void viewport_HScrollRequest(object sender, ScrollSurfaceRequestEventArgs e)
 {
     if (HScrollRequest != null)
     {
         HScrollRequest.Invoke(sender, e);
     }
 }
 protected void RaiseScrollChangedEvents(bool needVeritcal, bool needHorizontal)
 {
     if (this.HasVScrollChanged)
     {
         VScrollRequest.Invoke(this, new ScrollSurfaceRequestEventArgs(needVeritcal));
     }
     if (this.HasHScrollChanged)
     {
         HScrollRequest.Invoke(this, new ScrollSurfaceRequestEventArgs(needHorizontal));
     }
 }