Proposal: Non-XAML `ThemeResource`

[MartinZikmund]

Proposal: Non-XAML ThemeResource

Summary

Currently it is only possible to set ThemeResource from XAML, which is problematic in various scenarios, for example in Uno Platform's C# Markup:

• [MVUX/CSM] Resources don't update after changing Fluent Theme manually  unoplatform/uno.extensions#2429
• https://github.com/unoplatform/uno.csharpmarkup/issues/716

This is especially problemtai

Rationale

• It is not possible to retrieve a theme-dependent brush for a given theme from a ResourceDictionary in code
• It is not possible to set a property like Background or BorderBrush of a control to a theme-dependent brush in code

Scope

Capability	Priority
Retrieve a theme-dependent value of a resource from ResourceDictionary in code	Must
Set a control's dependency property to a theme-dependent resource in code	Must

Workarounds

There some very limited ways to work around this, like:

microsoft-ui-xaml/src/controls/dev/WebView2/WebView2.cpp

Lines 909 to 927 in 66a7b0a

	winrt::Color WebView2::GetThemeBackgroundColor()
	{
	bool isHighContrast = GetAccessibilitySettings().HighContrast();
	std::wstring helperBrushName = isHighContrast ? L"BrushForThemeBackgroundColor_HC" : L"BrushForThemeBackgroundColor";
	// Ensure helper SCB referencing appropriate color as a ThemeResource is added to WV2.Resources.
	// This is needed since programatic lookup of theme resources themselves does not account for active theme.
	if (!this->Resources().HasKey(box_value(helperBrushName)))
	{
	std::wstring resourceName = isHighContrast ? L"SystemColorWindowColor" : L"SolidBackgroundFillColorBase";
	winrt::SolidColorBrush helperBrush = winrt::XamlReader::Load(
	L"<SolidColorBrush xmlns='http://schemas.microsoft.com/winfx/2006/xaml/presentation' "
	L"xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml' "
	L"x:Name='" + helperBrushName + L"' "
	L"Color='{ThemeResource " + resourceName + L"}'/>"
	).as<winrt::SolidColorBrush>();
	this->Resources().Insert(box_value(helperBrushName), helperBrush);
	}

, but these are very inefficient and definitely not ideal.