private function onMouseMove(event:MouseEvent):void
    {
     x1 = mouseX * 2 - (x0 + x2) / 2;
     y1 = mouseY * 2 - (y0 + y2) / 2;
     graphics.clear();
     graphics.lineStyle(1);
     graphics.moveTo(x0, y0);
     graphics.curveTo(x1, y1, x2, y2);
    }
   }
  }