var loadingView = new StackLayout
{
    Padding = 6,
    Orientation = StackOrientation.Horizontal,
    BackgroundColor = Color.Gray,
    Children =
    {
        new ActivityIndicator
        {
            Color = Color.White,
            IsRunning = true,
            VerticalOptions = LayoutOptions.Center,
            WidthRequest = 20,
            HeightRequest = 20
        },
        new Label
        {
            TextColor = Color.White,
            Text = "Loading...",
            VerticalOptions = LayoutOptions.Center
        }
    }
};

var layout = new AbsoluteLayout
{
    Padding = 0,
    HorizontalOptions = LayoutOptions.FillAndExpand,
    VerticalOptions = LayoutOptions.FillAndExpand,
    BackgroundColor = Color.Red.MultiplyAlpha(0.33d),
    Children =
    {
        {
            new StackLayout
            {
                Orientation = StackOrientation.Vertical,
                BackgroundColor = Color.Blue.MultiplyAlpha(0.33d),
                Padding = 6,
                Children =
                {
                    new Entry{Placeholder = "Entry1"},
                    new Entry{Placeholder = "Entry2"},
                    new Button{Text="Ok"}
                }
            },
            new Rectangle(0.5, 0.5, 1, -1),
            AbsoluteLayoutFlags.PositionProportional | AbsoluteLayoutFlags.WidthProportional
        },
        {
            loadingView,
            new Rectangle(0.5, 0.5, -1, -1),
            AbsoluteLayoutFlags.PositionProportional
        }
    }
}