Xamarin.iOS screenshot with transparent background - xamarin.forms

Question

I build an application that users can load a report and draw on screen for highlight and share some information (as an image). I develop this feature using Rg.Plugins.Popup.Pages.PopupPage and SkiaSharp. The popup page has a transparent background and a sharecontent button. Users draw in the popup page.
In Android, this works fine because I can take a screeshot of the entire screen using currentview.Window.DecorView.RootView, but in iOS when I take a screenshot the image get a black background. I use the following code to take the screenshot:
public void ShareContent(string imageName)
{
UIGraphics.BeginImageContextWithOptions(new CoreGraphics.CGSize(UIScreen.MainScreen.Bounds.Width, UIScreen.MainScreen.Bounds.Height), false, 0);
UIImage capture = UIApplication.SharedApplication.KeyWindow.Capture();
UIGraphics.EndImageContext();
var activityItems = new[] { capture };
var activityController = new UIActivityViewController(activityItems, null);
var topController = UIApplication.SharedApplication.KeyWindow.RootViewController;
while (topController.PresentedViewController != null)
{
topController = topController.PresentedViewController;
}
topController.PresentViewController(activityController, true, () => { });
}
How can I draw an image with a transparent background in Xamarin iOS?
Codebehind of PopupPage in shared project
public partial class DrawAndSharePage : Rg.Plugins.Popup.Pages.PopupPage
{
Dictionary<long, SKPath> inProgressPaths = new Dictionary<long, SKPath>();
List<SKPath> completedPaths = new List<SKPath>();
SKPaint paint = new SKPaint
{
Style = SKPaintStyle.Stroke,
Color = SKColors.Blue,
StrokeWidth = 10,
StrokeCap = SKStrokeCap.Round,
StrokeJoin = SKStrokeJoin.Round
};
public DrawAndSharePage()
{
BackgroundColor = new Color(0, 0, 0, 0.1);
InitializeComponent();
}
private async void BtnClose_Clicked(object sender, System.EventArgs e)
{
await Rg.Plugins.Popup.Services.PopupNavigation.Instance.PopAllAsync();
}
void OnTouchEffectAction(object sender, TouchActionEventArgs args)
{
switch (args.Type)
{
case TouchActionType.Pressed:
if (!inProgressPaths.ContainsKey(args.Id))
{
SKPath path = new SKPath();
path.MoveTo(ConvertToPixel(args.Location));
inProgressPaths.Add(args.Id, path);
canvasView.InvalidateSurface();
}
break;
case TouchActionType.Moved:
if (inProgressPaths.ContainsKey(args.Id))
{
SKPath path = inProgressPaths[args.Id];
path.LineTo(ConvertToPixel(args.Location));
canvasView.InvalidateSurface();
}
break;
case TouchActionType.Released:
if (inProgressPaths.ContainsKey(args.Id))
{
completedPaths.Add(inProgressPaths[args.Id]);
inProgressPaths.Remove(args.Id);
canvasView.InvalidateSurface();
}
break;
case TouchActionType.Cancelled:
if (inProgressPaths.ContainsKey(args.Id))
{
inProgressPaths.Remove(args.Id);
canvasView.InvalidateSurface();
}
break;
}
}
void OnCanvasViewPaintSurface(object sender, SKPaintSurfaceEventArgs args)
{
SKCanvas canvas = args.Surface.Canvas;
canvas.Clear();
foreach (SKPath path in completedPaths)
{
canvas.DrawPath(path, paint);
}
foreach (SKPath path in inProgressPaths.Values)
{
canvas.DrawPath(path, paint);
}
}
SKPoint ConvertToPixel(Point pt)
{
return new SKPoint((float)(canvasView.CanvasSize.Width * pt.X / canvasView.Width),
(float)(canvasView.CanvasSize.Height * pt.Y / canvasView.Height));
}
private void OnTouchEffectAction(object sender, object args)
{
}
}