Android之使用Messenger進行IPC

看下官網對Messenger（信使）的介紹：

主要告訴我們可以參考Handler對message的使用一樣，通過Messenger對Message的使用，達到進程間通訊的效果。

爲了方便我們在一個工程裏通過將Service設置爲不同的進程，來進行IPC。
服務端的代碼如下：

public class MessengerService extends Service {

    private static final String TAG = "MessengerService";

    private static class MessengerHandler extends Handler{
        @Override
        public void handleMessage(Message msg) {
            switch (msg.what){
                case MSG_FROM_CLIENT:
                    Log.i(TAG , "receive msg from client:" + msg.getData().getString("msg"));
                    break;
                default:
                    super.handleMessage(msg);
            }
        }
    }

    private final Messenger mMessenger = new Messenger(new MessengerHandler());

    @Nullable
    @Override
    public IBinder onBind(Intent intent) {
        return mMessenger.getBinder();
    }

然後在Manifest裏聲明該Service，並指定爲另外一個進程：

<service android:name=".messenger.MessengerService"
            android:process=":remote"/>

接着是Actvity裏的代碼：

public class MessengerActivity extends AppCompatActivity {

    private static final String TAG = "MessengerActivity";
    private Messenger mService;

    private ServiceConnection mConnection = new ServiceConnection() {
        @Override
        public void onServiceConnected(ComponentName name, IBinder service) {
            mService = new Messenger(service);
            Message message = Message.obtain(null, Constant.MSG_FROM_CLIENT);
            Bundle data = new Bundle();
            data.putString("msg", "hello , this is client.");
            message.setData(data);
            try {
                mService.send(message);
            } catch (RemoteException e) {
                e.printStackTrace();
            }
        }

        @Override
        public void onServiceDisconnected(ComponentName name) {

        }
    };

    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_messenger);
        Intent intent = new Intent(this, MessengerService.class);
        bindService(intent, mConnection, Context.BIND_AUTO_CREATE);
    }

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

大概的流程是這樣的：
1.首先我們創建了一個服務MessengerService，用於處理接收客戶端發來的信息。在當我們新創建這個Service的時候，它只有一個onBind的方法，該方法需要我們返回一個IBinder，而在這裏我們通過Messenger的getBinder方法可以將其返回，以供到時客戶端去調用。
2.我們在MessengerService裏創建了一個Handler與Messenger關聯，是爲了處理客戶端到時傳過來的Message。
3.在客戶端裏我們通過bindService與MessengerService建立聯繫，一旦聯繫建立了，就會調用ServiceConnection裏的方法，我們可以看到onServiceConnected的參數裏有一個IBinder，那麼這個就是我們在MessengerService的onBind方法裏返回的。
4.此時我們根據服務端的這個IBinder對象來創建一個Messenger對象，然後攜帶Message進行發送，這樣服務端那邊也就會接收到客戶端發來的消息了。

---

那麼上面是客戶端發送消息給服務端，現在服務端接收到消息裏，需要給客戶端進行回覆，
在原來服務端的handleMessage裏的接收到客戶端的消息後，添加回應。

public class MessengerService extends Service {

    private static final String TAG = "MessengerService";

    private static class MessengerHandler extends Handler{
        @Override
        public void handleMessage(Message msg) {

            switch (msg.what){
                case MSG_FROM_CLIENT:
                    Log.i(TAG , "receive msg from client:" + msg.getData().getString("msg"));
                    //回覆客戶端
                    Messenger client = msg.replyTo;
                    Message replyMessage = Message.obtain(null , MSG_FROM_SERVICE);
                    Bundle bundle = new Bundle();
                    bundle.putString("reply" , "Well , I will reply you later");
                    replyMessage.setData(bundle);
                    try {
                        client.send(replyMessage);
                    } catch (RemoteException e) {
                        e.printStackTrace();
                    }

                    break;
                default:
                    super.handleMessage(msg);
            }

        }
    }

    private final Messenger mMessenger = new Messenger(new MessengerHandler());

    @Nullable
    @Override
    public IBinder onBind(Intent intent) {
        return mMessenger.getBinder();
    }
}

從msg.replyTo裏獲取到客戶端傳來的Messenger。
那麼客戶端要做如下修改：

public class MessengerActivity extends AppCompatActivity {

    private static final String TAG = "MessengerActivity";
    private Messenger mService;

    private Messenger mGetReplyMessenger = new Messenger(new MessengerHandler());
    //與在服務端的寫法一樣，都是爲了處理信息
    private static class MessengerHandler extends Handler {
        @Override
        public void handleMessage(Message msg) {
            switch (msg.what) {
                case Constant.MSG_FROM_SERVICE:
                    Log.i(TAG, "receive msg from Service:" + msg.getData().getString("reply"));
                    break;
                default:
                    super.handleMessage(msg);
            }

        }
    }


    private ServiceConnection mConnection = new ServiceConnection() {
        @Override
        public void onServiceConnected(ComponentName name, IBinder service) {
            mService = new Messenger(service);
            Message message = Message.obtain(null, Constant.MSG_FROM_CLIENT);
            Bundle data = new Bundle();
            data.putString("msg", "hello , this is client.");
            message.setData(data);
            //在原來的Message裏添加Messenger，用於給服務端指定回覆
            message.replyTo = mGetReplyMessenger;
            try {
                mService.send(message);
            } catch (RemoteException e) {
                e.printStackTrace();
            }
        }

        @Override
        public void onServiceDisconnected(ComponentName name) {

        }
    };

    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_messenger);
        Intent intent = new Intent(this, MessengerService.class);
        bindService(intent, mConnection, Context.BIND_AUTO_CREATE);
    }

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