[AS3]invalidate方法实现延迟计算

invalidate方法的主要工作就是实现延迟计算。查看LayoutManager的attachListeners(systemManager)方法，就会理解他是如何做到的：

public function attachListeners(systemManager:ISystemManager):void 
    { 
        if (!waitedAFrame) 
        { 
            systemManager.addEventListener(Event.ENTER_FRAME, waitAFrame); 
        } 
        else 
        { 
            systemManager.addEventListener(Event.ENTER_FRAME, doPhasedInstantiationCallback); 
            if (!usePhasedInstantiation) 
            { 
                if (systemManager && (systemManager.stage || usingBridge(systemManager))) 
                { 
                    systemManager.addEventListener(Event.RENDER, doPhasedInstantiationCallback); 
                    if (systemManager.stage) 
                        systemManager.stage.invalidate(); 
                } 
            } 
        } 
  
        listenersAttached = true; 
    }