Spec to using an Attached property

CornerRadiusXCT.cs

 public class CornerRadiusEffect
    {
        public static readonly BindableProperty CornerRadiusProperty = BindableProperty.CreateAttached(
    nameof(CornerRadius),
    typeof(CornerRadius),
    typeof(CornerRadiusEffect),
    default(CornerRadius),
    propertyChanged: OnCornerRadiusPropertyChanged);

        static void OnCornerRadiusPropertyChanged(BindableObject bindable, object oldValue, object newValue)
        {
            if (!(bindable is VisualElement elementView))
                return;

            elementView.SizeChanged -= ElementView_SizeChanged;
            elementView.SizeChanged += ElementView_SizeChanged;


            UpdateClip(elementView);

        }

        static void ElementView_SizeChanged(object sender, EventArgs e)
        {
            UpdateClip((VisualElement)sender);
        }

        public static CornerRadius GetCornerRadius(BindableObject bindable)
            => (CornerRadius)(bindable?.GetValue(CornerRadiusProperty) ?? throw new ArgumentNullException(nameof(bindable)));

        public static void SetCornerRadius(BindableObject bindable, CornerRadius value)
            => bindable?.SetValue(CornerRadiusProperty, value);

        static void UpdateClip(VisualElement elementView)
        {
            var rect = new Rect(0, 0, elementView.Width, elementView.Height);
            var cornerRadius = GetCornerRadius(rect, elementView);

            if (cornerRadius == default)
            {
                elementView.Clip = null;
                return;
            }

            if (!(elementView.Clip is RoundRectangleGeometry roundRectangleGeometry))
            {
                elementView.Clip = new RoundRectangleGeometry(cornerRadius, rect);
                return;
            }

            roundRectangleGeometry.CornerRadius = cornerRadius;
            roundRectangleGeometry.Rect = rect;
        }

        static CornerRadius GetCornerRadius(Rect rect, VisualElement elementView)
        {
            var maxCornerRadius = Math.Min(rect.Width, rect.Height) / 2;
            if (maxCornerRadius <= 0)
                return default;

            var cornerRadius = GetCornerRadius(elementView);
            if (cornerRadius.TopLeft > maxCornerRadius ||
                cornerRadius.TopRight > maxCornerRadius ||
                cornerRadius.BottomLeft > maxCornerRadius ||
                cornerRadius.BottomRight > maxCornerRadius)
            {
                return new CornerRadius(
                    Math.Min(cornerRadius.TopLeft, maxCornerRadius),
                    Math.Min(cornerRadius.TopRight, maxCornerRadius),
                    Math.Min(cornerRadius.BottomLeft, maxCornerRadius),
                    Math.Min(cornerRadius.BottomRight, maxCornerRadius));
            }

            return cornerRadius;
        }
    }