Handler and Post Message on Android

How to use handler and message when we need to control UI in another work thread.

note.  UI control is available only in main thread(UI thread).



1. create handler in main thread

import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.app.Activity;

public class main extends Activity {

  public Handler _handler = new Handler() {
   @Override
   public void handleMessage(Message msg) {
             // where the main activity thread receives messages
             // do what we want
    showMyDialog();
    super.handleMessage(msg);
   }

  };
  
 public void showMyDialog()
 {
  //show ui
  
 }
  
 public void onCreate(Bundle savedInstanceState)         
    {
  super.onCreate(savedInstanceState);
     //...
    }
}

2. use Message in other work thread

//in another thread
 public void sendMessageToShowDialog() {
//--------------------------
  //call showMyDialog() directly will cause exception because here is not in ui thread.
  //m_parents.showMyDialog();
  //-------------------------
  
  //-----------------------------
  //send message to show dialog
  Message msg = Message.obtain();
  //User-defined message code
  msg.what = 999;
  m_parents._handler.sendMessage(msg);
  //------------------------------
 }