feat: ThemeShadow support on iOS/macOS

Author: MartinZikmund

src/Uno.UI/UI/Xaml/Media/ThemeShadowManager.iOSmacOS.cs

@@ -0,0 +1,54 @@
+using System;
+using Windows.UI.Xaml;
+
+namespace Uno.UI.Xaml.Media
+{
+	internal static partial class ThemeShadowManager
+	{
+		static partial void UnsetShadow(UIElement uiElement)
+		{
+			var translation = uiElement.Translation;
+
+#if __MACOS__
+			if (uiElement is AppKit.NSView view)
+#else
+			if (uiElement is UIKit.UIView view)
+#endif
+			{
+				view.Layer.ShadowOpacity = 0;
+			}
+		}
+
+		static partial void SetShadow(UIElement uiElement)
+		{
+			var translation = uiElement.Translation;
+
+#if __MACOS__
+			if (uiElement is AppKit.NSView view)
+#else
+			if (uiElement is UIKit.UIView view)
+#endif
+			{
+				// Values for 1dp elevation according to https://material.io/guidelines/resources/shadows.html#shadows-illustrator
+				const float x = 0.25f;
+				const float y = 0.92f * 0.5f; // Looks more accurate than the recommended 0.92f.
+				const float blur = 0.5f;
+
+#if __MACOS__
+				view.WantsLayer = true;
+				view.Shadow ??= new AppKit.NSShadow();
+#endif
+				view.Layer.MasksToBounds = false;
+				view.Layer.ShadowOpacity = 1;
+#if __MACOS__
+				view.Layer.ShadowColor = AppKit.NSColor.Black.CGColor;
+#else
+				view.Layer.ShadowColor = UIKit.UIColor.Black.CGColor;
+#endif
+				view.Layer.ShadowRadius = (nfloat)(blur * translation.Z);
+				view.Layer.ShadowOffset = new CoreGraphics.CGSize(x * translation.Z, y * translation.Z);
+				//view.Layer.ShadowPath = path; - needed for rounded corners?
+			}
+		}
+	}
+}