A library to dual target system XAML and WinUI
This is a C++ header-only library that allows your app or library to seamlessly target either UWP XAML and WinUI 3. This is achieved via type and namespace aliases. Currently the library can target WinUI 3 Preview 2.
Your app can switch from UWP XAML to WinUI 3 by defining a pre-processor variable USE_WINUI3.
The syntax for packages.config does not allow runtime conditional behavior, so your project's packages.config will have to include WinUI 3 (and WinUI 2 if you use it) always, and then at build time one or the other will be used.
In your msbuild project (vcxproj) file:
<ItemDefinitionGroup>
<!-- Enlighten C++ code about WinUI3 -->
<ClCompile>
<PreprocessorDefinitions>USE_WINUI3;%(PreprocessorDefinitions)</PreprocessorDefinitions>
</ClCompile>
<!-- Enlighten IDL interfaces about WinUI3 -->
<Midl>
<PreprocessorDefinitions>USE_WINUI3;%(PreprocessorDefinitions)</PreprocessorDefinitions>
</Midl>
</ItemDefinitionGroup>Instead of including <winrt/Windows.UI.Xaml.h> etc., just include <UI.Xaml.h>.
Then when use the namespace aliases, e.g. instead of winrt::Windows::UI::Xaml (or WinUI 3's winrt::Microsoft::UI::Xaml) use the xaml alias.