Android – canvas will not be updated (invalidated) until the end of the entire while loop

I'm trying to move a ball on the canvas. A and B are similar to X and Y coordinate positions. Any way to try to get different values dynamically from my code. A and B are global variables. But it seems that "invalidate()" or screen refresh only occurs at the end of the whole cycle. Do you know why If I have to build it on another thread, please suggest me some simple code

private void shootBall(){
    while (a>b){
        a = getPositionX();
        b = getPositionY();
        invalidate();
        }
    }
}

resolvent:

Do this and use postinvalidate() instead:

private void shootBall(){

    new Thread(new Runnable() {
        public void run() {

            while (a>b){
                a = getPositionX();
                b = getPositionY();
                postInvalidate();
            }

        }
    }).start();
}

Edit: but as mentioned earlier, don't assume that the screen is invalid. If it is marked for redrawing, the UI thread will bypass it