为什么不画这个矩形?

public class Greeter extends MovieClip
{

    public function Greeter()
    {
        addEventListener(Event.ADDED_TO_STAGE, go);
    }

    private function go(evt:Event):void
    {
        removeEventListener(Event.ADDED_TO_STAGE, go);
        var _root:MovieClip = parent.parent as MovieClip;

        var sp:Sprite = new Sprite();

        // Desenhando com um objeto graphics
        var g:Graphics = sp.graphics;
        g.beginFill(0xFF0000, 1);
        g.drawRect(10, 10, 300, 200);
        g.endFill();

        _root.addChild(sp);
    }
}

mxml文件:

<?xml version="1.0" encoding="utf-8"?>
<s:Application xmlns:fx="http://ns.adobe.com/mxml/2009" 
               xmlns:s="library://ns.adobe.com/flex/spark" 
               xmlns:mx="library://ns.adobe.com/flex/mx" minWidth="1024" minHeight="768"
               creationComplete="initApp()">

        <fx:Script>
            <![CDATA[
                public function initApp():void
                {
                    var greeter:Greeter = new Greeter();
                }
            ]]>
        </fx:Script>

</s:Application>

--回答:

使用addElement和extends添加到stagespark.core.SpriteVisualElement.