ReactiveCocoa

ReactiveCocoa is an open-source framework for reactive programming on Apple platforms, designed to implement functional reactive programming (FRP) concepts in the Cocoa and Cocoa Touch ecosystems. It provides a set of abstractions for modeling, composing, and reacting to asynchronous events and user interactions in a declarative way.

At the core of ReactiveCocoa are signals, signal producers, and related operators. Signals represent streams of values over time, while signal producers create signals on demand. The framework also includes constructs such as RACCommand (for representing executable user actions), RACSubject (a flexible source of values), and various binding helpers that connect UI components to data sources. It offers a rich set of operators for transforming, combining, and filtering streams, enabling complex asynchronous logic to be expressed succinctly. In addition, it provides mechanisms for bridging Cocoa's key-value observing (KVO) and UI controls to reactive streams.

ReactiveCocoa began with support for Objective-C and has since been adapted for Swift, with a modular architecture that separates language bindings from Cocoa-specific features. The Swift layer leverages ReactiveSwift, a pure Swift FRP library, while ReactiveCocoa supplies the Cocoa-oriented extensions and UI bindings on top of that foundation. This structure allows developers to write code that reacts to changes in models, UI state, and user input in a cohesive, testable manner.

The project is maintained as an open-source effort with contributions from the community. While Apple’s Combine framework offers a native FRP alternative, ReactiveCocoa and ReactiveSwift remain in use in many projects, valued for their mature API and extensive Cocoa bindings.