其实在 skin里非常容易实现，不过没有用 flash 的 帧来实现，

只用计数器，不知耗性能如何，有兴趣谁去测试回头转告下哈！

看图先吧：

ApplicationSkin1.mxml

<? xml version="1.0" encoding="utf-8" ?>

<!-- ADOBE SYSTEMS INCORPORATED
    Copyright 2008 Adobe Systems Incorporated
    All Rights Reserved.

NOTICE: Adobe permits you to use, modify, and distribute this file
    in accordance with the terms of the license agreement accompanying it. -->

<!-- - The default skin class for the Spark Application component. 
        
      @langversion 3.0
      @playerversion Flash 10
      @playerversion AIR 1.5
      @productversion Flex 4 -->
< s:Skin xmlns:fx ="http://ns.adobe.com/mxml/2009" xmlns:s ="library://ns.adobe.com/flex/spark" alpha.disabled ="0.5" creationComplete ="onStart()" >

< fx:Metadata >
     <![CDATA[ /** 
     * A strongly typed property that references the component to which this skin is applied.
     */
        [HostComponent("spark.components.WindowedApplication")] ]]>
     </ fx:Metadata > 
    
     < fx:Script >
         <![CDATA[ import flash.utils.Timer;
            
            private var ticker:Timer;
            
            public function onStart():void
            {
                if(movingCloud.left<contentGroup.width+580)
                {
                    movingCloud.left +=0.5;
                }
                else
                {
                    movingCloud.left = -580;
                }
                ticker=new Timer(40, 1);
                ticker.addEventListener(TimerEvent.TIMER_COMPLETE, onTimerComplete);
                ticker.start();
            }
            public function onTimerComplete(event:TimerEvent):void
            {
                onStart();
            } ]]>
     </ fx:Script >

< s:states >
         < s:State name ="normal" />
         < s:State name ="disabled" />
         < s:State name ="normalAndInactive" />
         < s:State name ="normalWithControlBar" />
         < s:State name ="disabledWithControlBar" />
         < s:State name ="disabledAndInactive" />
     </ s:states >
    
     <!-- fill -->
     <!-- - 
        A rectangle with a solid color fill that forms the background of the application.
        The color of the fill is set to the Application's backgroundColor property. -->
     < s:Rect id ="backgroundRect" left ="0" right ="0" top ="0" bottom ="0" >
         < s:fill >
            <!-- <s:SolidColor color="{getStyle('backgroundColor')}" /> -->
             < s:BitmapFill source ="@Embed(source='assets/img/cloud.jpg')" smooth ="true" />            
         </ s:fill >
     </ s:Rect >
    
     < s:Group width ="100%" height ="100%" minWidth ="0" minHeight ="0" top ="-10" left ="-580" >
         < s:BitmapImage id ="movingCloud" source ="assets/img/cloud1.png" smooth ="true" />
     </ s:Group >
        
     < s:Group left ="0" right ="0" top ="0" bottom ="0" >
         < s:layout >
             < s:VerticalLayout gap ="0" horizontalAlign ="justify" />
         </ s:layout >
        
         <!-- - 
            Application Control Bar -->
         < s:Group id ="topGroup" minWidth ="0" minHeight ="0" includeIn ="normalWithControlBar, disabledWithControlBar" >

<!-- layer 0: control bar highlight -->
             < s:Rect left ="0" right ="0" top ="0" bottom ="1" >
                < s:stroke >
                     < s:LinearGradientStroke rotation ="90" weight ="1" >
                         < s:GradientEntry color ="0xFFFFFF" />
                         < s:GradientEntry color ="0xD8D8D8" />
                     </ s:LinearGradientStroke >
                </ s:stroke >
             </ s:Rect >

<!-- layer 1: control bar fill -->
             < s:Rect left ="1" right ="1" top ="1" bottom ="2" >
                < s:fill >
                     < s:LinearGradient rotation ="90" >
                         < s:GradientEntry color ="0xEDEDED" />
                         < s:GradientEntry color ="0xCDCDCD" />
                     </ s:LinearGradient >
                </ s:fill >
             </ s:Rect >

<!-- layer 2: control bar divider line -->
             < s:Rect left ="0" right ="0" bottom ="0" height ="1" alpha ="0.55" >
                 < s:fill >
                     < s:SolidColor color ="0x000000" />
                 </ s:fill >
             </ s:Rect >

<!-- layer 3: control bar -->
             < s:Group id ="controlBarGroup" left ="0" right ="0" top ="1" bottom ="1" minWidth ="0" minHeight ="0" >
                 < s:layout >
                     < s:HorizontalLayout paddingLeft ="10" paddingRight ="10" paddingTop ="7" paddingBottom ="7" gap ="10" />
                 </ s:layout >
             </ s:Group >
         </ s:Group >

<!-- - 
            @copy spark.components.SkinnableContainer#contentGroup -->
         < s:Group id ="contentGroup" width ="100%" height ="100%" minWidth ="0" minHeight ="0" />
        
     </ s:Group >

</ s:Skin >

贴就贴完整点吧，虽然简单

工程文件：

<? 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:views ="views.*" skinClass ="ApplicationSkin1" >
     < fx:Declarations >
         <!-- 将非可视元素（例如服务、值对象）放在此处 -->
     </ fx:Declarations >
     <!-- <views:vApps x="0" y="0">
    </views:vApps> -->

</ s:WindowedApplication >