feat(theme): Limited support for FrameworkElement.RequestedTheme

Setting FrameworkElement.RequestedTheme will apply the nominated RequestedTheme to the entire application. This does not allow setting a different RequestedTheme for individual pages or controls, but does support the use case of overriding the system theme while the application is running.

src/Uno.UI.Tests/Windows_UI_Xaml/Given_ResourceDictionary.cs

@@ -221,13 +221,13 @@ public void When_Has_Multiple_Themes()
 #if !NETFX_CORE //Not legal on UWP to change theme after app has launched
 			try
 			{
-				Application.Current.SetRequestedTheme(ApplicationTheme.Dark);
+				Application.Current.SetExplicitRequestedTheme(ApplicationTheme.Dark);
 				var retrieved2 = rd["Blu"];
 				Assert.AreEqual(Colors.DarkBlue, ((SolidColorBrush)retrieved2).Color);
 			}
 			finally
 			{
-				Application.Current.SetRequestedTheme(ApplicationTheme.Light);
+				Application.Current.SetExplicitRequestedTheme(ApplicationTheme.Light);
 			}
 #endif
 		}

src/Uno.UI.Tests/Windows_UI_Xaml/Given_ThemeResource.cs

@@ -41,6 +41,28 @@ public async Task When_Themed_Color_Theme_Changed()
 			Assert.AreEqual(Colors.DarkBlue, (page.TestBorder.Background as SolidColorBrush).Color);
 		}
 
+		[TestMethod]
+		public void When_Element_Theme_Changed()
+		{
+			var page = new ThemeResource_Themed_Color_Page();
+			var app = UnitTestsApp.App.EnsureApplication();
+			app.HostView.Children.Add(page);
+
+			Assert.AreEqual(Colors.LightBlue, (page.TestBorder.Background as SolidColorBrush).Color);
+
+			page.RequestedTheme = ElementTheme.Dark;
+			Assert.AreEqual(Colors.DarkBlue, (page.TestBorder.Background as SolidColorBrush).Color);
+
+			page.RequestedTheme = ElementTheme.Light;
+			Assert.AreEqual(Colors.LightBlue, (page.TestBorder.Background as SolidColorBrush).Color);
+
+			page.RequestedTheme = ElementTheme.Dark;
+			Assert.AreEqual(Colors.DarkBlue, (page.TestBorder.Background as SolidColorBrush).Color);
+
+			page.RequestedTheme = ElementTheme.Default;
+			Assert.AreEqual(Colors.LightBlue, (page.TestBorder.Background as SolidColorBrush).Color);
+		}
+
 		[TestMethod]
 		public async Task When_Visual_States_Keyframe_Theme_Changed_Reapplied()
 		{
@@ -471,7 +493,7 @@ private static async Task<bool> SwapSystemTheme()
 
 			await _swapTask;
 #else
-			Application.Current.SetRequestedTheme(targetTheme);
+			Application.Current.SetExplicitRequestedTheme(targetTheme);
 #endif
 			Assert.AreEqual(targetTheme, Application.Current.RequestedTheme);
 			return true;

src/Uno.UI/Extensions/ElementThemeExtensions.cs

@@ -0,0 +1,22 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using Uno.Extensions;
+using Windows.UI.Xaml;
+using Windows.UI.Xaml.Controls;
+
+namespace Uno.UI.Extensions
+{
+	internal static class ElementThemeExtensions
+	{
+		public static ApplicationTheme? ToApplicationThemeOrDefault(this ElementTheme elementTheme)
+			=> elementTheme switch
+			{
+				ElementTheme.Default => null,
+				ElementTheme.Light => ApplicationTheme.Light,
+				ElementTheme.Dark => ApplicationTheme.Dark,
+				_ => throw new ArgumentException()
+			};
+	}
+}

src/Uno.UI/Generated/3.0.0.0/Windows.UI.Xaml/ElementTheme.cs

@@ -2,21 +2,21 @@
 #pragma warning disable 114 // new keyword hiding
 namespace Windows.UI.Xaml
 {
-	#if __ANDROID__ || __IOS__ || NET461 || __WASM__ || __SKIA__ || __NETSTD_REFERENCE__ || __MACOS__
-	#if __ANDROID__ || __IOS__ || NET461 || __WASM__ || __SKIA__ || __NETSTD_REFERENCE__ || __MACOS__
+#if false
+#if false
 	[global::Uno.NotImplemented]
-	#endif
+#endif
 	public   enum ElementTheme 
 	{
-		#if __ANDROID__ || __IOS__ || NET461 || __WASM__ || __SKIA__ || __NETSTD_REFERENCE__ || __MACOS__
+#if false
 		Default,
-		#endif
-		#if __ANDROID__ || __IOS__ || NET461 || __WASM__ || __SKIA__ || __NETSTD_REFERENCE__ || __MACOS__
+#endif
+#if false
 		Light,
-		#endif
-		#if __ANDROID__ || __IOS__ || NET461 || __WASM__ || __SKIA__ || __NETSTD_REFERENCE__ || __MACOS__
+#endif
+#if false
 		Dark,
-		#endif
+#endif
 	}
-	#endif
+#endif
 }

