Xamarin.Forms - How to overlay an ActivityIndicator in the middle of a StackLayout programmatically

If you have problems with RelativeLayout, you can also use AbsoluteLayout which works in a similar context. Sample code below:

var overlay = new AbsoluteLayout();
var content = new StackLayout();
var loadingIndicator = new ActivityIndicator();
AbsoluteLayout.SetLayoutFlags(content, AbsoluteLayoutFlags.PositionProportional);
AbsoluteLayout.SetLayoutBounds(content, new Rectangle(0f, 0f, AbsoluteLayout.AutoSize, AbsoluteLayout.AutoSize));
AbsoluteLayout.SetLayoutFlags(loadingIndicator, AbsoluteLayoutFlags.PositionProportional);
AbsoluteLayout.SetLayoutBounds(loadingIndicator, new Rectangle(0.5, 0.5, AbsoluteLayout.AutoSize, AbsoluteLayout.AutoSize));
overlay.Children.Add(content);
overlay.Children.Add(loadingIndicator);

If you would like a full working example, I have made one available @ https://github.com/teamtam/xamarin-forms-timesheet

There's an acceptable answer but I wrote an extensions for all contentpage and think it could be nice.

public static void AddProgressDisplay (this ContentPage page)
{
    var content = page.Content;

    var grid = new Grid();
    grid.Children.Add(content);
    var gridProgress = new Grid { BackgroundColor = Color.FromHex("#64FFE0B2"), Padding = new Thickness(50) };
    gridProgress.RowDefinitions.Add(new RowDefinition { Height = new GridLength(1, GridUnitType.Star) });
    gridProgress.RowDefinitions.Add(new RowDefinition { Height = new GridLength(1, GridUnitType.Auto) });
    gridProgress.RowDefinitions.Add(new RowDefinition { Height = new GridLength(1, GridUnitType.Star) });
    gridProgress.SetBinding(VisualElement.IsVisibleProperty, "IsWorking");
    var activity = new ActivityIndicator
    {
        IsEnabled = true,
        IsVisible = true,
        HorizontalOptions = LayoutOptions.FillAndExpand,
        IsRunning = true
    };
    gridProgress.Children.Add(activity, 0, 1);
    grid.Children.Add(gridProgress);
    page.Content = grid;
}

Remark: IsWorking must be member of ViewModel (implemented INotifyPropertyChanged) property.

Call extension page ctor last statement.

this.AddProgressDisplay();

You need to use absolute layout. Follow the following hierarchy of the layout:

<AbsoluteLayout HorizontalOptions="FillAndExpand" VerticalOptions="FillAndExpand">
    <StackLayout AbsoluteLayout.LayoutFlags="All" AbsoluteLayout.LayoutBounds="0,0,1,1">

        <!--< Your control design goes here >-->

    </StackLayout>

    <StackLayout IsVisible="{Binding IsBusy}" Padding="12"
             AbsoluteLayout.LayoutFlags="PositionProportional"
             AbsoluteLayout.LayoutBounds="0.5,0.5,-1,-1">

        <ActivityIndicator IsRunning="{Binding IsBusy}" Color ="#80000000"/>

        <Label Text="Loading..." HorizontalOptions="Center" TextColor="White"/>

    </StackLayout>

</AbsoluteLayout>

With this the activity indicator will be overlaid in the middle of a stack layout.