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>