Combine Reactive — Expert Decisions

Expert decision frameworks for Combine choices. Claude knows Combine syntax — this skill provides judgment calls for when Combine adds value vs async/await and how to avoid common pitfalls.

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Decision Trees

Combine vs Async/Await

What's your data flow pattern?
├─ Single async operation (fetch once)
│  └─ async/await
│     Simpler, built into language
│
├─ Stream of values over time
│  └─ Is it UI-driven (user events)?
│     ├─ YES → Combine (debounce, throttle shine here)
│     └─ NO → AsyncSequence may suffice
│
├─ Combining multiple data sources
│  └─ How many sources?
│     ├─ 2-3 → Combine's CombineLatest, Zip
│     └─ Many → Consider structured concurrency (TaskGroup)
│
└─ Existing codebase uses Combine?
   └─ Maintain consistency unless migrating

The trap: Using Combine for simple one-shot async. async/await is cleaner and doesn't need cancellable management.

Subject Type Selection

Do you need to access current value?
├─ YES → CurrentValueSubject
│  Can read .value synchronously
│  New subscribers get current value immediately
│
└─ NO → Is it event-based (discrete occurrences)?
   ├─ YES → PassthroughSubject
   │  No stored value, subscribers only get new emissions
   │
   └─ NO (need replay of past values) →
      Consider external state management
      Combine has no built-in ReplaySubject

Operator Chain Design

What transformation is needed?
├─ Value transformation
│  └─ map (simple), compactMap (filter nils), flatMap (nested publishers)
│
├─ Filtering
│  └─ filter, removeDuplicates, first, dropFirst
│
├─ Timing
│  └─ User input → debounce (wait for pause)
│     Scrolling → throttle (rate limit)
│
├─ Error handling
│  └─ Can provide fallback? → catch, replaceError
│     Need retry? → retry(n)
│
└─ Combining streams
   └─ Need all values paired? → zip
      Need latest from each? → combineLatest
      Merge into one stream? → merge

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NEVER Do

Subscription Management

NEVER forget to store subscriptions:

// ❌ Subscription cancelled immediately
func loadData() {
    publisher.sink { value in
        self.data = value  // Never called!
    }
    // sink returns AnyCancellable that's immediately deallocated
}

// ✅ Store in Set<AnyCancellable>
private var cancellables = Set<AnyCancellable>()

func loadData() {
    publisher.sink { value in
        self.data = value
    }
    .store(in: &cancellables)
}

NEVER capture self strongly in sink closures:

// ❌ Retain cycle — ViewModel never deallocates
publisher
    .sink { value in
        self.updateUI(value)  // Strong capture!
    }
    .store(in: &cancellables)

// ✅ Use [weak self]
publisher
    .sink { [weak self] value in
        self?.updateUI(value)
    }
    .store(in: &cancellables)

NEVER use assign(to:on:) with classes:

// ❌ Strong reference to self — retain cycle
publisher
    .assign(to: \.text, on: self)  // Retains self!
    .store(in: &cancellables)

// ✅ Use sink with weak self
publisher
    .sink { [weak self] value in
        self?.text = value
    }
    .store(in: &cancellables)

// Or use assign(to:) with @Published (no retain cycle)
publisher
    .assign(to: &$text)  // Safe — doesn't return cancellable

Subject Misuse

NEVER expose subjects directly:

// ❌ External code can send values
class ViewModel {
    let users = PassthroughSubject<[User], Never>()  // Public subject!
}

// External code:
viewModel.users.send([])  // Breaks encapsulation

// ✅ Expose as Publisher
class ViewModel {
    private let usersSubject = PassthroughSubject<[User], Never>()

    var users: AnyPublisher<[User], Never> {
        usersSubject.eraseToAnyPublisher()
    }
}

NEVER send on subject from multiple threads without care:

// ❌ Race condition — undefined behavior
DispatchQueue.global().async {
    subject.send(value1)
}
DispatchQueue.global().async {
    subject.send(value2)
}

// ✅ Serialize access
let subject = PassthroughSubject<Value, Never>()
let serialQueue = DispatchQueue(label: "subject.serial")

serialQueue.async {
    subject.send(value)
}

// Or use .receive(on:) to ensure delivery on specific scheduler

Operator Mistakes

NEVER use flatMap when you mean map:

// ❌ Confusing — flatMap for non-publisher transformation
publisher
    .flatMap { user -> AnyPublisher<String, Never> in
        Just(user.name).eraseToAnyPublisher()  // Overkill!
    }

// ✅ Use map for simple transformation
publisher
    .map { user in user.name }

NEVER forget to handle errors in sink:

// ❌ Compiler allows this but errors are ignored
publisher  // Has Error type
    .sink { value in
        // Only handles values, error terminates silently
    }

// ✅ Handle both completion and value
publisher
    .sink(
        receiveCompletion: { completion in
            if case .failure(let error) = completion {
                handleError(error)
            }
        },
        receiveValue: { value in
            handleValue(value)
        }
    )

NEVER debounce without scheduler on main:

