Service 和 Activity 如果想互发 Message

Service 如何给 Activity 发送 Message 见《Activity 调用 Service 中的方法都有哪些方式?》题目中 Activity 跟 Service 绑定的第二种方式:Using a Messenger。
考虑到官方只给出了 Activity 给 Service 发送 Message 的代码，在这里我给出一个 Activity 跟 Service 之间互相 发送 Message 通信的示例代码:

1. MainActivity 代码

package com.example.serviceAndActivity; 
import android.os.Bundle;
import android.os.Handler;
import android.os.IBinder;
import android.os.Message;
import android.os.Messenger;
import android.os.RemoteException;
import android.app.Activity; 
import android.app.Service;
import android.content.ComponentName;
import android.content.Intent;
import android.content.ServiceConnection;
import android.util.Log;
import android.view.View;
import android.widget.Toast;
/**
* Activity 和 Service 互发 Message *
* @author wzy 2015-11-25
*
*/
public class MainActivity extends Activity {
private Messenger messenger;
//将该 Handler 发送 Service
private Messenger mOutMessenger = new Messenger(new OutgoingHandler());
@Override
protected void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      setContentView(R.layout.activity_main);
}
// 绑定服务
public void click1(View view) {
    Intent intent = new Intent(this, MessengerService.class);
    ServiceConnection conn = new MyServiceConnection();
    bindService(intent, conn, Service.BIND_AUTO_CREATE);
}
// 发送消息
public void click2(View view) throws RemoteException {
    if (messenger == null) {
        Toast.makeText(this, "服务不可用!", Toast.LENGTH_SHORT).show();
return; }
        Message message = new Message();
message.obj="长江长江我是黄河"; message.what =0; messenger.send(message);
}
class OutgoingHandler extends Handler{
    @Override
    public void handleMessage(Message msg) {
            Log.d("tag", msg.toString());
} }
class MyServiceConnection implements ServiceConnection {
    @Override
    public void onServiceConnected(ComponentName name, IBinder service) {
          Toast.makeText(MainActivity.this, "连接成功!", Toast.LENGTH_SHORT).show(); messenger = new Messenger(service);
            Message message=new Message();
            message.what = 1;
            //Activity 绑定 Service 的时候给 Service 发送一个消息，该消息的 obj 属性是一个 Messenger 对象 message.obj = mOutMessenger;
            try {
                messenger.send(message);
            } catch (RemoteException e) {
                e.printStackTrace();
        }
  }
    @Override
    public void onServiceDisconnected(ComponentName name) {
        Toast.makeText(MainActivity.this, "连接已经断开!", Toast.LENGTH_SHORT).show(); }
      }
 }

2. MessengerService 代码

package com.example.serviceAndActivity;
import android.app.Service;
import android.content.Intent;
import android.os.Handler;
import android.os.IBinder;
import android.os.Message;
import android.os.Messenger;
import android.os.RemoteException;
import android.util.Log;
/**
* 该 Service 接收到 Activity 的消息后会在返回一条消息
 * @author wzy 2015-11-25
 *
 */
public class MessengerService extends Service {
private Messenger messenger = new Messenger(new IncomingHandler()); private Messenger mActivityMessenger ;
    @Override
    public IBinder onBind(Intent intent) {
        IBinder binder = messenger.getBinder();
        return binder;
    }
//1.定义一个 Handler 对象，该 Handler 处理 Activity 发送过来的消息 class IncomingHandler extends Handler{
        @Override
        public void handleMessage(Message msg) {
            switch (msg.what) {
              case 0:
                    Log.d("tag", msg.toString());
                    if (mActivityMessenger!=null) {
                           Message message= new Message();
                           message.what = 2;
                          message.obj="地瓜地瓜我是土豆"; 
                        try {
                           mActivityMessenger.send(message);
                       } catch (RemoteException e) {
                           e.printStackTrace();
                       }
              }
                  break;
              case 1:
                      mActivityMessenger = (Messenger) msg.obj;
                      Log.d("tag", "已经获取到 Activity 发送了的 Messenger 对象");
                  break;
              default:
                  break;
           }
        } }