Android Service详解之Messenger双向通信的实现

[TOC]

文章参考：https://blog.csdn.net/SuperBigLw/article/details/53158164

一、概述

Messenger实现IPC通信,底层也是使用了AIDL方式。和AIDL方式不同的是, Messenger方式是利用Handler形式处理,因此,它是线程安全的,这也表示它不支持并发处理;而AIDL方式是非线程安全的,支持并发处理,因此,我们使用,AIDL方式时需要保证代码的线程安全。
大部分情况下, 我们应用中不需要并发处理。 因此, 我们通常只需要使用Messenger方式。
思想:在进程A中创建一个Message,将这个Message对象通过Messenger.send(message)方法传递到进程B(当然, Message对象本身是无法被传递到进程B的, send(message)方法会使用一个Parcel对象对Message对象编集,再将Parcel对象传递到进程B中,然后解编集,得到一个和进程,A中Message对象内容一样的对象) ,再把Message对象加入到进程B的消息队列里, Handler会去处理它。

二、具体步骤，双向通信的实现

1.首先创建一个Service服务，并实现一个 Handler，由其接收来自客户端的每个调用的回调
（这个是接收时的代码）

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();
                System.out.println(msg.getData().getString("info"));
                
                break;
            default:
                super.handleMessage(msg);
        }
    }
}

（实现双向通信的代码，接收后发送给客户端）

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();
                 System.out.println(msg.getData().getString("info"));
                 //实现双向通讯
                 Messenger client = msg.replyTo;
                 Message replyMessage = Message.obtain(null, MSG_SAY_HELLO);
                 for(int i = 0;i<20;i++){
                     try {
                         Thread.sleep(500);
                         Bundle bundle = new Bundle();
                         bundle.putString("reply", "嗯嗯，我已经收到你的第"+i+"条信息，稍后回复你。");
                         replyMessage.setData(bundle);
                         try {
                             client.send(replyMessage);
                         } catch (RemoteException e) {
                             e.printStackTrace();
                         }
                     } catch (InterruptedException e) {
                         e.printStackTrace();
                     }
                 }
 
                 break;
             default:
                 super.handleMessage(msg);
         }
     }
 }

2.Handler 用于创建 Messenger 对象（对 Handler 的引用）

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

3.Messenger 创建一个 IBinder，服务通过 onBind() 使其返回客户端

@Override
    public IBinder onBind(Intent intent) {
        Toast.makeText(getApplicationContext(), "binding", Toast.LENGTH_SHORT).show();
        return mMessenger.getBinder();
    }

4.上述完整代码

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();
                    System.out.println(msg.getData().getString("info"));
                    //实现双向通讯
                    Messenger client = msg.replyTo;
                    Message replyMessage = Message.obtain(null, MSG_SAY_HELLO);
                    for(int i = 0;i<20;i++){
                        try {
                            Thread.sleep(500);
                            Bundle bundle = new Bundle();
                            bundle.putString("reply", "嗯嗯，我已经收到你的第"+i+"条信息，稍后回复你。");
                            replyMessage.setData(bundle);
                            try {
                                client.send(replyMessage);
                            } catch (RemoteException e) {
                                e.printStackTrace();
                            }
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }
 
                    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();
    }
}

5.客户端使用 IBinder 将 Messenger（引用服务的 Handler）实例化，然后使用后者将 Message 对象发送给服务
（1）定义一个Messenger对象，Messenger mService = null;并在绑定成功后实例化

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;
}

（2）声明一个Handler用来处理服务端的数据

private class MessengerHandler extends Handler {
       @Override
       public void handleMessage(Message msg) {
           switch (msg.what) {
               case MessengerService.MSG_SAY_HELLO:
                   System.out.println("receive from Service:" + msg.getData().get("reply"));
                   TextView textView = new TextView(getApplicationContext());
                   textView.setTextColor(Color.BLACK);
                   textView.setText("receive from Service:" + msg.getData().get("reply"));
                   linearLayout.addView(textView);
                   break;
               default:
                   super.handleMessage(msg);
           }
       }
   }

（3）声明一个Messenger对象

private Messenger mGetReplyMessenger = new Messenger(new MessengerHandler());

（4）创建一个Message对象并用Messenger发送给服务

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);
        Bundle bundle = new Bundle();
        bundle.putString("info","呵呵哒");
        msg.setData(bundle);
        msg.replyTo = mGetReplyMessenger;
        try {
            mService.send(msg);
        } catch (RemoteException e) {
            e.printStackTrace();
        }
    }

并用msg.replyTo得到Messenger实例