as3.0划线带撤销功能

package com{
    import flash.display.MovieClip;
    import flash.display.SimpleButton;
    import flash.events.MouseEvent;
    import flash.display.Shape;
    import flash.geom.ColorTransform;
    import flash.geom.Rectangle;

    public class Drow extends MovieClip {

        private var emptyMc:MovieClip;
        private var blackboard:MovieClip;
        private var thisMc:MovieClip;
        private var shape:Shape;
        private var colorNum:Number
        public function Drow(_emptyMc:MovieClip,_thisMc:MovieClip,_blackboard:MovieClip) {
            emptyMc=_emptyMc;
            thisMc=_thisMc;
            blackboard=_blackboard;
        }
        private var penBtn:SimpleButton;
        private var revokeBtn:SimpleButton;
        private var eraserBtn:SimpleButton;
        public function toolFun(_penBtn:SimpleButton,_revokeBtn:SimpleButton,_eraserBtn:SimpleButton) {
            penBtn=_penBtn;
            revokeBtn=_revokeBtn;
            eraserBtn=_eraserBtn;
            eraserBtn.addEventListener(MouseEvent.CLICK,onClick);
            revokeBtn.addEventListener(MouseEvent.CLICK,onClick);
            penBtn.addEventListener(MouseEvent.CLICK,onClick);
        }
        private function onClick(e:MouseEvent) {

            var str:String=e.target.name;
            if (str=="penBtn") {
                if (penBtn.visible==true) {
                    colorNum=0
                    penBtn.visible=false;
                    blackboard.addEventListener(MouseEvent.MOUSE_DOWN,downFun);
                } else {
                    penBtn.visible=true;
                    blackboard.removeEventListener(MouseEvent.MOUSE_DOWN,downFun);
                }
            } else if (str=="revokeBtn") {
                if(emptyMc.numChildren-1>=0)
                emptyMc.removeChildAt(emptyMc.numChildren-1);
            } else if (str=="eraserBtn") {
                if (eraserBtn.visible==true) {
                    eraserBtn.visible=false;
                    penBtn.visible=true;
                    colorNum=0xFFFFFF
                } else {
                    eraserBtn.visible=true;
                }
            }

        }
        private function downFun(e:MouseEvent) {
            
            shape=new Shape()
            emptyMc.addChild(shape)            
            shape.graphics.clear();
            shape.graphics.lineStyle(5,colorNum);
            shape.graphics.moveTo(mouseX,mouseY);
            blackboard.addEventListener(MouseEvent.MOUSE_MOVE,moveFun);
            blackboard.addEventListener(MouseEvent.MOUSE_UP,upFun);
        }
        private function moveFun(e:MouseEvent) {
            shape.graphics.lineTo(mouseX,mouseY);
            e.updateAfterEvent();
        }
        private function upFun(e:MouseEvent) {
            blackboard.removeEventListener(MouseEvent.MOUSE_MOVE,moveFun);
            blackboard.removeEventListener(MouseEvent.MOUSE_UP,upFun);
        }

    }

}