Windows App SDK
Windows App SDK (formerly known as Project Reunion) is a software development kit (SDK) from Microsoft that provides a unified set of APIs and components that can be used to develop desktop applications for both Windows 11 and Windows 10 version 1809 and later. The purpose of this project is to offer a decoupled implementation of capabilities which were previously tightly-coupled to the UWP app model. Windows App SDK allows native Win32 (USER32/GDI32) or .NET (WPF/WinForms) developers alike a path forward to enhance their apps with modern features. It follows that Windows App SDK is not intended to replace the Windows SDK. By exposing a common application programming interface (API) primarily using the Windows Runtime (WinRT) through generated WinMD metadata, the tradeoffs which once characterized either app model are largely eliminated. NuGet packages for version 1.4 were released in August 2023 after approximately four months of development. Features and components
While Microsoft has developed a number of new features, some of the features listed below are abstractions of functionality provided by existing APIs. WinUI 3 
Most of the investment into the decoupled UI stack has gone towards bug fixes, improvements to the debugging experience, and simplifying the window management capabilities made possible by switching from CoreWindow. An API abstracting USER32/GDI32 primitives known as AppWindow was introduced to expose a unified set of windowing capabilities and enable support for custom window controls. WebView2 
A replacement for the UWP WebView control was announced early on. This is because it was based on an unsupported browser engine. A new Chromium-based control, named WebView2, was developed and can be used from WinUI as well as other supported app types. Packaging 
While MSIX is included in the Windows App SDK and considered to be the recommended application packaging format, a design goal was to allow for unpackaged apps. These apps can be deployed as self-contained or framework-dependent. Support for dynamic loading of app dependencies is included for both packaged and unpackaged apps. Graphics 
DWriteCore is being developed as a decoupled and device-independent solution for high-quality text rendering. Win2D has also been made available to WinUI 3 apps. Resource management 
MRT Core allows for management of app resources for purposes such as localization. It is a decoupled version of the resource management system from UWP. App lifecycle 
With the stable releases delivered after its initial launch, Windows App SDK now supports several app lifecycle features which previously required a considerable amount of effort for developers to implement in Win32 applications. These features include power management notifications, rich activation, multiple instances, and programmatic app restart. Notifications 
Support for push notifications was initially implemented as a limited-access, preview feature. However, the APIs for it have since been stabilized and push notifications can be delivered to app users. Official documentation states that access to the feature can be revoked by Microsoft at their discretion. Additionally, apps can now easily display local app notifications without the need to create an XML payload. Widgets 
Third-party integration with the Windows Widgets system in Windows 11 has been included as part of the stable release channel. Developers can design custom widgets for their app using adaptive cards and surface them on the widgets board. See also
DWriteCore
Uno Platform
Windows Driver Kit (WDK)

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External links

Free and open-source software
Microsoft development tools
Microsoft free software
Software development kits
Windows APIs
Windows-only free software