axiom-combine-patterns

Combine Patterns

Overview

Combine remains embedded in massive production codebases — UIKit delegates, NotificationCenter bridging, KVO observation, and @Published properties are everywhere. New code prefers async/await, but interop and maintenance of existing Combine pipelines is daily work. This skill covers the decisions and pitfalls that matter: when to use Combine vs async/await, how to avoid memory leaks, and how to bridge between the two paradigms.

Core principle: Combine is not dead — it's mature. The question isn't "should I use Combine?" but "is Combine the right tool for THIS specific data flow?"

When to Use This Skill

• Working with existing Combine pipelines
• Deciding between Combine and async/await for a new data flow
• Debugging AnyCancellable memory leaks or silent pipeline failures
• Using @Published or ObservableObject
• Bridging Combine publishers with async/await code
• Working with Subjects (PassthroughSubject, CurrentValueSubject)

When NOT to Use This Skill

• Timer.publish patterns → route via axiom-ios-concurrency to timer-patterns skill (dedicated timer lifecycle coverage)
• @Observable migration from ObservableObject → use axiom-swift-concurrency (modern observation)
• UIKit ↔ SwiftUI bridging → route via axiom-ios-ui (view wrapping, not data flow)
• General async/await patterns → use axiom-swift-concurrency

Example Prompts

• "Should I use Combine or async/await for this?"
• "My Combine pipeline silently stops producing values"
• "How do I convert a publisher to an async sequence?"
• "AnyCancellable is leaking — where do I store it?"
• "What's the difference between combineLatest and zip?"
• "How do I debounce a text field with Combine?"
• "My @Published property update isn't reaching the view"
• "How do I bridge a Combine publisher into async/await code?"

---

Part 1: Combine vs async/await Decision Tree

Use Case	Combine	async/await	Why
One-shot network call	No	Yes	async/await is simpler, no cancellable management
Stream of values over time	Yes	AsyncStream	Combine's operators (debounce, combineLatest) are richer
Debounce/throttle user input	Yes	Awkward	Combine has built-in debounce/throttle; AsyncStream requires manual implementation
Merge multiple sources	Yes	TaskGroup	Combine's merge/combineLatest handle heterogeneous streams naturally
Existing UIKit KVO/Notification	Yes	Bridge	publisher(for:) and NotificationCenter.default.publisher are idiomatic Combine
New project iOS 17+	No	Yes	@Observable + async/await is the modern pattern
Existing codebase with Combine	Maintain	Migrate incrementally	Don't rewrite working pipelines — bridge at boundaries

Quick Decision

Is it a one-shot operation (network call, file read)?
├─ Yes → async/await (simpler, no cancellable management)
│
Does it need time-based operators (debounce, throttle, delay)?
├─ Yes → Combine (built-in operators, no manual implementation)
│
Are you combining multiple ongoing streams?
├─ Yes → Combine (combineLatest, merge, zip are purpose-built)
│
Is this new code on iOS 17+?
├─ Yes → async/await + @Observable (modern pattern)
│
Is it existing Combine code that works?
└─ Yes → Keep it. Bridge at boundaries when async/await code needs the data.

---

Part 2: Publisher/Subscriber Lifecycle

AnyCancellable Storage Rules

AnyCancellable cancels its subscription when deallocated. If you don't store it, the pipeline is cancelled immediately after setup.

❌ Pipeline dies instantly

func setupPipeline() {
    publisher
        .sink { value in
            self.handle(value)  // Never called
        }
    // AnyCancellable returned by sink is discarded → subscription cancelled
}

✅ Store in Set

private var cancellables = Set<AnyCancellable>()

func setupPipeline() {
    publisher
        .sink { [weak self] value in
            self?.handle(value)
        }
        .store(in: &cancellables)
}

Why Set, Not Array

Set<AnyCancellable> is the idiomatic choice because:

• store(in:) works with both Set and RangeReplaceableCollection (including Array), but Set is conventional
• Order doesn't matter for subscriptions
• Prevents accidental duplicates if setup runs twice

