combine-framework

Combine Framework

Reactive programming with Apple's Combine framework.

See also: Combine is the legacy reactive stack. For new SwiftUI state, prefer the Observation framework (@Observable) — see the swiftui-observation skill. Reach for Combine for existing ObservableObject/@Published code, custom publisher pipelines, and event streams.

Core Concepts

Publisher & Subscriber

// ✅ Basic subscription
let publisher = Just("Hello")
let cancellable = publisher.sink { value in
    print(value)  // "Hello"
}  // sink returns AnyCancellable, not a Subscriber

Creating Publishers

// ✅ Just - single value
Just(42)

// ✅ Future - one async result. Eager: the closure runs immediately at creation
// and the result is cached. Wrap in Deferred for lazy, per-subscriber execution.
Deferred {
    Future<Int, URLError> { promise in
        promise(.success(42))
        // or promise(.failure(URLError(.badServerResponse)))
    }
}

// ✅ PassthroughSubject - broadcast
let subject = PassthroughSubject<String, Never>()
subject.send("Hello")
subject.send(completion: .finished)

// ✅ CurrentValueSubject - holds current value
let currentValue = CurrentValueSubject<Int, Never>(0)
currentValue.send(1)
print(currentValue.value)  // 1

// ✅ @Published in ObservableObject
class ViewModel: ObservableObject {
    @Published var count = 0
}

Common Operators

Transform

// ✅ map - transform values
[1, 2, 3].publisher
    .map { $0 * 2 }
    .sink { print($0) }  // 2, 4, 6

// ✅ flatMap - nested publishers
[1, 2, 3].publisher
    .flatMap { value in
        Future<Int, Never> { promise in
            promise(.success(value * 2))
        }
    }
    .sink { print($0) }

// ✅ scan - accumulate
(1...10).publisher
    .scan(0, +)
    .sink { print($0) }  // 1, 3, 6, 10, 15...

Filter

// ✅ filter
(1...10).publisher
    .filter { $0 % 2 == 0 }
    .sink { print($0) }  // 2, 4, 6, 8, 10

// ✅ removeDuplicates
[1, 1, 2, 2, 3].publisher
    .removeDuplicates()
    .sink { print($0) }  // 1, 2, 3

// ✅ debounce - wait for pause. Prefer DispatchQueue.main over RunLoop.main:
// RunLoop.main does not fire while the run loop is in tracking mode (scrolling).
searchTextPublisher
    .debounce(for: .milliseconds(300), scheduler: DispatchQueue.main)
    .sink { text in
        performSearch(text)
    }