fix(xamlreader): Load ThemeResources properly

This change ensures that ThemeResources are loaded through the common flow for resource loading, so that the values coming from actual theme dictionaries, and parent element lazy dictionaries are applied properly.

Author: jeromelaban

src/Uno.UI.Tests/Windows_UI_Xaml_Markup/XamlReaderTests/Given_XamlReader.cs

@@ -240,6 +240,64 @@ public void When_StaticResource()
 			app.Resources.Remove("StaticHeight");
 		}
 
+		[TestMethod]
+		public void When_ThemeResource()
+		{
+			var app = UnitTestsApp.App.EnsureApplication();
+
+			if (app.Resources.ThemeDictionaries.TryGetValue("Light", out var themeDictionary)
+				&& themeDictionary is ResourceDictionary dictionary)
+			{
+				dictionary["StaticRow"] = 42;
+				dictionary["StaticWidth"] = 42.0;
+				dictionary["StaticHeight"] = 44.0;
+			}
+
+			var s = GetContent(nameof(When_ThemeResource));
+			var r = Windows.UI.Xaml.Markup.XamlReader.Load(s) as UserControl;
+
+			Assert.IsNotNull(r);
+
+			var panel = r.FindName("innerPanel") as StackPanel;
+			Assert.IsNotNull(panel);
+
+			Assert.AreEqual(42, Grid.GetRow(panel));
+			Assert.AreEqual(42.0, panel.Width);
+			Assert.AreEqual(44.0, panel.Height);
+
+			if (app.Resources.ThemeDictionaries.TryGetValue("Light", out var themeDictionary2)
+				&& themeDictionary is ResourceDictionary dictionary2)
+			{
+				dictionary2.Remove("StaticRow");
+				dictionary2.Remove("StaticWidth");
+				dictionary2.Remove("StaticHeight");
+			}
+		}
+
+		[TestMethod]
+		public void When_ThemeResource_Lazy()
+		{
+			var app = UnitTestsApp.App.EnsureApplication();
+
+			var s = GetContent(nameof(When_ThemeResource_Lazy));
+			var r = Windows.UI.Xaml.Markup.XamlReader.Load(s) as UserControl;
+
+			Assert.IsNotNull(r);
+
+			var panel = r.FindName("innerPanel") as StackPanel;
+			Assert.IsNotNull(panel);
+
+			Assert.AreEqual(0, Grid.GetRow(panel));
+			Assert.AreEqual(double.NaN, panel.Width);
+			Assert.AreEqual(double.NaN, panel.Height);
+
+			r.ForceLoaded();
+
+			Assert.AreEqual(42, Grid.GetRow(panel));
+			Assert.AreEqual(42.0, panel.Width);
+			Assert.AreEqual(44.0, panel.Height);
+		}
+
 		[TestMethod]
 		public void When_TextBlock_Basic()
 		{
@@ -306,6 +364,8 @@ public void When_Style_ControlTemplate()
 			var r = Windows.UI.Xaml.Markup.XamlReader.Load(s) as UserControl;
 			Assert.IsNotNull(r);
 
+			r.ForceLoaded();
+
 			var innerContent = r.Content as ContentControl;
 			Assert.IsNotNull(innerContent);
 
@@ -587,6 +647,8 @@ public void When_StaticResource_Style_And_Binding()
 			var s = GetContent(nameof(When_StaticResource_Style_And_Binding));
 			var r = Windows.UI.Xaml.Markup.XamlReader.Load(s) as UserControl;
 
+			r.ForceLoaded();
+
 			Assert.IsTrue(r.Resources.ContainsKey("test"));
 
 			var tb1 = r.FindName("tb1") as ToggleButton;

src/Uno.UI.Tests/Windows_UI_Xaml_Markup/XamlReaderTests/When_ThemeResource.xamltest

