/**
   * 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);
  }
/*最后我們再看post中調(diào)用的另外一個(gè)方法，源碼中沒有注釋，但我們很容易看出來，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更新問題handler創(chuàng)建方式*/
  private Handler handler_post = new Handler();
  /*sendMessage方法解決UI更新問題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();
  }