Swift Academy · Lesson

Bridging Result to async/await

Converting callback-based Result APIs into async throwing functions.

Lesson 4 of 412 steps

Bridging Result to async/await is a free Swift Academy lesson on CoddyKit. This is lesson 4 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

Legacy callback-based APIs return `Result`. Modern code uses async/await. Bridging between them is a common task — Swift has clean patterns for both directions.

withCheckedThrowingContinuation

```swift func fetchAsync(url: URL) async throws -> Data { try await withCheckedThrowingContinuation { cont in fetchCallback(url) { result in cont.resume(with: result) } } } ``` `cont.resume(with: result)` accepts a `Result` directly.

withCheckedContinuation (Non-Throwing)

```swift func fetchAsync(url: URL) async -> Result { await withCheckedContinuation { cont in fetchCallback(url) { result in cont.resume(returning: result) } } } ```

Converting async to Result

```swift func asResult(of operation: () async throws -> T) async -> Result { do { return .success(try await operation()) } catch { return .failure(error) } } let r = await asResult { try await fetch(url) } ```

Result.get() in async Context

```swift func process() async { let result: Result = await fetchAsync(url) do { let data = try result.get() handle(data) } catch { handle(error) } } ```

Task { } for Bridging

```swift func load() { Task { let data = try await fetchAsync(url) await MainActor.run { self.render(data) } } } ```

AsyncStream from Callbacks

```swift func events() -> AsyncStream { AsyncStream { cont in let token = subscribe { event in cont.yield(event) } cont.onTermination = { _ in unsubscribe(token) } } } ```

Continuation Safety Rules

• Call `resume` exactly once — calling twice is undefined behaviour • Do not resume after the function has returned • Use `withCheckedContinuation` in debug (detects mistakes); `withUnsafeContinuation` in release if needed for performance

MainActor for UI Updates

```swift await MainActor.run { label.text = data.description } // or: @MainActor func updateUI(_ d: Data) { label.text = ... } await updateUI(data) ```

Testing Async+Result

```swift func testFetch() async throws { let result = await asResult { try await fetchAsync(url) } switch result { case .success(let d): XCTAssertFalse(d.isEmpty) case .failure(let e): XCTFail(e.localizedDescription) } } ```

Quick Check

Which continuation type detects misuse like double-resume in debug builds?

Answer it in the interactive lesson

Recap

Key takeaways: • `withCheckedThrowingContinuation` bridges callbacks to async throws • `cont.resume(with: result)` accepts a Result directly • Wrap `async throws` in `Result` with a helper function • Call `resume` exactly once • `MainActor.run { }` for safe UI updates from async context Course complete! Next: SwiftUI basics.

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Frequently Asked Questions

Is the “Bridging Result to async/await” lesson free?

Yes — any 3 lessons of the Swift Academy learning path, including “Bridging Result to async/await”, 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 “Bridging Result to async/await”?

Converting callback-based Result APIs into async throwing functions. 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 4 of 4.

How long does the “Bridging Result to async/await” 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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