Android IPC通信之Messenger

Messenger可以译为信使，顾名思义，可以通过它在不同进程中传递Message对象，在Message中放入我们需要传递的数据，就可以轻松地实现数据的进程间传递了。Messenger是一种轻量级的IPC方案，底层实现是AIDL，使用起来也很简单。首先需要在服务端进程中创建一个Service来处理客户端的连接请求，同时创建出一个Handler并通过它来创建一个Messenger对象，然后在Service的onBind中返回这个Messenger对象底层的Binder即可；接着在客户端进程中绑定服务端的Service，绑定成功后用服务端的IBinder对象创建出一个Messenger，通过这个Messenger就可以向服务端发送消息了，发消息类型为Message对象。如果需要服务端能够回复客户端，就和服务端一样，还需要创建一个Handler并创建一个Messenger，并把这个Messenger对象通过Message的replyTo参数传递给服务端，服务端通过这个replyTo参数就可以回复客户端。下面通过简单的例子来讲解：

//服务端代码，需要在AndroidManifest里面注册服务
public class MessengerService extends Service {
    private static final String TAG = "MessengerService";
    //Handler处理客户端发过来的消息
    private static class MessengerHandler extends Handler {
        @Override
        public void handleMessage(Message msg) {
            switch (msg.what) {
                case 0:
                    Log.e(TAG,"receive msg fron client:"+msg.getData().getString("msg"));
                    //拿到客户端Messenger对象，回复消息
                    Messenger client = msg.replyTo;
                    Message replyMessage = Message.obtain(null,1);
                    Bundle bundle = new Bundle();
                    bundle.putString("reply","okay, i have got you request");
                    replyMessage.setData(bundle);
                    try {
                        client.send(replyMessage);
                    } catch (RemoteException e) {
                        e.printStackTrace();
                    }
                    break;
                default:
                    super.handleMessage(msg);
            }

        }
    }
    //创建Messenger对象，构造函数如下
    //public Messenger(Handler target) {
    //    mTarget = target.getIMessenger();
    //}
    private final Messenger messenger = new Messenger(new MessengerHandler());

    //返回Messenger对象的底层Binder
    @Nullable
    @Override
    public IBinder onBind(Intent intent) {
        return messenger.getBinder();
    }
}

// 客户端代码
public class MainActivity extends AppCompatActivity {
    private static final String TAG = "MainActivity";
    private Messenger mService;
    //实例化ServiceConnection对象
    private ServiceConnection con = new ServiceConnection() {
        @Override
        public void onServiceConnected(ComponentName name, IBinder service) {
            //创建Messenger对象，构造函数如下
            //public Messenger(IBinder target) {
            //mTarget = IMessenger.Stub.asInterface(target);
            //}
            mService = new Messenger(service);

            Message msg = Message.obtain(null,0);
            Bundle bundle = new Bundle();
            bundle.putString("msg","hello,this is from client");
            msg.setData(bundle);
            msg.replyTo = replyMessenger;
            try {
                mService.send(msg);
            } catch (RemoteException e) {
                e.printStackTrace();
            }
        }
        @Override
        public void onServiceDisconnected(ComponentName name) {

        }
    };

    //Messenger对象接收服务端消息
    private Messenger replyMessenger = new Messenger(new MessengerHandler());

    private static class MessengerHandler extends Handler {
        @Override
        public void handleMessage(Message msg) {
            super.handleMessage(msg);
            if (msg.what == 1) {
                Log.e(TAG,"receive msg fron server:"+msg.getData().getString("reply"));
            }
        }
    }
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Intent intent = new Intent(this,MessengerService.class);
        bindService(intent,con, Context.BIND_AUTO_CREATE);
    }

    @Override
    protected void onDestroy() {
        unbindService(con);
        super.onDestroy();
    }
}

如下为Messenger原理图：