4 Memory Leak Patterns

Leak 1: Strong self in sink

// ❌ LEAK: sink closure captures self strongly
publisher
    .sink { value in
        self.handle(value)  // Strong capture → retain cycle
    }
    .store(in: &cancellables)

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

Leak 2: Missing store(in:)

// ❌ LEAK: cancellable assigned to local var, not stored
let cancellable = publisher.sink { handle($0) }
// cancellable deallocated at end of scope → pipeline cancelled

// ✅ FIX: store in instance property
publisher.sink { [weak self] in self?.handle($0) }
    .store(in: &cancellables)

Leak 3: Over-retained cancellables

// ❌ LEAK: cancellables set never cleared, old pipelines accumulate
func refreshData() {
    // Each call adds another subscription without removing the previous one
    dataPublisher
        .sink { [weak self] in self?.update($0) }
        .store(in: &cancellables)
}

// ✅ FIX: clear before re-subscribing
func refreshData() {
    cancellables.removeAll()  // Cancel previous subscriptions
    dataPublisher
        .sink { [weak self] in self?.update($0) }
        .store(in: &cancellables)
}

Leak 4: assign(to:on:) strong capture

assign(to:on:) captures the on: parameter strongly. When the target is self, you get a retain cycle: self → cancellables → subscription → self.

// ❌ LEAK: assign(to:on:) retains self strongly — deinit never called
userPublisher
    .map { $0.name }
    .assign(to: \.displayName, on: self)
    .store(in: &cancellables)

// ✅ FIX: use assign(to:) with @Published projected value (iOS 14+)
userPublisher
    .map { $0.name }
    .assign(to: &$displayName)
// No store(in:) needed — subscription tied to @Published property lifetime

Key difference: assign(to: &$prop) does NOT return an AnyCancellable — the subscription is managed internally and cancelled when the @Published property's owner deallocates. No retain cycle, no cancellable storage needed.

If you must support iOS 13, use sink with [weak self] instead.

---

Part 3: Essential Operators

One canonical example per group. These cover 90% of real-world usage.

Transform

// map: transform each value
publisher.map { $0.name }

// compactMap: transform + filter nil
publisher.compactMap { Int($0) }

// flatMap: one-to-many (each value produces a new publisher)
searchText
    .flatMap { query in
        api.search(query)  // Returns a publisher
    }

flatMap gotcha: Without .switchToLatest() or maxPublishers: .max(1), flatMap creates a new inner publisher for every upstream value. For search-as-you-type, use map + switchToLatest instead:

searchText
    .map { query in api.search(query) }
    .switchToLatest()  // Cancels previous search when new query arrives

Combine Multiple Sources

// combineLatest: latest value from each, fires when ANY changes
Publishers.CombineLatest(namePublisher, agePublisher)
    .map { name, age in "\(name), \(age)" }

// merge: interleave values from same-type publishers
Publishers.Merge(localUpdates, remoteUpdates)
    .sink { update in handle(update) }

// zip: pairs values 1:1 (waits for both to produce)
Publishers.Zip(requestA, requestB)
    .sink { responseA, responseB in /* both complete */ }

Operator	Fires When	Use Case
combineLatest	Any input changes	Form validation (all fields)
merge	Any input produces	Combining event streams
zip	All inputs produce one value	Parallel requests that must complete together

Time-Based

// debounce: wait until values stop arriving (search-as-you-type)
searchTextPublisher
    .debounce(for: .milliseconds(300), scheduler: RunLoop.main)
    .sink { [weak self] query in self?.search(query) }
    .store(in: &cancellables)

// throttle: emit at most once per interval (scroll position)
scrollOffsetPublisher
    .throttle(for: .milliseconds(100), scheduler: RunLoop.main, latest: true)
    .sink { [weak self] offset in self?.updateHeader(offset) }
    .store(in: &cancellables)

Operator	Behavior	Use Case
debounce	Waits for silence, then emits last value	Search fields, auto-save
throttle(latest: true)	Emits latest value at fixed intervals	Scroll tracking, sensor data
throttle(latest: false)	Emits first value at fixed intervals	Rate-limiting button taps

Error Handling

// tryMap: transform that can throw
publisher.tryMap { data in
    try JSONDecoder().decode(Model.self, from: data)
}

