ReactiveX

ReactiveX, or Reactive Extensions, is a cross-language API for composing asynchronous and event-based programs using observable sequences. It provides a standard set of interfaces and operators that allow developers to work with streams of data as they arrive over time. The project originated as Reactive Extensions for the .NET platform developed by Microsoft around 2009–2010 and later expanded into a family of language-specific implementations under the ReactiveX umbrella.

In ReactiveX, an observable represents a push-based sequence of values. An observer subscribes to an observable to receive notifications via onNext, onError, and onCompleted. Operators transform, filter, combine, or otherwise manipulate these streams, enabling complex data flows to be expressed declaratively. Concurrency is controlled through schedulers, and some implementations introduce backpressure-aware types to handle high-volume streams.

Popular implementations include RxJava for Java and Android, RxJS for JavaScript, RxSwift for Swift, and Rx.NET for C#. These libraries share the same fundamental concepts and operators, but they may differ in details and performance characteristics across languages.

ReactiveX is widely used for asynchronous programming, user interface event handling, network requests, real-time data processing, and other scenarios where data arrives over time. While it emphasizes composition and functional-style operators, it is compatible with traditional imperative code and can interoperate with promises or futures in many contexts.

The ReactiveX ecosystem continues to evolve, with ongoing development of new operators and language bindings, as well as discussions about backpressure, cancellation, and error handling semantics across platforms.