// ❌ Debouncing on background scheduler, updating UI
searchText
    .debounce(for: .milliseconds(300), scheduler: DispatchQueue.global())
    .sink { [weak self] text in
        self?.results = search(text)  // UI update on background thread!
    }

// ✅ Receive on main for UI updates
searchText
    .debounce(for: .milliseconds(300), scheduler: DispatchQueue.main)
    .sink { [weak self] text in
        self?.results = search(text)
    }

// Or explicitly receive on main
searchText
    .debounce(for: .milliseconds(300), scheduler: DispatchQueue.global())
    .receive(on: DispatchQueue.main)
    .sink { ... }

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Essential Patterns

ViewModel with Combine

@MainActor
final class SearchViewModel: ObservableObject {
    @Published var searchText = ""
    @Published private(set) var results: [SearchResult] = []
    @Published private(set) var isLoading = false
    @Published private(set) var error: Error?

    private let searchService: SearchServiceProtocol
    private var cancellables = Set<AnyCancellable>()

    init(searchService: SearchServiceProtocol) {
        self.searchService = searchService
        setupSearch()
    }

    private func setupSearch() {
        $searchText
            .debounce(for: .milliseconds(300), scheduler: DispatchQueue.main)
            .removeDuplicates()
            .filter { !$0.isEmpty }
            .handleEvents(receiveOutput: { [weak self] _ in
                self?.isLoading = true
                self?.error = nil
            })
            .flatMap { [weak self] query -> AnyPublisher<[SearchResult], Never> in
                guard let self = self else {
                    return Just([]).eraseToAnyPublisher()
                }
                return self.searchService.search(query: query)
                    .catch { [weak self] error -> Just<[SearchResult]> in
                        self?.error = error
                        return Just([])
                    }
                    .eraseToAnyPublisher()
            }
            .receive(on: DispatchQueue.main)
            .sink { [weak self] results in
                self?.results = results
                self?.isLoading = false
            }
            .store(in: &cancellables)
    }
}

Combine to Async Bridge

extension Publisher where Failure == Error {
    func async() async throws -> Output {
        try await withCheckedThrowingContinuation { continuation in
            var cancellable: AnyCancellable?
            var didResume = false

            cancellable = first()
                .sink(
                    receiveCompletion: { completion in
                        guard !didResume else { return }
                        didResume = true

                        switch completion {
                        case .finished:
                            break  // Value handled in receiveValue
                        case .failure(let error):
                            continuation.resume(throwing: error)
                        }
                        cancellable?.cancel()
                    },
                    receiveValue: { value in
                        guard !didResume else { return }
                        didResume = true
                        continuation.resume(returning: value)
                    }
                )
        }
    }
}

// Usage
Task {
    let user = try await fetchUserPublisher.async()
}

Event Bus Pattern

final class EventBus {
    static let shared = EventBus()

    // Private subjects
    private let userLoggedInSubject = PassthroughSubject<User, Never>()
    private let userLoggedOutSubject = PassthroughSubject<Void, Never>()
    private let cartUpdatedSubject = PassthroughSubject<Cart, Never>()

    // Public publishers (read-only)
    var userLoggedIn: AnyPublisher<User, Never> {
        userLoggedInSubject.eraseToAnyPublisher()
    }

    var userLoggedOut: AnyPublisher<Void, Never> {
        userLoggedOutSubject.eraseToAnyPublisher()
    }

    var cartUpdated: AnyPublisher<Cart, Never> {
        cartUpdatedSubject.eraseToAnyPublisher()
    }

    // Send methods
    func send(userLoggedIn user: User) {
        userLoggedInSubject.send(user)
    }

    func sendUserLoggedOut() {
        userLoggedOutSubject.send()
    }

    func send(cartUpdated cart: Cart) {
        cartUpdatedSubject.send(cart)
    }
}

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Quick Reference

Combine vs Async/Await Decision

Scenario	Prefer
Single async call	async/await
Stream of values	Combine
User input debouncing	Combine
Combining multiple API calls	Either (async let or CombineLatest)
Existing Combine codebase	Combine for consistency
New project, simple needs	async/await

Subject Comparison

Subject	Stores Value	New Subscribers Get
PassthroughSubject	No	Only new values
CurrentValueSubject	Yes	Current + new values

Common Operators

Category	Operators
Transform	map, flatMap, compactMap, scan
Filter	filter, removeDuplicates, first, dropFirst
Combine	combineLatest, zip, merge
Timing	debounce, throttle, delay
Error	catch, retry, replaceError

Red Flags

Smell	Problem	Fix
Subscription not stored	Immediate cancellation	.store(in: &cancellables)
Strong self in sink	Retain cycle	[weak self]
assign(to:on:self)	Retain cycle	Use sink or assign(to:&$property)
Public Subject	Encapsulation broken	Expose as AnyPublisher
flatMap for simple transform	Overkill	Use map
Debounce on background, sink updates UI	Threading bug	receive(on: .main)
Empty receiveCompletion	Errors ignored	Handle .failure case