flash简单画画actionscript3.0

as3.0
../drawingapplication.fla
                     document class设置为ch6.drawingApplication
../ch6/drawingApplication.as
package ch6 {
    import flash.display.Sprite;
    import flash.events.MouseEvent;
   
    public class DrawingApplication extends Sprite {
       
        private var _canvas:Sprite;
        public function DrawingApplication() {
            _canvas = new Sprite();
           
            _canvas.graphics.beginFill(0xf0f0f0);
            _canvas.graphics.drawRect(0, 0, stage.stageWidth, stage.stageHeight);
            _canvas.graphics.endFill();
           
            _canvas.graphics.lineStyle(2, 0x000000);
           
            addChild(_canvas);
           
            _canvas.addEventListener(MouseEvent.MOUSE_DOWN, onCanvasMouseDown);
            _canvas.addEventListener(MouseEvent.MOUSE_UP, onCanvasMouseUP);
        }
        private function onCanvasMouseDown(event:MouseEvent): void {
            _canvas.graphics.moveTo(event.localX, event.localY);
            _canvas.addEventLis