fix(theme): source dictionary not getting theme updated

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

@@ -1,11 +1,18 @@
 using Microsoft.VisualStudio.TestTools.UnitTesting;
+using Windows.UI;
 using Windows.UI.Xaml;
+using Windows.UI.Xaml.Markup;
+using Windows.UI.Xaml.Media;
+
+#if HAS_UNO
+using Uno.UI.Xaml;
+#endif
 
 namespace Uno.UI.RuntimeTests.Tests.Windows_UI_Xaml
 {
 	[TestClass]
 	public class Given_ResourceDictionary
-    {
+	{
 		[TestMethod]
 		[RunsOnUIThread]
 		public void When_Key_Overwritten()
@@ -20,5 +27,73 @@ public void When_Key_Overwritten()
 
 			Assert.AreEqual(newValue, resourceDictionary[key]);
 		}
+
+#if HAS_UNO // uses uno specifics code
+		[TestMethod]
+		[RunsOnUIThread]
+		public void When_LinkedResDict_ThemeUpdated()
+		{
+			const string TestBrush = nameof(TestBrush);
+			const string TestThemeColor = nameof(TestThemeColor);
+			const string Light = nameof(Light);
+			const string Dark = nameof(Dark);
+
+			var theme = ResourceDictionary.GetActiveTheme();
+			try
+			{
+				// setup
+				ResourceDictionary.SetActiveTheme("Light");
+
+				// initialize source res-dict
+				var parserContext = new XamlParseContext();
+				var dontcare = new object();
+				var source = new ResourceDictionary()
+				{
+					IsParsing = true,
+					ThemeDictionaries =
+					{
+						[Light] = new WeakResourceInitializer(dontcare, that => new ResourceDictionary { [TestThemeColor] = Colors.Red, }),
+						[Dark] = new WeakResourceInitializer(dontcare, that => new ResourceDictionary { [TestThemeColor] = Colors.Blue, }),
+					},
+					[TestBrush] = new WeakResourceInitializer(dontcare, that =>
+					{
+						var brush = new SolidColorBrush();
+						ResourceResolverSingleton.Instance.ApplyResource(brush, SolidColorBrush.ColorProperty, TestThemeColor, true, parserContext);
+
+						return brush;
+					})
+				};
+				source.CreationComplete();
+
+				// making a copy from source
+				var copy = new ResourceDictionary();
+				copy.CopyFrom(source);
+
+				// retrieve the "TestBrush" from each res-dict while also materializing it in both the source and in the copy
+				var materialized1 = (SolidColorBrush)source[TestBrush];
+				var materialized2 = (SolidColorBrush)copy[TestBrush];
+				var materialized2Color = materialized2.Color;
+
+				// set active theme and update the copy res-dict
+				ResourceDictionary.SetActiveTheme(Dark);
+				copy.UpdateThemeBindings(Windows.UI.Xaml.Data.ResourceUpdateReason.ThemeResource);
+
+				// retrieve the "TestBrush" again from each res-dict
+				var materialized3 = (SolidColorBrush)source[TestBrush];
+				var materialized4 = (SolidColorBrush)copy[TestBrush];
+
+				// validation
+				Assert.AreEqual(false, ReferenceEquals(materialized1, materialized2)); // we are expecting these to be different, as the CopyFrom should copy them as WeakResourceInitializer...
+				Assert.AreEqual(false, ReferenceEquals(materialized3, materialized4)); // ^same
+				Assert.AreNotEqual(materialized2Color, materialized4.Color); // check the theme change is actually applied (otherwise it would void the next check)
+				Assert.AreEqual(materialized3.Color, materialized4.Color); // check the theme change is propagated to the source res-dict
+			}
+			finally
+			{
+				// clean up
+				ResourceDictionary.SetActiveTheme(theme);
+			}
+		}
+#endif
 	}
 }

src/Uno.UI/UI/Xaml/ResourceDictionary.cs

@@ -13,6 +13,7 @@
 using ResourceKey = Windows.UI.Xaml.SpecializedResourceDictionary.ResourceKey;
 using System.Runtime.CompilerServices;
 using Windows.UI.Xaml.Data;
+using Uno.UI.DataBinding;
 
 namespace Windows.UI.Xaml
 {
@@ -21,6 +22,7 @@ public partial class ResourceDictionary : DependencyObject, IDependencyObjectPar
 		private readonly SpecializedResourceDictionary _values = new SpecializedResourceDictionary();
 		private readonly List<ResourceDictionary> _mergedDictionaries = new List<ResourceDictionary>();
 		private ResourceDictionary _themeDictionaries;
+		private ManagedWeakReference _sourceDictionary;
 
 		/// <summary>
 		/// This event is fired when a key that has value of type <see cref="ResourceDictionary"/> is added or changed in the current <see cref="ResourceDictionary" />
@@ -424,6 +426,11 @@ internal void CopyFrom(ResourceDictionary source)
 			_mergedDictionaries.Clear();
 			_themeDictionaries?.Clear();
 
+			// In a foreign library that uses merged-dict with 'Source=...', there can be multiple instances of res-dict from one single xaml file.
+			// And, the instance referenced in App.xaml may not be the same one used in merged-dict of library-a\textbox.xaml.
+			// In order to ensure the theme updates covers all of them, we need to keep track of this source instance for theme updates before it is lost.
+			_sourceDictionary = WeakReferencePool.RentWeakReference(this, source);
+
 			_values.AddRange(source._values);
 			_mergedDictionaries.AddRange(source._mergedDictionaries);
 			if (source._themeDictionaries != null)
@@ -556,6 +563,11 @@ internal void UpdateThemeBindings(ResourceUpdateReason updateReason)
 			{
 				mergedDict.UpdateThemeBindings(updateReason);
 			}
+
+			if (_sourceDictionary?.Target is ResourceDictionary target)
+			{
+				target.UpdateThemeBindings(updateReason);
+			}
 		}
 
 		[EditorBrowsable(EditorBrowsableState.Never)]
@@ -596,6 +608,8 @@ public StaticResourceAliasRedirect(string resourceKey, XamlParseContext parseCon
 
 		internal static object GetStaticResourceAliasPassthrough(string resourceKey, XamlParseContext parseContext) => new StaticResourceAliasRedirect(resourceKey, parseContext);
 
+		internal static ResourceKey GetActiveTheme() => Themes.Active;
+
 		internal static void SetActiveTheme(SpecializedResourceDictionary.ResourceKey key)
 			=> Themes.Active = key;