Swift Academy · Lesson

@Published and Observation in Combine

How @Published works under the hood to emit changes through Combine publishers.

Lesson 3 of 412 steps

@Published and Observation in Combine is a free Swift Academy lesson on CoddyKit. This is lesson 3 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

`@Published` is a property wrapper from the Combine framework that emits a new value through a publisher whenever the property changes.

@Published Basics

```swift import Combine class Counter: ObservableObject { @Published var count = 0 } let c = Counter() let cancel = c.$count.sink { print("Count: \($0)") } c.count += 1 // "Count: 1" c.count += 1 // "Count: 2" ```

ObservableObject Protocol

```swift class ViewModel: ObservableObject { @Published var title = "Initial" @Published var items: [String] = [] } ``` Any class conforming to `ObservableObject` gets an `objectWillChange` publisher for free.

objectWillChange Publisher

```swift class VM: ObservableObject { @Published var name = "" } let vm = VM() let cancel = vm.objectWillChange.sink { print("Will change") } vm.name = "Alice" // "Will change" printed before assignment ```

@Published with SwiftUI @StateObject

```swift @StateObject var vm = ViewModel() // In View body: Text(vm.title) // Re-renders when vm.title changes ``` `@StateObject` owns the ViewModel; `@ObservedObject` observes an externally owned one.

Chaining Publishers from @Published

```swift class Search: ObservableObject { @Published var query = "" @Published var results: [String] = [] private var cancellables = Set() init() { $query .debounce(for: 0.3, scheduler: RunLoop.main) .sink { [weak self] q in self?.search(q) } .store(in: &cancellables) } } ```

@Published Fires Before Assignment

`objectWillChange` fires *before* the new value is stored: ```swift vm.objectWillChange.sink { _ in print("will change") } $count.sink { n in print("is \(n)") } // Order: "will change" then "is 1" ```

Storing Subscriptions

```swift var cancellables = Set() c.$count .sink { print("\($0)") } .store(in: &cancellables) // cancellables keeps subscription alive ```

@Published Limitations

• Only on classes (reference types) • Cannot use on `let` constants • Does not work across actor boundaries without explicit isolation • Value published *before* assignment completes (willSet semantics)

Observation Macro (Swift 5.9+)

Swift 5.9 introduces `@Observable` as a simpler alternative: ```swift @Observable class Counter { var count = 0 } // No Combine import needed; SwiftUI auto-subscribes ```

Quick Check

When does `@Published` emit its value — before or after the property is set?

Answer it in the interactive lesson

Recap

Key takeaways: • `@Published var x` exposes `$x` as a `Publisher` • `ObservableObject` gets a free `objectWillChange` publisher • Fires before the new value is stored (willSet) • Store subscriptions in `Set` • Swift 5.9: `@Observable` replaces most `@Published` needs Next: composing property wrappers.

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Frequently Asked Questions

Is the “@Published and Observation in Combine” lesson free?

Yes — any 3 lessons of the Swift Academy learning path, including “@Published and Observation in Combine”, 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 “@Published and Observation in Combine”?

How @Published works under the hood to emit changes through Combine publishers. 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 3 of 4.

How long does the “@Published and Observation in Combine” 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?

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