判断sprite是否出界 出界后删除sprite

<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" layout="absolute" creationComplete="init()">
    <mx:Canvas x="29" y="145" width="661" height="362">
        <mx:Canvas id="canvas_1" x="118" y="30" width="457" height="117" backgroundColor="#F0E6E6">
            <mx:Image id="hair" x="150" y="32" source="../images/hair_1_0.gif"/>
        </mx:Canvas>
        <mx:Canvas id="canvas_2" x="118" y="175" width="457" height="185" borderColor="#B2BCC3" backgroundColor="#B1D6F6" horizontalScrollPolicy="off" verticalScrollPolicy="off">
        </mx:Canvas>
    </mx:Canvas>
    <mx:Script>
        <![CDATA[
            import mx.controls.Image;
            import mx.core.DragSource;
            import mx.managers.DragManager;
            import mx.events.DragEvent;
            private var sp_1:Sprite;
            private var sp_2:Sprite;
            private var del_per:Number=0.3;
            private var del_alpha:Number = 0.1;      
          
            internal function init():void {
              
                sp_1 = new Sprite();
                sp_1.graphics.lineStyle(3,0x243435,1);
                sp_1.graphics.beginFill(0x556433,0.5);
                sp_1.graphics.drawCircle(40,40,40);
                sp_1.graphics.endFill();
                this.canvas_2.rawChildren.addChild(sp_1);
                sp_1.addEventListener(MouseEvent.MOUSE_DOWN,spDownHandler);
            }
        /***=========移动sprite============****/  
        private function spDownHandler(e:MouseEvent):void {
            sp_1.startDrag();
            sp_1.addEventListener(MouseEvent.MOUSE_UP,spUpHandler);
            sp_1.addEventListener(MouseEvent.MOUSE_MOVE,spMoveHandler);
        }
        private function spMoveHandler(e:MouseEvent):void {
            var sp:Sprite = Sprite(e.currentTarget);
            /**横向判断**/
            if(sp.x<(0-sp.width+sp.width*del_per)) {
                sp.alpha=del_alpha;
            } else if(sp.x>(this.canvas_2.width-sp.width*del_per)) {
                sp.alpha=del_alpha;
            } else if(sp.y<(0-sp.height+sp.height*del_per)) {/**纵向判断**/
                sp.alpha=del_alpha;
            } else if(sp.y>(this.canvas_2.height-sp.height*del_per)) {
                sp.alpha=del_alpha;
            } else {
                sp.alpha=1;
            }
        }
        private function spUpHandler(e:MouseEvent):void {
            var sp:Sprite = Sprite(e.currentTarget);
            sp_1.stopDrag();
            sp_1.removeEventListener(MouseEvent.MOUSE_UP,spUpHandler);
            sp_1.removeEventListener(MouseEvent.MOUSE_MOVE,spMoveHandler);
            if(sp.alpha<=del_alpha) {
                sp.graphics.clear();
                sp = null;
            }
        }

        ]]>
    </mx:Script>
</mx:Application>