使用Handler实现Activity和Service之间的交互

在开发应用的时候，我们有时候需要Service来处理后台任务。同时需要两者之间的实时交互，比如说下载的时候我们需要实时地更新Notification的下载进度。我们可以通过BroadcastReceiver来实现Service和Activity的交互，但是今天我们来介绍另外一种实现方法。使用Handler实现Activity和Service之间的交互。

实现的原理：

1.在Service中的onCreate方法中创建Service端的Handler和Messenger对象，并且在handleMessage方法中获取Activity端的Messenger

2.在Service的onBind方法中返回Messenger的Binder对象

3.在Activity中创建一个Handler对象，用来处理消息

4.在Activity中创建一个ServiceConnection对象，并且在onServiceConnected方法中，获取Service端的Messenger对象

5.在Activity创建Messenger，并封装在Message中传递给Service端

文字不理解可以看图并结合代码来思考

public class MessengerService extends Service{

    private Messenger mActivityMessenger;

    private Handler handler;

    private Messenger mServiceMessenger;


    @Override
    public void onCreate() {
        super.onCreate();
        Log.i("TAG", "onCreate()");

        /**
         * HandlerThread是Android系统专门为Handler封装的一个线程类，
           通过HandlerThread创建的Hanlder便可以进行耗时操作了
         * HandlerThread是一个子线程,在调用handlerThread.getLooper()之前必须先执行
         * HandlerThread的start方法。
         */
        HandlerThread handlerThread = new HandlerThread("serviceCalculate");
        handlerThread.start();

        
        handler = new Handler(handlerThread.getLooper()){
            @Override
            public void handleMessage(Message msg) {
                if(msg.what == 0x11){

                    if(mActivityMessenger == null) {
                        mActivityMessenger = msg.replyTo;
                    }

                    //模拟耗时任务
                    try {
                        Thread.sleep(10000);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }


                    //发送结果回Activity
                    Message message = this.obtainMessage();
                    message.what = 0x12;
                    message.arg1 = msg.arg1 + msg.arg2;
                    try {
                        mActivityMessenger.send(message);
                    } catch (RemoteException e) {
                        e.printStackTrace();
                    }
                }
            }
        };

        mServiceMessenger = new Messenger(handler);
    }

    @Override
    public IBinder onBind(Intent intent) {
        Log.i("TAG","onBind()");
        return mServiceMessenger.getBinder();
    }
}

public class ScrollingActivity extends AppCompatActivity {
    private Button sendToServiceBtn;

    //Service端的Messenger对象
    private Messenger mServiceMessenger;

    //Activity端的Messenger对象
    private Messenger mActivityMessenger;

    /**
     * Activity端的Handler处理Service中的消息
     */
    private Handler handler = new Handler(){
        @Override
        public void handleMessage(Message msg) {
            if(msg.what == 0x12){
                Toast.makeText(ScrollingActivity.this, "Service发送过来的结果是......"
        + msg.arg1,Toast.LENGTH_SHORT).show();
            }
        }
    };

    /**
     * Service绑定状态的监听
     */
    private ServiceConnection connection = new ServiceConnection() {
        @Override
        public void onServiceConnected(ComponentName name, IBinder service) {
            //获取Service端的Messenger
            mServiceMessenger = new Messenger(service);
        }


        @Override
        public void onServiceDisconnected(ComponentName name) {

        }
    };


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_scrolling);

        sendToServiceBtn = (Button) findViewById(R.id.btn_sendToService);
        sendToServiceBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                //Activity端的Messenger
                if(mActivityMessenger == null) {
                    mActivityMessenger = new Messenger(handler);
                }


                //创建消息
                Message message = Message.obtain();
                message.what = 0x11;
                message.arg1 = 2016;
                message.arg2 = 1;

                //设定消息要回应的Messenger
                message.replyTo = mActivityMessenger;

                try {
                    //通过ServiceMessenger将消息发送到Service中的Handler
                    mServiceMessenger.send(message);
                } catch (RemoteException e) {
                    e.printStackTrace();
                }
            }
        });

        //绑定Service
        Intent intent = new Intent(ScrollingActivity.this, MessengerService.class);
        bindService(intent, connection, Service.BIND_AUTO_CREATE);

    }

}

接下来几篇文章会介绍Message MessageQueue Looper Handler的流程和源代码