private function init():void {
     ball = new Sprite();
     addChild(ball);
     ball.graphics.beginFill(0xff0000);
     ball.graphics.drawCircle(0, 0, 40);
     ball.graphics.endFill();
     ball.x = stage.stageWidth / 2;
     ball.y = stage.stageHeight / 2;
     stage.addEventListener(KeyboardEvent.KEY_DOWN, onKeyboardEvent);
    }