Microsoft is forging ahead with its plan to try to unify the Windows app development space with its “Project Reunion” effort. This week, Microsoft took the wraps off the next milestone on this path: The second preview of WinUI 3. And officials also provided a few updated dates for its Reunion roadmap.

What is WinUI? Microsoft execs provided a handy slide (which I saw via @gcaughey), dedicated to answering that. WinUI is the native UI platform for WIindows 10, which can be used to build .NET and C++ apps for Windows 10 devices. WinUI is part of the Windows and Xbox OS shells, and also “many apps, plus platforms like Xamarin.Forms and React Native for Windows,” officials said.

WinUI 3 is under development and is one of the key components of Project Reunion. Microsoft released Preview 2 of WInUI 3 on July 15. Preview 2 includes a bunch of fixes, along with updated documentation and walkthroughs.

Microsoft has faced a big Windows developer problem since launching the Universal Windows Platform. The company didn’t convince the majority of developers to build new UWP apps and/or update their existing Win32 apps with UWP elements. Microsoft ended up with two, siloed native app platforms, which had uneven support between them. The Windows development team has been trying to devise a way to fix this and enable developers to simply build “Windows apps” that work on the 1 billion-plus Windows 10 devices out there.

As of yesterday, Microsoft officials said they are planning to make WinUI 3 Preview 3, which will include new features and capabilities and not just fixes, available in September, concurrent with Ignite. A WinUI 3 November release (name to be determined) will be the first “go-live” preview of WinUI 3, meaning it can be used in production apps. Microsoft also moved the date when it plans to open source the WinUI code to November.

In a Q&A session following a call about WinUI 3, Microsoft officials said that the Windows Presentation Foundation (WPF) team now sits in the same organization as WinUI. ARM64 support for WPF is planned for 21H1, officials said. Officials also said that they are working on plans to improve the XAML engine in the new unified dev platform, adding that this is one of the main reasons they’re separating WinUI from the core OS. (Thanks, again to @gcaughey, for tweeting the Qs and As.)

Microsoft’s long term vision for Project Reunion is to create a Unified App platform (rather than UWP, Unified Windows Platform) which will be applicable for Win32 and UWP apps. The team wants to offer developers a platform on which they can build state-separated, cloud-powered modern apps that will work across all the existing one billion-plus Windows PCs. 

The first announced components of Reunion are WinUI 3, Edge/Chromium-backed WebView 2, React Native for Windows, composition and input layers, C++/WinRT, Rust/WinRT and C# WinRT libraries, along with MSIX-Core for app distribution.