render事件来提高程序的运行效率(as3.0)

public function addItem(item:DisplayObject):voide {
	......
	stage.invalidate()
}

package {
	
import flash.display.DisplayObject;
import flash.display.Sprite;
import flash.events.Event;

public class List extends Sprite {
	
	private var changed:Boolean;
	
	public function List() {
		super();
		addEventListener(Event.ADDED_TO_STAGE, __addToStage);
	}
	
	public function addItem(item:DisplayObject):void {
		addChild(item);
		requireLayout();
	}
	
	public function removeItem(item:DisplayObject):void {
		if(contains(item)) {
			removeChild(item);
			requireLayout();
		}
	}
	
	private function requireLayout():void {
		changed = true;
		if(stage != null) stage.invalidate();
	}
	
	//对内部项目进行排列，可以是任意的排列算法
	protected function layoutContents():void {
		trace("do layout");
		
		var y:Number = 0;
				
		var num:int = numChildren;
		for(var i:int=0; i<num; i++) {
			
			var child:DisplayObject = getChildAt(i);
			child.x = 0;
			child.y = y;
			
			y += child.height+2;
		}
	}
	
	private function __addToStage(e:Event):void {
		stage.addEventListener(Event.RENDER, __render);
		if(changed) stage.invalidate();
	}
	
	private function __render(e:Event):void {
		if(changed) {
			layoutContents();
			changed = false;
		}
	}
}
}