// mapError: convert error types
publisher.mapError { error in
    AppError.network(error)
}

// replaceError: provide fallback value (terminates error path)
publisher.replaceError(with: defaultValue)

// retry: re-subscribe on failure
publisher.retry(3)  // Retry up to 3 times before propagating error

Error handling order matters: retry should come before replaceError. Retry re-subscribes to the upstream publisher; replaceError terminates the error and makes the pipeline infallible.

api.fetchData()
    .retry(3)                    // Try 3 more times on failure
    .replaceError(with: cached)  // If all retries fail, use cache
    .sink { data in update(data) }
    .store(in: &cancellables)

replaceError after flatMap kills the outer pipeline: If replaceError is downstream of flatMap, a single inner publisher error terminates the entire pipeline — not just that one request. Move error handling inside flatMap so each inner publisher handles its own errors:

// ❌ One API error kills the entire pipeline
$searchText
    .flatMap { query in api.search(query) }
    .replaceError(with: [])  // Pipeline completes on first error
    .sink { ... }

// ✅ Each search handles its own errors independently
$searchText
    .flatMap { query in
        api.search(query)
            .replaceError(with: [])  // Only this search affected
    }
    .sink { ... }

---

Part 4: @Published + ObservableObject

willSet Timing

@Published fires its publisher in willSet, not didSet. This means subscribers see the new value before the property has actually been set on the object.

class ViewModel: ObservableObject {
    @Published var count = 0

    init() {
        $count.sink { newValue in
            // 'newValue' is the incoming value
            // BUT self.count is still the OLD value here
            print("New: \(newValue), Current: \(self.count)")
            // Prints "New: 1, Current: 0" when count is set to 1
        }
        .store(in: &cancellables)
    }
}

If you need to read the property's value after it's been set, don't subscribe to $count — use a didSet observer instead, or read self.count after a brief deferral. The $ publisher is designed for reacting to the incoming value, not for reading post-mutation state.

Nested ObservableObject Trap

SwiftUI does NOT observe nested ObservableObject changes. Only the top-level object's objectWillChange triggers view updates.

// ❌ View won't update when settings.theme changes
class AppState: ObservableObject {
    @Published var settings = Settings()  // Settings is also ObservableObject
}

class Settings: ObservableObject {
    @Published var theme = "light"  // Changes here don't propagate
}

// ✅ FIX: Forward objectWillChange manually
class AppState: ObservableObject {
    @Published var settings = Settings()
    private var cancellables = Set<AnyCancellable>()

    init() {
        settings.objectWillChange
            .sink { [weak self] _ in
                self?.objectWillChange.send()
            }
            .store(in: &cancellables)
    }
}

Better fix for iOS 17+: Migrate to @Observable, which handles nested observation automatically. See axiom-swift-concurrency for migration patterns.

Thread Safety Warning

@Published is NOT thread-safe. Setting a @Published property from a background thread triggers objectWillChange off the main thread, which can crash SwiftUI views.

// ❌ CRASH: @Published set from background thread
class ViewModel: ObservableObject {
    @Published var data: [Item] = []

    func fetch() {
        Task {
            let items = await api.fetchItems()
            data = items  // Background thread → crash
        }
    }
}

// ✅ FIX: Ensure main thread
@MainActor
class ViewModel: ObservableObject {
    @Published var data: [Item] = []

    func fetch() {
        Task {
            let items = await api.fetchItems()
            data = items  // Safe — @MainActor ensures main thread
        }
    }
}

---

Part 5: Bridging Combine and async/await

Publisher → AsyncSequence

Use .values to consume any publisher as an async sequence:

let cancellable = notificationPublisher
    .sink { notification in handle(notification) }

// ✅ Modern equivalent using .values
for await notification in notificationPublisher.values {
    handle(notification)
}

Caveats with .values:

