ACSignal

ACSignal is a lightweight abstraction used in reactive programming libraries to model an asynchronous sequence of values over time. An ACSignal may emit zero or more values, followed by a completion event or an error. Signals decouple producers from consumers: the code that creates a signal does not need to know which code will handle its emissions. Signals can deliver events synchronously or asynchronously, depending on configuration or the scheduler in use.

Subscribers attach handlers by subscribing to the signal. A subscription provides callbacks for next values, errors, and completion. Signals are typically immutable and composed with operators to transform, filter, merge, or combine streams. Common operators include map, filter, and flatMap. The lifecycle of a subscription is managed by a disposable token; disposing cancels work and stops further events. Many implementations support delivering events on a specific thread or queue to coordinate UI updates or background processing.

ACSignal is used in various reactive programming ecosystems. It is conceptually similar to signal types in other frameworks, such as RACSignal in ReactiveCocoa and Publisher types in newer reactive libraries. Advocates cite improved composability and testability, while critics note added indirection. Adoption typically involves selecting a library that provides ACSignal-compatible operators and schedulers and integrating with existing asynchronous code.