android - Android How to call invalidate in SurfaceView

Question

I have created a class from SurfaceView.I wanted it that when i touch on the screen then the screen will be refreshed by calling invalidate() function. But in my implementation invalidate () is not called when i touched on the screen.

Here is my code.

public class GameBoard extends SurfaceView implements Callback {

private SurfaceHolder holder;

int clicked =0;

public GameBoard(Context context) 
{
     super(context);
     holder = getHolder();
     holder.addCallback(this);

     setFocusable(true);
     requestFocus();
}


@Override
public void surfaceCreated(SurfaceHolder holder) 
{
    Canvas c = holder.lockCanvas(null);
    onDraw(c);
    holder.unlockCanvasAndPost(c);

}

@Override
public void surfaceDestroyed(SurfaceHolder holder) 
{
    // TODO Auto-generated method stub

}

@Override
public void surfaceChanged(SurfaceHolder holder, int format, int width, int height) 
{ 

}

public boolean onTouchEvent(MotionEvent event) 
{ 
    int action = event.getAction();


    if (action==MotionEvent.ACTION_UP)
    {

        clicked++;

//          Canvas c = holder.lockCanvas(null);
//          onDraw(c);
//          holder.unlockCanvasAndPost(c);


        invalidate();

    }

    return true;
}

 @Override
 protected void onDraw(Canvas canvas) 
 {
       canvas.drawColor(0xFF00ff00);    

        Paint p = new Paint();
        p.setTextSize(20);
        p.setColor( Color.RED );

        p.setAntiAlias(true);

        canvas.drawText(""+clicked, 100, 500, p);  

 }


}