@@ -0,0 +1,13 @@
+<Page xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+      xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
+      xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+      xmlns:controls="using:Microsoft.Toolkit.Uwp.UI.Controls"
+			Name="rootPage"
+      mc:Ignorable="d">
+
+	<StackPanel x:Name="innerPanel" 
+				Width="{ThemeResource StaticWidth}"
+				Height="{ThemeResource StaticHeight}"
+				Grid.Row="{ThemeResource StaticRow}" />
+</Page>

src/Uno.UI.Tests/Windows_UI_Xaml_Markup/XamlReaderTests/When_ThemeResource_Lazy.xamltest

@@ -0,0 +1,19 @@
+<Page xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+      xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
+      xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+      xmlns:controls="using:Microsoft.Toolkit.Uwp.UI.Controls"
+			Name="rootPage"
+      mc:Ignorable="d">
+  
+  <Page.Resources>
+	<x:Double x:Key="StaticWidth">42.0</x:Double>
+	<x:Double x:Key="StaticHeight">44.0</x:Double>
+	<x:Int32 x:Key="StaticRow">42</x:Int32>
+  </Page.Resources>
+
+	<StackPanel x:Name="innerPanel" 
+				Width="{ThemeResource StaticWidth}"
+				Height="{ThemeResource StaticHeight}"
+				Grid.Row="{ThemeResource StaticRow}" />
+</Page>

src/Uno.UI/UI/Xaml/Markup/Reader/XamlObjectBuilder.cs

@@ -7,6 +7,7 @@
 using System.Text.RegularExpressions;
 using Uno.Extensions;
 using Uno.UI;
+using Uno.UI.Xaml;
 using Uno.Xaml;
 using Windows.UI.Xaml.Controls;
 using Windows.UI.Xaml.Data;
@@ -453,7 +454,7 @@ private void ProcessMemberMarkupExtension(object instance, XamlMemberDefinition
 			{
 				ProcessBindingMarkupNode(instance, member);
 			}
-			else if (IsStaticResourceMarkupNode(member))
+			else if (IsStaticResourceMarkupNode(member) || IsThemeResourceMarkupNode(member))
 			{
 				ProcessStaticResourceMarkupNode(instance, member);
 			}
@@ -465,25 +466,26 @@ private void ProcessStaticResourceMarkupNode(object instance, XamlMemberDefiniti
 
 			if (resourceNode != null)
 			{
-				string keyName = resourceNode.Members.FirstOrDefault()?.Value?.ToString();
+				var keyName = resourceNode.Members.FirstOrDefault()?.Value?.ToString();
 				var dependencyProperty = TypeResolver.FindDependencyProperty(member);
 
-				if (keyName != null && dependencyProperty != null)
+				if (keyName != null
+					&& dependencyProperty != null
+					&& instance is DependencyObject dependencyObject)
 				{
-					void ResolveResource()
-					{
-						object staticResource = ResolveStaticResource(instance, keyName);
+					ResourceResolver.ApplyResource(
+						dependencyObject,
+						dependencyProperty,
+						keyName,
+						isThemeResourceExtension: IsThemeResourceMarkupNode(member));
 
-						if (staticResource != null)
+					if (instance is FrameworkElement fe)
+					{
+						fe.Loading += delegate
 						{
-							instance.SetValue(
-								dependencyProperty,
-								staticResource
-							);
-						}
+							fe.UpdateResourceBindings();
+						};
 					}
-
-					_postActions.Enqueue(ResolveResource);
 				}
 				else
 				{
@@ -513,7 +515,10 @@ private static object ResolveStaticResource(object instance, string keyName)
 		}
 
 		private bool IsStaticResourceMarkupNode(XamlMemberDefinition member)
-			=> member.Objects.Any(o => o.Type.Name == "StaticResource" || o.Type.Name == "ThemeResource");
+			=> member.Objects.Any(o => o.Type.Name == "StaticResource");
+
+		private bool IsThemeResourceMarkupNode(XamlMemberDefinition member)
+			=> member.Objects.Any(o => o.Type.Name == "ThemeResource");
 
 		private void ProcessBindingMarkupNode(object instance, XamlMemberDefinition member)
 		{