MovieClipSWFLoader.play() and stop() cannot work.

playswf.mxml

<?xml version="1.0" encoding="utf-8"?>
<s:WindowedApplication xmlns:fx="http://ns.adobe.com/mxml/2009" 
                       xmlns:s="library://ns.adobe.com/flex/spark" 
                       xmlns:mx="library://ns.adobe.com/flex/mx" xmlns:local="*">
    <fx:Declarations>
        <local:PlaySwfHelper id="helper"/>
    </fx:Declarations>
    <s:VGroup width="100%" height="100%"
              paddingTop="8" paddingBottom="8"
              paddingLeft="8" paddingRight="8">
        <s:MovieClipSWFLoader id="loader" width="100%" height="100%"
                              verticalAlign="middle" horizontalAlign="center"
                              autoLoad="false"/>
        <s:HGroup width="100%">
            <s:Button id="playButton" label="Play" width="100%"/>
            <s:Button id="stopButton" label="Stop" width="100%"/>
        </s:HGroup>
    </s:VGroup>
</s:WindowedApplication>

PlaySwfHelper.as

package {
    import flash.events.MouseEvent;

    import mx.core.IMXMLObject;
    import mx.events.FlexEvent;

    public class PlaySwfHelper implements IMXMLObject {
        private var root: playswf;

        public function initialized(object: Object, id: String): void {
            root = object as playswf;
            root.addEventListener(FlexEvent.CREATION_COMPLETE, onCreationComplete);
        }
        
        private function onCreationComplete(event: FlexEvent): void {
            root.loader.source = "asset/example.swf";
            root.playButton.addEventListener(MouseEvent.CLICK, onPlayButtonClick);
            root.stopButton.addEventListener(MouseEvent.CLICK, onStopButtonClick);
        }

        private function onPlayButtonClick(event: MouseEvent): void {
            if (root.loader.content == null) {
                root.loader.load();
            } else {
                trace(root.loader.content); // this code prints "[object AVM1Movie]"
                root.loader.movieClip.play();
            }
        }

        private function onStopButtonClick(event: MouseEvent): void {
            root.loader.movieClip.stop();
        }
    }
}