Handler 接收客户端每一次调用的回调。
2. Handler 用来创建一个Messenger对象，它是一个Handler的引用。
3. Messenger创建一个 IBinder，service从 onBind()中把它返回给客户端。
4. 客户端使用这个IBinder来实例化Messenger （service的Handler的引用），客户端使用它来向service发送Message对象。
5. service在它的Handler中接收每一个Message对象，在它的 handleMessage()方法中。
Code
public class MessengerService extends Service
{
/** Command to the service to display a message */
static final int MSG_SAY_HELLO = 1;
/**
* Handler of incoming messages from clients.
*/
class IncomingHandler extends Handler
{
@Override
public void handleMessage(Message msg)
{
switch (msg.what)
{
case MSG_SAY_HELLO:
Toast.makeText(getApplicationContext(), "hello!",
Toast.LENGTH_SHORT).show();
break;
default:
super.handleMessage(msg);
}
}
}
/**
* Target we publish for clients to send messages to IncomingHandler.
*/
final Messenger mMessenger = new Messenger(new IncomingHandler());
/**
* When binding to the service, we return an interface to our messenger for
* sending messages to the service.
*/
@Override
public IBinder onBind(Intent intent)
{
Toast.makeText(getApplicationContext(), "binding", Toast.LENGTH_SHORT)
.show();
return mMessenger.getBinder();
}
}
注意 Handler中的 handleMessage() 方法是service接收到来的 Message并且决定做什么的地方。
客户端需要做的仅仅是创建一个基于service所返回的 IBinder的 Messenger，然后用 send()方法发送信息。
比如，这里有一个简单的activity和service绑定并且发送信息给service：
public class ActivityMessenger extends Activity
{
/** Messenger for communicating with the service. */
Messenger mService = null;
/** Flag indicating whether we have called bind on the service. */
boolean mBound;
/**
* 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 object we can use to
// interact with the service. We are communicating with the
// service using a Messenger, so here we get a client-side
// representation of that from the raw IBinder object.
mService = new Messenger(service);
mBound = true;
}
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;
mBound = false;
}
};
public void sayHello(View v)
{
if (!mBound)
return;
// Create and send a message to the service, using a supported 'what'
// value
Message msg = Message
.obtain(null, MessengerService.MSG_SAY_HELLO, 0, 0);
try
{
mService.send(msg);
}
catch (RemoteException e)
{
e.printStackTrace();
}
}
@Override
protected void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
}
@Override
protected void onStart()
{
super.onStart();
// Bind to the service
bindService(new Intent(this, MessengerService.class), mConnection,
Context.BIND_AUTO_CREATE);
}
@Override
protected void onStop()
{
super.onStop();
// Unbind from the service
if (mBound)
{
unbindService(mConnection);
mBound = false;
}
}
}
注意这个例子并没有展示service如何响应客户端，如果你想要service响应，你需要在客户端中创建一个 Messenger。
然后当客户端接收到onServiceConnected()回调方法时，它会发送一个 Message到service，在它的send() 方法的replyTo参数中包含了客户端的Messenger。