package {
    import flash.display.Sprite;
    import flash.events.Event;
    import flash.events.MouseEvent;
    import flash.ui.Mouse;
    import flash.ui.MouseCursor;
     
    public class Spring2 extends Sprite {
        private var ball:Ball;
        private var spring:Number=0.1;
        private var targetX:Number=stage.stageWidth/2;
 
        private var targetY:Number=stage.stageHeight/2;
        private var vx:Number=0;
        private var vy:Number=0;
        private var friction:Number=0.95;
         
        public function Spring2() {
            init();
        }
        private function init():void {
            ball = new Ball(5);
            ball.x = Math.random() * stage.stageWidth;
            ball.y = Math.random() * stage.stageHeight;         
            addChild(ball);
            addEventListener(Event.ENTER_FRAME, EnterFrameHandler);
            stage.addEventListener(MouseEvent.MOUSE_DOWN,MouseDownHandler); 
            MouseDownHandler(null);
            Mouse.cursor = MouseCursor.BUTTON;
        }
         
        function MouseDownHandler(e:MouseEvent):void{
            graphics.clear();
            ball.x = Math.random() * stage.stageWidth;
            ball.y = Math.random() * stage.stageHeight;
            ball.vx = (Math.random()*2-1) * 50;
            ball.vy = (Math.random()*2-1) * 50;
            graphics.moveTo(ball.x,ball.y);         
        }
         
        private function EnterFrameHandler(event:Event):void {
            //去掉下面的注释，就变成鼠标跟随的弹性运动
            //targetX = mouseX;
            //targetY = mouseY;
            var dx:Number=targetX-ball.x;
            var dy:Number=targetY-ball.y;
            var ax:Number=dx*spring;
            var ay:Number=dy*spring;
            vx+=ax;
            vy+=ay;
            vx*=friction;
            vy*=friction;
            ball.x+=vx;
            ball.y+=vy;
            graphics.lineStyle(0.3,0xbbbbbb,1);
            graphics.lineTo(ball.x,ball.y);
        }
    }
}