小球碰撞

package
{
 import flash.display.MovieClip;
 import flash.events.Event;

 public class Main extends MovieClip
 {
  var yd1:Number = 9;
  var yd2:Number = 9;
  public function Main()
  {
   xq_mc.addEventListener(Event.ENTER_FRAME,ydhs);
  }
  private function ydhs(e:Event):void
  {
   xq_mc.x +=  yd2;
   xq_mc.y +=  yd1;
   if (xq_mc.y > stage.stageHeight - xq_mc.height / 2)
   {
    yd1 *=  -1;
   }
   if (xq_mc.y < xq_mc.height / 2)
   {
    yd1 *=  -1;
   }
   if (xq_mc.x > stage.stageWidth - xq_mc.width / 2)
   {
    yd2 *=  -1;
   }
   if (xq_mc.x < xq_mc.width / 2)
   {
    yd2 *=  -1;
   }
  }
 }
}