handler

handler假如在主线程（ui线程）默认启动Looper，Looper会调用Looper.prepare()和Looper.looper()方法进行循环。
1.但是在子线程的话需要手动调用prepaer和looper(不用刷新ui,只是处理消息。)

Looper.prepare();
mHandler = new Handler(){
    @Override
  public void handleMessage(Message msg) {
            Log.d(TAG," mHandler is coming");
  handler_main.sendEmptyMessage(1);
  }
};
mHandler.sendEmptyMessage(1);
Looper.loop();

2.1.要刷新UI 在其它线程

Handler handler = new Handler(Looper.getMainLooper());