private void ScrollView_Scrolled(object sender, Xamarin.Forms.ScrolledEventArgs e)
 {
     System.Diagnostics.Debug.WriteLine(e.ScrollY);
     // sample to close form when scroll down reaches certian limit
     if (e.ScrollY < 100)
     {
         PopupNavigation.Instance.PopAsync();
     }
 }
 void ScrollView_Scrolled(object sender, Xamarin.Forms.ScrolledEventArgs e)
 => appFrame.ShadowOpacity = e.ScrollY / 50 > 1 ? 1 : e.ScrollY / 50;