Flex 中addChild没法显示元素

<?xml version="1.0" encoding="utf-8"?>
<s:Group xmlns:fx="http://ns.adobe.com/mxml/2009" 
         xmlns:s="library://ns.adobe.com/flex/spark" 
         xmlns:mx="library://ns.adobe.com/flex/mx" width="400" height="300">
    <fx:Script>
        <![CDATA[
            import mx.controls.Label;
            import mx.core.*;

            
            //显示网格线
            //width : 地图总宽度
            //height : 地图总高度
            //size : 单元格尺寸
            public function ShowGrid(xlen : int,ylen : int,size : int) : void
            {
                var sp : Sprite = new Sprite();
                var comp: UIComponent = new UIComponent(); 
                comp.addChild(sp);
                var la : Label = new Label();
                la.text = "demo";
                comp.addChild(la);
                la.x = 50;
                la.y = 50;
                this.addElement(comp);
            }
        ]]>
    </fx:Script>

    <fx:Declarations>
        <!-- 将非可视元素（例如服务、值对象）放在此处 -->        
    </fx:Declarations>
</s:Group>