• The for await loop runs indefinitely until the publisher completes or the Task is cancelled
• Errors thrown by the publisher terminate the loop
• Only one consumer — if two for await loops consume the same .values, behavior is undefined

async/await → Publisher

Wrap an async function in Future for Combine consumption:

func fetchUser(id: String) async throws -> User { ... }

// Wrap as a Combine publisher
let userPublisher = Future<User, Error> { promise in
    Task {
        do {
            let user = try await fetchUser(id: "123")
            promise(.success(user))
        } catch {
            promise(.failure(error))
        }
    }
}

Future executes immediately — it runs its closure when created, not when subscribed. Wrap in Deferred if you need lazy execution:

let lazyPublisher = Deferred {
    Future<User, Error> { promise in
        Task {
            do {
                let user = try await fetchUser(id: "123")
                promise(.success(user))
            } catch {
                promise(.failure(error))
            }
        }
    }
}

Gradual Migration Strategy

Don't rewrite working Combine code. Bridge at the boundary:

Combine pipeline  →  .values  →  async/await code
                     (bridge)

async function    →  Future   →  Combine pipeline
                     (bridge)

Migration priority:

1. New code: write in async/await
2. Boundary: bridge with .values or Future
3. Existing Combine: leave working pipelines alone
4. Rewrite: only when the pipeline needs significant changes anyway

---

Part 6: Subjects

PassthroughSubject vs CurrentValueSubject

Feature	PassthroughSubject	CurrentValueSubject
Initial value	None	Required
Late subscribers	Miss previous values	Get current value immediately
.value property	No	Yes (read current value)
Use case	Events (button taps, notifications)	State (current selection, loading status)

// Event-driven: no initial value, late subscribers miss past events
let taps = PassthroughSubject<Void, Never>()
taps.send()

// State-driven: always has a current value
let isLoading = CurrentValueSubject<Bool, Never>(false)
isLoading.value = true  // Direct access
isLoading.send(false)   // Also works

Send-After-Completion Pitfall

Once a Subject receives a completion event, all subsequent send() calls are silently ignored. No crash, no error — just silence.

let subject = PassthroughSubject<Int, Never>()

subject.send(1)           // Delivered
subject.send(completion: .finished)
subject.send(2)           // Silently ignored — no crash, no warning

// This is the most common cause of "my pipeline stopped working"

Diagnosis: If a pipeline silently stops producing values, check whether anything upstream sent a .finished or .failure completion. Once complete, the pipeline is dead.

---

Part 7: Cold vs Hot Publishers (share/multicast)

Most Combine publishers are cold — they start work when subscribed and each subscriber gets its own independent execution. URLSession.dataTaskPublisher fires a new HTTP request per subscriber.

// ❌ Two subscribers = two network requests
let publisher = URLSession.shared
    .dataTaskPublisher(for: url)
    .map(\.data)
    .eraseToAnyPublisher()

publisher.sink { cache.store($0) }.store(in: &cancellables)  // Request 1
publisher.sink { display($0) }.store(in: &cancellables)       // Request 2

share()

.share() makes a cold publisher hot — the first subscriber triggers the work, subsequent subscribers share the output:

// ✅ One request, shared result
let publisher = URLSession.shared
    .dataTaskPublisher(for: url)
    .map(\.data)
    .share()
    .eraseToAnyPublisher()

publisher.sink { cache.store($0) }.store(in: &cancellables)  // Triggers request
publisher.sink { display($0) }.store(in: &cancellables)       // Shares result

share() Gotchas

Gotcha	Effect	Fix
Late subscribers miss values	share() uses PassthroughSubject — no replay	Attach all subscribers before the first value arrives, or use multicast with CurrentValueSubject
Upstream completed before subscriber attaches	Late subscriber immediately gets .finished with no values	Ensure subscription order, or cache the result outside Combine
All subscribers cancel → upstream cancels	New subscriber after that triggers a NEW upstream execution	Expected behavior, but surprising if you assumed the result was cached

When to use share()

Multiple subscribers to the same expensive publisher?
├─ No → Don't use share() (unnecessary complexity)
│
├─ Yes, all subscribe at the same time?
│   └─ Yes → share() works
│
└─ Yes, subscribers attach at different times?
    └─ Use multicast(subject:) with CurrentValueSubject, or cache the result in a property

