ReactiveUI

ReactiveUI is an open-source, cross-platform MVVM framework for .NET that builds on top of Reactive Extensions (Rx). It provides a way to compose user interfaces from observable sequences, enabling declarative data binding and asynchronous operations across different UI frameworks.

The framework centers on a set of reactive primitives. ReactiveObject implements INotifyPropertyChanged and serves as a base class for view models. ReactiveCommand encapsulates asynchronous or long-running operations as observable sequences, handling execution, can-execute logic, and error handling. WhenAnyValue monitors property changes and ObservableAsPropertyHelper helps expose derived, read-only properties from observables. Activation supports managing the lifecycle of view models, and Interaction provides a mechanism for view models to request user input or display dialogs without tightly coupling to a view.

ReactiveUI offers binding utilities that connect view models to views in a platform-agnostic way. Bind and BindCommand enable two-way data binding and command binding between view and view model, while WhenActivated helps manage subscriptions tied to the view’s life cycle. The design emphasizes testability, allowing developers to model UI interactions as observable sequences and to verify behavior with Rx testing utilities.

Platform support and ecosystem: ReactiveUI is designed to work across multiple UI platforms, including Windows desktop (WPF, WinForms), mobile (Xamarin.Forms, MAUI), and cross-platform toolkits such as Avalonia and Uno Platform. The framework is modular, with platform-specific packages, and is maintained by a community of contributors on GitHub.

Related concepts include the broader Reactive Extensions and the MVVM architectural pattern, with ReactiveUI providing a practical implementation that favors reactive data flows and composable UI logic.