Android : Handler Update UI Thread

Handler allows us to send and process Message and Runnable Objects associated with a thread's MessageQueue.

Why Handler?:
There are two main uses for a Handler:
a)- To Schedule Messages and Runnables to be executed as some point in the future.
b)- To enqueue an action to be performed on a different Thread than our own.

Message?:
Defines a message containing a description and orbitrary data object that can be sent to a Handler.this object contaions two extra fields.

Looper :
It's designed to make an ordinary thread into a Looper Thread.We need this this type of thread because we often need a thread which continuously cycle.if there is a new task execute them and continue to wait for the next task. Looper internally maintains a message queue(Linked Queue).

     NOTE: A Thread can only have a Looper object.

Class used to run a message loop for a thread. Threads by default do not have a Message loop associated with them; to create one, call prepare() in the thread that is to run loop, and then loop() to have it process messages until the loop is stopped.

          Class LooperThread extends Thread{
            public Handler mHandler;
            public void run(){
              Looper.prepare();
              mHandler=new Hanlder(){
                public void handleMessage(Message msg){
                   //process incomming messages here
                 }
              };
              Looper.loop();
            }
          }

Each Handler instance is associated with a single thread and that thread's message queue.

The Handler is the middleman between a new thread and the message queue.


   Note:- When we Create a new Handler,it's bound to the Thread/ MessageQueue of the Thread that is creating it-from that point on,it will deliver messages and runnables to that message queue and execute them as they come out of the queue.


Handler Ball Moving Example:

HandlerUIExample.java :

package com.imrankhanandroid.HandlerExample;

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

public class HandlerUIExample extends Activity {
    /** Called when the activity is first created. */
 protected static final int UiUpadte=0x1001;
 Thread myThread=null;
 BounceBallView UiBounceBallview=null;
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        this.requestWindowFeature(Window.FEATURE_NO_TITLE);
        this.UiBounceBallview=new BounceBallView(this);
        setContentView(this.UiBounceBallview);
        new Thread(new myThread()).start();
    }
    Handler Testhandler=new Handler(){
     public void handleMessage(Message msg){
      switch(msg.what)
      {
      case HandlerUIExample.UiUpadte:
       UiBounceBallview.invalidate();
       break;
      }
      super.handleMessage(msg);
     }
    };
    class myThread implements Runnable{
    /*Worker Thread for Updating UI */
  @Override
  public void run() {
   // TODO Auto-generated method stub
   while(!Thread.currentThread().isInterrupted()){
    Message mesg=new Message();
    mesg.what=HandlerUIExample.UiUpadte;
    HandlerUIExample.this.Testhandler.
                                sendMessage(mesg);
    try{
     Thread.sleep(100);
    }
    catch (InterruptedException  e) {
     // TODO: handle exception
     Thread.currentThread().interrupt();
    }
   }
  }
     
    }
}

BounceBallView.java :

package com.imrankhanandroid.HandlerExample;

import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.view.View;

public class BounceBallView extends View {
 float x=20;
 float y=260;
 float xy=230;
 float yx=140;
 float yz=60;
 float yt=0;

 public BounceBallView(Context context) {
  super(context);
  // TODO Auto-generated constructor stub
 }
  @Override
  protected void onDraw(Canvas canvas)
  {

   Paint mPaint =new Paint();
   mPaint.setAntiAlias(true);
   mPaint.setColor(Color.RED);
   canvas.drawCircle(x, yz, 30, mPaint);
   mPaint.setColor(Color.GREEN);
   canvas.drawCircle(x, yx, 30, mPaint);
   mPaint.setColor(Color.YELLOW);
   canvas.drawCircle(x, xy, 30, mPaint);
   if(x==y & x>20 )
   {
    x=y-20;
    y-=20;
    
    yt=yz;
    yz=yx;
    yx=yt; 
   }
   else
   {
    yt=yz;
    yz=yx;
    yx=yt;
    x+=10;
    y=260;
   }
  }
}

Download Source Code: HandlerExample