/**   * Causes the Runnable r to be added to the message queue.   * 這里源碼注釋的意思是：把r這個(gè)任務(wù)對(duì)象添加到消息隊(duì)列中。   */  public final boolean post(Runnable r)  {    return sendMessageDelayed(getPostMessage(r), 0);  }/**   * Enqueue a message into the message queue after all pending messages*然后我們接著看post方法中直接調(diào)用到的發(fā)送延時(shí)消息的方法，源碼注釋*的意思是把這個(gè)消息放入消息隊(duì)列，   */  public final boolean sendMessageDelayed(Message msg, long delayMillis)  {    if (delayMillis < 0) {      delayMillis = 0;    }    return sendMessageAtTime(msg, SystemClock.uptimeMillis() + delayMillis);  }/*最后我們?cè)倏磒ost中調(diào)用的另外一個(gè)方法，源碼中沒(méi)有注釋，但我們很容易看出來(lái)，這個(gè)方法就是把r這個(gè)任務(wù)包裝成了一個(gè)空的消息并且返回*/private static Message getPostMessage(Runnable r) {    Message m = Message.obtain();    m.callback = r;    return m;}

private TextView tv_up;  private String new_str = "";  /*post方法解決UI更新問(wèn)題handler創(chuàng)建方式*/  private Handler handler_post = new Handler();  /*sendMessage方法解決UI更新問(wèn)題handler創(chuàng)建方式*/  Handler handler_senM = new Handler() {    public void handleMessage(Message msg) {      if (msg.what == 1) {        /*sendMessage方法更新UI的操作必須在handler的handleMessage回調(diào)中完成*/        tv_up.setText(new_str);      }    };  };  @Override  protected void onCreate(Bundle savedInstanceState) {    super.onCreate(savedInstanceState);    setContentView(R.layout.activity_main);    new Thread(new Runnable() {      @Override      public void run() {        new_str = "更新UI";        /*sendMessage方法解決UI更新發(fā)送消息給handler（主線程中的handler）*/        handler_senM.sendEmptyMessage(1);        /*post方法解決UI更新，直接在runnable里面完成更新操作，這個(gè)任務(wù)會(huì)被添加到handler所在線程的消息隊(duì)列中，即主線程的消息隊(duì)列中*/        handler_post.post(new Runnable() {          @Override          public void run() {            tv_up.setText(new_str);          }        });      }    }).start();  }