feat: Support ThemeResource in TargetNullValue and FallbackValue

src/SourceGenerators/Uno.UI.SourceGenerators/XamlGenerator/XamlFileGenerator.cs

@@ -4048,6 +4048,22 @@ void WriteBinding(bool isTemplateBindingAttachedProperty, string? prefix = null)
 							}
 						}
 
+						foreach (var option in bindingOptions)
+						{
+							var themeResourceCandidate = option.Objects.FirstOrDefault();
+							if (themeResourceCandidate?.Type is { PreferredXamlNamespace: XamlConstants.PresentationXamlXmlNamespace, Name: "ThemeResource" })
+							{
+								if (option.Member.Name == "TargetNullValue" && themeResourceCandidate.Members.FirstOrDefault().Value is string targetNullValueKey)
+								{
+									writer.AppendLineIndented($".ApplyTargetNullValueThemeResource(@\"{targetNullValueKey}\", {ParseContextPropertyAccess})");
+								}
+								else if (option.Member.Name == "FallbackValue" && themeResourceCandidate.Members.FirstOrDefault().Value is string fallbackValueKey)
+								{
+									writer.AppendLineIndented($".ApplyFallbackValueThemeResource(@\"{fallbackValueKey}\", {ParseContextPropertyAccess})");
+								}
+							}
+						}
+
 						// xbind initialization
 						if (bindNode != null && !isBindingType)
 						{

src/Uno.UI/Helpers/BindingExtensions.cs

@@ -0,0 +1,23 @@
+using System.ComponentModel;
+using Microsoft.UI.Xaml.Data;
+
+namespace Uno.UI.Helpers.Xaml;
+
+public static class BindingExtensions
+{
+	[EditorBrowsable(EditorBrowsableState.Never)]
+	public static Binding ApplyTargetNullValueThemeResource(this Binding binding, string themeResourceName, object parseContext)
+	{
+		binding.TargetNullValueThemeResource = themeResourceName;
+		binding.ParseContext = parseContext;
+		return binding;
+	}
+
+	[EditorBrowsable(EditorBrowsableState.Never)]
+	public static Binding ApplyFallbackValueThemeResource(this Binding binding, string themeResourceName, object parseContext)
+	{
+		binding.FallbackValueThemeResource = themeResourceName;
+		binding.ParseContext = parseContext;
+		return binding;
+	}
+}

src/Uno.UI/UI/Xaml/Data/Binding.cs

@@ -123,6 +123,8 @@ public object FallbackValue
 			}
 		}
 
+		internal string FallbackValueThemeResource { get; set; }
+
 		/// <summary>
 		/// Gets or sets a value that indicates the direction of the data flow in the binding.
 		/// </summary>
@@ -180,6 +182,10 @@ public object Source
 		/// <value>The target null value.</value>
 		public object TargetNullValue { get; set; }
 
+		internal string TargetNullValueThemeResource { get; set; }
+
+		internal object ParseContext { get; set; }
+
 		/// <summary>
 		/// Gets or sets a value that determines the timing of binding source updates for two-way bindings.
 		/// </summary>

src/Uno.UI/UI/Xaml/DependencyObjectStore.cs

@@ -1289,13 +1289,31 @@ internal void UpdateResourceBindings(ResourceUpdateReason updateReason, Resource
 				throw new ArgumentException();
 			}
 
+			ResourceDictionary[]? dictionariesInScope = null;
+
+			if (updateReason == ResourceUpdateReason.ThemeResource)
+			{
+				dictionariesInScope = GetResourceDictionaries(includeAppResources: false, containingDictionary).ToArray();
+				for (var i = dictionariesInScope.Length - 1; i >= 0; i--)
+				{
+					ResourceResolver.PushSourceToScope(dictionariesInScope[i]);
+				}
+
+				_properties.UpdateBindingExpressions();
+
+				foreach (var dict in dictionariesInScope)
+				{
+					ResourceResolver.PopSourceFromScope();
+				}
+			}
+
 			if (_resourceBindings == null || !_resourceBindings.HasBindings)
 			{
 				UpdateChildResourceBindings(updateReason);
 				return;
 			}
 
-			var dictionariesInScope = GetResourceDictionaries(includeAppResources: false, containingDictionary).ToArray();
+			dictionariesInScope ??= GetResourceDictionaries(includeAppResources: false, containingDictionary).ToArray();
 
 			var bindings = _resourceBindings.GetAllBindings();
 
@@ -1428,6 +1446,8 @@ private void UpdateChildResourceBindings(ResourceUpdateReason updateReason)
 					// Call OnThemeChanged after bindings of descendants have been updated
 					themeChangeAware.OnThemeChanged();
 				}
+
+				_properties.OnThemeChanged();
 			}
 		}
 

src/Uno.UI/UI/Xaml/DependencyPropertyDetailsCollection.Bindings.cs

@@ -1,11 +1,12 @@
 using System;
 using System.Collections.Generic;
 using System.Text;
+using Microsoft.UI.Xaml.Data;
 using Uno.Collections;
 using Uno.Extensions;
 using Uno.Foundation.Logging;
+using Uno.UI;
 using Uno.UI.DataBinding;
-using Microsoft.UI.Xaml.Data;
 
 namespace Microsoft.UI.Xaml
 {
@@ -219,5 +220,58 @@ internal BindingExpression GetBindingExpression(DependencyProperty dependencyPro
 
 			return null;
 		}
+
+		internal void UpdateBindingExpressions()
+		{
+			foreach (var binding in _bindings)
+			{
+				UpdateBindingPropertiesFromThemeResources(binding.ParentBinding);
+			}
+
+			foreach (var binding in _templateBindings)
+			{
+				UpdateBindingPropertiesFromThemeResources(binding.ParentBinding);
+			}
+		}
+
+		private static void UpdateBindingPropertiesFromThemeResources(Binding binding)
+		{
+			if (binding.TargetNullValueThemeResource is { } targetNullValueThemeResourceKey)
+			{
+				binding.TargetNullValue = (object)ResourceResolverSingleton.Instance.ResolveResourceStatic(targetNullValueThemeResourceKey, typeof(object), context: binding.ParseContext);
+			}
+
+			if (binding.FallbackValueThemeResource is { } fallbackValueThemeResourceKey)
+			{
+				binding.FallbackValue = (object)ResourceResolverSingleton.Instance.ResolveResourceStatic(fallbackValueThemeResourceKey, typeof(object), context: binding.ParseContext);
+			}
+		}
+
+		internal void OnThemeChanged()
+		{
+			foreach (var binding in _bindings)
+			{
+				RefreshBindingValueIfNecessary(binding);
+			}
+
+			foreach (var binding in _templateBindings)
+			{
+				RefreshBindingValueIfNecessary(binding);
+			}
+		}
+
+		private void RefreshBindingValueIfNecessary(BindingExpression binding)
+		{
+			if (binding.ParentBinding.TargetNullValueThemeResource is not null ||
+				binding.ParentBinding.FallbackValueThemeResource is not null)
+			{
+				// Note: This may refresh the binding more than really necessary.
+				// For example, if TargetNullValue is set to a theme resource, but the binding is not null
+				// In this case, a change to TargetNullValue should probably not refresh the binding.
+				// Another case is when the ThemeResource evaluates the same between light/dark themes.
+				// For now, it's not necessary.
+				binding.RefreshTarget();
+			}
+		}
 	}
 }