[ActionScript 3.0] AS3.0 对象在矩形范围随机运动

package com.views
{

    import flash.display.Bitmap;
    import flash.display.MovieClip;
    import flash.display.Sprite;
    import flash.events.Event;

    /**
     * @author:Frost.Yen
     * @E-mail:871979853@qq.com
     * @create:    2016-7-7 下午2:33:56
     *
     */
    public class LogoView extends Sprite
    {
        private var _stageW:Number = 1920;
        private var _stageH:Number = 1080;
        private var _mc:MovieClip = new MovieClip();
        public function LogoView(bmp:Bitmap)
        {
            _mc.addChild(bmp);
            bmp.x = -bmp.width*0.5;
            bmp.y = -bmp.height*0.5;
            initLogo();
        }

        private function initLogo():void{

            //随机位置
            _mc.x = Math.random()*_stageW;
            _mc.y = Math.random()*_stageH;
            //随机速度和方向
            Math.random()>0.5?_mc.speedX = Math.random()*5+1 : _mc.speedX = -1 * Math.random()*5+1
            Math.random()>0.5?_mc.speedY = Math.random()*5+1 : _mc.speedY = -1 * Math.random()*5+1
            //因为logo的坐标原点是logo的中心，所以用该函数用来调整logo位置，防止移出边界
            adjustBall(_mc);
            //添加logo到显示列表
            addChild(_mc);
            //用ENTER_FRAME侦听器使logo运动
            //startMove();
        }

        private function adjustBall(mc:MovieClip):void{
            if(mc.x > _stageW-mc.width/2){mc.x = _stageW-mc.width/2;}
            if(mc.x < mc.width/2){mc.x = mc.width/2};
            if(mc.y > _stageH-mc.height/2){mc.y = _stageH-mc.height/2;}
            if(mc.y < mc.height/2){mc.y = mc.height/2;}
        }

        private function onEnterFrame(evt:Event):void{
            //改变logo的坐标让小球运动
            evt.target.x += evt.target.speedX;
            evt.target.y += evt.target.speedY;
            //检测logo是否碰到舞台边界
            if(evt.target.x >= _stageW-evt.target.width/2 || evt.target.x <= evt.target.width/2){
                evt.target.speedX *= -1;
            }
            if(evt.target.y >= _stageH-evt.target.height/2 || evt.target.y <= evt.target.height/2){
                evt.target.speedY *= -1;
            }
        }
        public function startMove():void
        {
            _mc.addEventListener(Event.ENTER_FRAME,onEnterFrame);
        }
        public function stopMove():void
        {
            _mc.removeEventListener(Event.ENTER_FRAME,onEnterFrame);
        }
    }
}