什么是Handler(四)

1.Handler的post(Runnable)方法

2.Runnable的运行机制

3.post方法和sendMessage方法的区别

1.Handler的post(Runnable)方法

public static class PlaceholderFragment extends Fragment {
        
        private Button button;
        private Handler handler = new Handler();  //没有像以前那样写MyHandler方法

        public PlaceholderFragment() {
        }

        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container,
                Bundle savedInstanceState) {
            View rootView = inflater.inflate(R.layout.fragment_main, container, false);
            
            button = (Button)rootView.findViewById(R.id.buttonId);
            button.setOnClickListener(new OnClickListener() {                
                @Override
                public void onClick(View v) {
                    TestThread tt = new TestThread();
                    tt.start();
                }
            });
            
            return rootView;
        }
        
        class TestThread extends Thread{

            @Override
            public void run() {
                Runnable r = new Runnable() {                    
                    @Override
                    public void run() {
                        Log.i("tag","CuurentThread--->" + Thread.currentThread().getName());                        
                    }
                };
                handler.post(r);
            }         
        }
    }

　　打印结果很劲爆

　　

　　意味着Runnable竟然运行在主线程中！！！

　　看源码, Handler的post方法

public final boolean post(Runnable r)
    {
       return  sendMessageDelayed(getPostMessage(r), 0);
    }

　　先看getPostMessage(r)

private static Message getPostMessage(Runnable r) {
        //该方法完成两个操作：1、生成了一个Message对象。2、将r赋值给Message的callback属性
        Message m = Message.obtain();
        m.callback = r;  //Message的callback属性
        return m;
    }

　　因此Handler的post方法相当于下面

public final boolean post(Runnable r)
    {
        //return sendMessageAtTime(getPostMessage(r), 0);
        Meesage msg = getPostMessage(r);
        return sendMessageDelayed(msg, 0);
    }

　　上述中sendMessageDelayed是干嘛的呢

     * Enqueue a message into the message queue after all pending messages
     * before (current time + delayMillis). You will receive it in
     * {@link #handleMessage}, in the thread attached to this handler.
     *  
     * @return Returns true if the message was successfully placed in to the 
     *         message queue.  Returns false on failure, usually because the
     *         looper processing the message queue is exiting.  Note that a
     *         result of true does not mean the message will be processed -- if
     *         the looper is quit before the delivery time of the message
     *         occurs then the message will be dropped.
     */
    public final boolean sendMessageDelayed(Message msg, long delayMillis)
    {
        if (delayMillis < 0) {
            delayMillis = 0;
        }
        return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);
    }

　　也就是延迟发送, 当delayMillis=0即相当于直接发送

　　综上：生成一个Message对象, 将r赋值给Message对象的callback属性, 然后再将Message对象放置在消息队列当中

　　　　   Looper取出了携带有r对象的Message对象之后, 调用dispatchMessage方法, 判断Message的callback属性是否有值, 此时callback属性有值,

　　　　　所以执行了handCallback(Message msg),在该方法中执行msg.callback.run();

　　　　  TestThread中从网络取回数据, 定义一个Runnable对象, 在其run()方法中直接写更新UI的代码,

　　　　  再用主线程中生成的handler进行post出去, post方法中生成Message对象, r赋值给Meesage的callback属性,

　　　　  Message放入消息队列之中, Looper取出, 调用dispatchMessage方法,

              在dispatchMessage中调用handlercallback方法, 在handlercallback中会执行Runnable的run()方法。

　　        因为Looper在主线程, 所以dispatchMessage啊, handlercallback啊, 也都会在主线程.同样run()方法也是的