src/Uno.UI/Generated/3.0.0.0/Windows.UI.Xaml/FrameworkElement.cs

@@ -179,8 +179,8 @@ public  double Height
 			}
 		}
 		#endif
-		#if __ANDROID__ || __IOS__ || NET461 || __WASM__ || __SKIA__ || __NETSTD_REFERENCE__ || __MACOS__
-		[global::Uno.NotImplemented("__ANDROID__", "__IOS__", "NET461", "__WASM__", "__SKIA__", "__NETSTD_REFERENCE__", "__MACOS__")]
+		#if false
+		[global::Uno.NotImplemented]
 		public  global::Windows.UI.Xaml.ElementTheme RequestedTheme
 		{
 			get
@@ -416,8 +416,7 @@ public  bool AllowFocusOnInteraction
 			typeof(global::Windows.UI.Xaml.FrameworkElement), 
 			new FrameworkPropertyMetadata(default(double)));
 		#endif
-		#if __ANDROID__ || __IOS__ || NET461 || __WASM__ || __SKIA__ || __NETSTD_REFERENCE__ || __MACOS__
-		[global::Uno.NotImplemented("__ANDROID__", "__IOS__", "NET461", "__WASM__", "__SKIA__", "__NETSTD_REFERENCE__", "__MACOS__")]
+		#if false
 		public static global::Windows.UI.Xaml.DependencyProperty RequestedThemeProperty { get; } = 
 		Windows.UI.Xaml.DependencyProperty.Register(
 			nameof(RequestedTheme), typeof(global::Windows.UI.Xaml.ElementTheme), 

src/Uno.UI/UI/Xaml/Application.cs

@@ -77,12 +77,18 @@ public ApplicationTheme RequestedTheme
 				{
 					throw new NotSupportedException("Operation not supported");
 				}
-				// this flag makes sure the app will not respond to OS events	
-				_themeSetExplicitly = true;
-				SetRequestedTheme(value);
+				SetExplicitRequestedTheme(value);
 			}
 		}
 
+		internal void SetExplicitRequestedTheme(ApplicationTheme? explicitTheme)
+		{
+			// this flag makes sure the app will not respond to OS events
+			_themeSetExplicitly = explicitTheme.HasValue;
+			var theme = explicitTheme ?? GetDefaultSystemTheme();
+			SetRequestedTheme(theme);
+		}
+
 		public ResourceDictionary Resources { get; } = new ResourceDictionary();
 
 #pragma warning disable CS0067 // The event is never used
@@ -174,7 +180,7 @@ internal void RaiseWindowCreated(Window window)
 			OnWindowCreated(new WindowCreatedEventArgs(window));
 		}
 
-		internal void SetRequestedTheme(ApplicationTheme requestedTheme)
+		private void SetRequestedTheme(ApplicationTheme requestedTheme)
 		{
 			if (requestedTheme != _requestedTheme)
 			{

src/Uno.UI/UI/Xaml/ElementTheme.cs

@@ -0,0 +1,9 @@
+namespace Windows.UI.Xaml
+{
+	public enum ElementTheme
+	{
+		Default,
+		Light,
+		Dark,
+	}
+}

src/Uno.UI/UI/Xaml/FrameworkElement.cs

@@ -17,6 +17,7 @@
 using System.ComponentModel;
 using Uno.UI.DataBinding;
 using Uno.UI.Xaml;
+using Uno.UI.Extensions;
 #if XAMARIN_ANDROID
 using View = Android.Views.View;
 #elif XAMARIN_IOS_UNIFIED
@@ -378,6 +379,31 @@ private void UpdateActiveStyle()
 
 		private Style ResolveImplicitStyle() => (this as IDependencyObjectStoreProvider).Store.GetImplicitStyle();
 
+		#region Requested theme dependency property
+
+		public ElementTheme RequestedTheme
+		{
+			get => (ElementTheme)GetValue(RequestedThemeProperty);
+			set => SetValue(RequestedThemeProperty, value);
+		}
+
+		public static DependencyProperty RequestedThemeProperty { get; } =
+			DependencyProperty.Register(
+				nameof(RequestedTheme),
+				typeof(ElementTheme),
+				typeof(FrameworkElement),
+				new PropertyMetadata(
+					ElementTheme.Default,
+					(o, e) => ((FrameworkElement)o).OnRequestedThemeChanged((ElementTheme)e.OldValue, (ElementTheme)e.NewValue)));
+
+		private void OnRequestedThemeChanged(ElementTheme oldValue, ElementTheme newValue)
+			// This is an ultra-naive implementation... but nonetheless enables the common use case of overriding the system theme for
+			// the entire visual tree (since Application.RequestedTheme cannot be set after launch)
+			=> Application.Current.SetExplicitRequestedTheme(newValue.ToApplicationThemeOrDefault());
+
+
+		#endregion
+
 		/// <summary>
 		/// Replace previous style with new style, at nominated precedence. This method is called separately for the user-determined
 		/// 'active style' and for the baked-in 'default style'.