Flex脚本实例 动画加速效果

package {
    import flash.display.Sprite;
    import flash.events.Event;
   
    public class Main extends Sprite {
        private var _sprite:Sprite = new Sprite(  );
        private var _ax:Number = .2;
        private var _ay:Number = .1;
        private var _vx:Number = 0;
        private var _vy:Number = 0;
       
        public function Main(  ) {
            _sprite.graphics.beginFill(0x0000ff, 100);
            _sprite.graphics.drawCircle(0, 0, 25);
            _sprite.x = 5;
            _sprite.y = 10;
            addChild(_sprite);
            addEventListener(Event.ENTER_FRAME, onEnterFrame);
        }
       
        public function onEnterFrame(event:Event):void {
            _vx += _ax;
            _vy += _ay;
            _sprite.x += _vx;
            _sprite.y += _vy;
        }
    }   
}