ActionScript　キーボードイベント

package
{
    import flash.display.Sprite;
    import flash.events.KeyboardEvent;
    import flash.events.Event;
    import flash.ui.Keyboard;

    public class Main extends Sprite
    {
        //カーソル位置
        var pos_x:Number;
        var pos_y:Number;

        public function Main()
        {
            //キーボードイベントを登録
            this.stage.addEventListener(KeyboardEvent.KEY_DOWN, onKeyDown);

            pos_x = 200;
            pos_y = 150;

            Draw();
        }

               private function onKeyDown(e:KeyboardEvent):void
        {
            switch ( e.keyCode ) {
                case Keyboard.LEFT:
                    pos_x -= 1;
                    break;
                case Keyboard.RIGHT:
                    pos_x += 1;
                    break;
                case Keyboard.UP:
                    pos_y -= 1;
                    break;
                case Keyboard.DOWN:
                    pos_y += 1;
                    break;
            }

            Draw();
        }

        private function Draw():void
        {
            //丸を描く
            this.graphics.clear();
            this.graphics.lineStyle(3, 0x0000FF);
            this.graphics.drawCircle(pos_x,pos_y,3);
        }
    }
}