Swift Academy · Lesson

Publishers and Subscribers

The Publisher/Subscriber contract, sink and assign built-in subscribers.

Lesson 1 of 412 steps

Publishers and Subscribers is a free Swift Academy lesson on CoddyKit. This is lesson 1 of 4. Any 3 lessons of this learning path are free to read in full — after that, CoddyKit PRO unlocks every lesson, plus hands-on practice with a built-in code editor and a 24/7 AI tutor. It is part of the Swift Academy learning path, and your progress syncs across the web and the CoddyKit app. The Swift Academy course includes 4 lessons in total.

Welcome

Combine is Apple's reactive framework. At its core are Publishers (produce values) and Subscribers (consume them).

Publisher Protocol

A `Publisher` emits values over time and eventually completes or fails: ```swift publisher .sink { print("Received: \($0)") } ``` The `Output` type is the value type; `Failure` is the error type.

Subscriber: sink

```swift let nums = [1,2,3].publisher let cancel = nums.sink { print($0) } // 1, 2, 3 ```

Subscriber: assign

```swift class ViewModel { @Published var status = "" } let vm = ViewModel() statusPublisher .assign(to: &vm.$status) ```

AnyCancellable

Every subscription returns `AnyCancellable`. Store it to keep the subscription alive: ```swift var cancellables = Set() pub.sink { print($0) }.store(in: &cancellables) ```

Just Publisher

```swift Just(42).sink { print($0) } // emits once then completes ```

Notification Publisher

```swift NotificationCenter.default .publisher(for: UIApplication.didBecomeActiveNotification) .sink { _ in print("Active!") } .store(in: &cancellables) ```

Timer Publisher

```swift Timer.publish(every: 1, on: .main, in: .common) .autoconnect() .sink { date in print(date) } .store(in: &cancellables) ```

Completion Events

```swift pub.sink( receiveCompletion: { completion in switch completion { case .finished: print("Done") case .failure(let e): print("Error: \(e)") } }, receiveValue: { print($0) } ) ```

Backpressure

Combine supports backpressure — subscribers can request specific numbers of values. The `.unlimited` demand is the default for `sink`.

Quick Check

What type must you store to keep a Combine subscription alive?

Answer it in the interactive lesson

Recap

Key takeaways: • Publishers emit values over time; Subscribers receive them • `sink` receives values and completion; `assign` binds to a property • Store `AnyCancellable` to keep subscriptions alive • Built-in publishers: `Just`, `Timer.publish`, NotificationCenter Next: Operators.

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Frequently Asked Questions

Is the “Publishers and Subscribers” lesson free?

Yes — any 3 lessons of the Swift Academy learning path, including “Publishers and Subscribers”, are free to read in full here on the web. After that, CoddyKit PRO unlocks every lesson, plus interactive practice with a built-in code editor and a 24/7 AI tutor. The Swift Academy course includes 4 lessons in total.

What will I learn in “Publishers and Subscribers”?

The Publisher/Subscriber contract, sink and assign built-in subscribers. You practise Swift Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Swift Academy?

No prior experience is required. Swift Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 1 of 4.

How long does the “Publishers and Subscribers” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Swift Academy lesson?

Yes. Every Swift Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

All lessons in this course

Publishers and Subscribers
Operators: map, filter, combineLatest
PassthroughSubject and CurrentValueSubject
Networking with Combine and URLSession

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