---

Anti-Rationalization

Thought	Reality
"Combine is dead, just use async/await"	Combine has no deprecation notice. Thousands of production apps use it. Rewriting working pipelines wastes time and introduces bugs. Bridge incrementally instead.
"I'll just use .sink everywhere"	Without [weak self] and proper store(in:), every sink is a potential memory leak. The lifecycle rules in Part 2 prevent the top 4 leak patterns.
"assign(to:on:) is fine, it's the standard API"	It captures on: strongly — retain cycle if target is self. Use assign(to: &$prop) instead (Part 2, Leak 4).
"debounce and throttle are the same thing"	debounce waits for silence; throttle emits at intervals. Using the wrong one causes either delayed responses or missed events. Part 3 has the decision table.
"I know how @Published works"	@Published fires on willSet, not didSet. Nested ObservableObject doesn't propagate. Background thread access crashes. Part 4 covers all three traps.
"I'll migrate everything to async/await at once"	Full rewrites of working Combine code introduce bugs and waste time. Bridge at boundaries (Part 5). Rewrite only when the pipeline needs significant changes anyway.

---

Pressure Scenarios

Scenario 1: "Let's migrate all Combine code to async/await"

Setup: Tech lead wants to modernize the codebase. "Combine is legacy, let's rip it out."

Pressure: Authority + scope creep. The entire data layer uses Combine publishers, @Published properties, and operator chains.

Expected with skill: Push back with the gradual migration strategy (Part 5). New code uses async/await. Boundaries use .values and Future. Existing working pipelines stay until they need changes. Full rewrite is the most expensive option with the least benefit.

Pushback template: "Combine isn't deprecated — Apple still ships it in every SDK. A full rewrite of working pipelines introduces bugs we don't have today. Let's bridge at boundaries: new code in async/await, .values to consume existing publishers, and we only rewrite a pipeline when we're already changing it significantly."

---

Scenario 2: "Pipeline silently stopped — just recreate it"

Setup: A Combine pipeline stopped producing values after a refactor. No crash, no error.

Pressure: Time pressure. "Just tear it down and rebuild."

Expected with skill: Diagnose before rebuilding. Check: (1) Was a completion sent upstream? (send-after-completion, Part 6). (2) Is the AnyCancellable still alive? (storage rules, Part 2). (3) Did the publisher error without handling? (replaceError / catch, Part 3). These three causes cover 90% of silent pipeline failures.

Diagnostic checklist:

1. Is the AnyCancellable still stored? (Set not cleared, not deallocated)
2. Did anything upstream send .finished or .failure?
3. Is there a tryMap or other throwing operator without error handling?
4. Was switchToLatest used where the outer publisher completed?

Pushback template: "Before rebuilding, let me check four things: cancellable lifecycle, upstream completions, unhandled errors, and switchToLatest completion. One of these is almost always the cause. It takes 5 minutes to diagnose vs 30 minutes to rebuild and test."

---

Scenario 3: "Settings changes aren't updating the UI"

Setup: A settings screen uses a nested ObservableObject. The parent AppState holds a Settings object. When the user changes settings.theme, the UI doesn't update.

Pressure: "The binding works in isolation, it must be a SwiftUI bug. Let me just force a refresh with objectWillChange.send()."

Expected with skill: Recognize the nested ObservableObject trap (Part 4). SwiftUI does NOT observe nested ObservableObject changes — only the top-level object's objectWillChange triggers view updates. The fix is either forwarding objectWillChange from the nested object, or migrating to @Observable (iOS 17+) which handles nesting automatically.

Anti-pattern without skill: Sprinkling objectWillChange.send() calls throughout the code, adding @Published to every nested property (which doesn't help), or restructuring the model to flatten everything into one object (losing separation of concerns).

Pushback template: "SwiftUI only observes the top-level ObservableObject. Nested objects need their objectWillChange forwarded to the parent. Part 4 has the exact pattern — it's a 5-line fix in the parent's init, not a SwiftUI bug."

---

Resources

WWDC: 2019-722, 2019-721, 2020-10034

Docs: /combine, /combine/anycancellable, /combine/published

Skills: swift-concurrency, memory-debugging