What is Combine and its basic entities?

Middle PRO

The essence

Combine is Apple's declarative framework (iOS 13) for working with streams of values over time: UI events, network responses, notifications, property changes. Instead of callbacks you describe a chain of transformations, and the framework manages subscription, delivery and completion.

The basic entities:

• Publisher — the source. Parameterized by Output (the value type) and Failure (the error type, Never if it cannot fail). Emits zero or more values, then exactly one completion: .finished or .failure.
• Subscriber — the consumer. Receives a Subscription, requests demand, accepts values and completion. Built-in ones: sink, assign.
• Subscription — the link between them; through it the subscriber requests values (request(_:)) and cancels the stream (cancel()). Exposed outside as AnyCancellable.
• Operator — a Publisher method returning a new Publisher (map, filter, debounce, flatMap, combineLatest…). Operators are pure composition; they do nothing until subscribed.
• Subject (PassthroughSubject, CurrentValueSubject) — a Publisher you can feed imperatively; the bridge from non-reactive code.
• Scheduler — where and when work runs (DispatchQueue, RunLoop, OperationQueue).