ModalActivityIndicator

public class ModalActivityIndicator : RelativeLayout
    {
        public ModalActivityIndicator ()
        {
            this.SetBinding (StackLayout.IsVisibleProperty, "IsBusy");
            this.SetBinding (StackLayout.IsEnabledProperty, "IsBusy");

            Children.Add (
                view: new BoxView {
                    Opacity = .4,
                    BackgroundColor = Color.FromHex("#ccc")
                },
                widthConstraint: Constraint.RelativeToParent ((parent) => {
                    return parent.Width;
                }),
                heightConstraint: Constraint.RelativeToParent ((parent) => {
                    return parent.Height;
                })
            );

            var content = new StackLayout {
                BackgroundColor = Color.White,
                Spacing = 10,
                Padding = new Thickness(
                    horizontalSize: 10,
                    verticalSize: 20
                )
            };
            content.Children.Add (new ActivityIndicator { IsRunning = true });
            var label = new Label { HorizontalOptions = LayoutOptions.CenterAndExpand };
            label.SetBinding (Label.TextProperty, "BusyText");
            content.Children.Add (label);

            Children.Add (
                view: content,
                widthConstraint: Constraint.RelativeToParent ((parent) => {
                    return parent.Width / 2;
                }),
                heightConstraint: Constraint.RelativeToParent ((parent) => {
                    return parent.Width / 3;
                }),
                xConstraint: Constraint.RelativeToParent ((parent) => {
                    return parent.Width / 4;
                }),
                yConstraint: Constraint.RelativeToParent ((parent) => {
                    return (parent.Height / 2) - (parent.Width / 6);
                })
            );
        }
    }