Swift Academy · Lesson

async let and Concurrent Child Tasks

Running independent async work concurrently with async let bindings.

Lesson 3 of 412 steps

async let and Concurrent Child Tasks 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

`async let` runs multiple async operations concurrently — all start immediately, and `await` waits for their results when needed.

Sequential vs Concurrent

```swift // Sequential — slow: let a = try await fetchA() let b = try await fetchB() // Concurrent with async let — fast: async let a = fetchA() async let b = fetchB() let (resultA, resultB) = try await (a, b) ```

async let Syntax

```swift async let title = fetchTitle(id) async let author = fetchAuthor(id) async let content = fetchContent(id) let article = Article( title: try await title, author: try await author, content: try await content ) ```

All or Nothing

If any `async let` binding throws, the error propagates. The system automatically cancels sibling tasks.

async let in Loops

```swift var results: [Data] = [] await withTaskGroup(of: Data.self) { group in for url in urls { group.addTask { try await URLSession.shared.data(from: url).0 } } for await data in group { results.append(data) } } ```

TaskGroup

```swift await withTaskGroup(of: Int.self) { group in for i in 1...5 { group.addTask { i * i } } var squares: [Int] = [] for await square in group { squares.append(square) } print(squares.sorted()) // [1,4,9,16,25] } ```

ThrowingTaskGroup

```swift try await withThrowingTaskGroup(of: User.self) { group in for id in ids { group.addTask { try await fetchUser(id: id) } } for try await user in group { process(user) } } ```

Structured Concurrency Tree

`async let` and TaskGroup form a tree of tasks. Parent tasks wait for all children. Cancelling the parent cancels all children.

When to Use async let vs TaskGroup

• `async let` — fixed number of concurrent operations known at compile time • `TaskGroup` — dynamic number of tasks, e.g. processing an array

Limit Concurrency with TaskGroup

```swift await withTaskGroup(of: Void.self) { group in let concurrency = 4 var running = 0 for url in urls { if running >= concurrency { await group.next() } group.addTask { await process(url) } running += 1 } } ```

Quick Check

What happens to sibling `async let` tasks if one throws?

Answer it in the interactive lesson

Recap

Key takeaways: • `async let x = asyncWork()` — starts immediately, concurrent • `try await x` — waits for the result • All-or-nothing: failure cancels siblings • `withTaskGroup` — dynamic set of concurrent tasks • Structured concurrency = automatic child management Next: Actors.

Free to start

Learn Swift with an AI tutor — free

Write and run real code in your browser, get instant help from a 24/7 AI tutor, and pick up where you left off on the web or in the app.

Courses: 122
Lessons: 409

Start learning free View the full course

Frequently Asked Questions

Is the “async let and Concurrent Child Tasks” lesson free?

Yes — any 3 lessons of the Swift Academy learning path, including “async let and Concurrent Child Tasks”, 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 “async let and Concurrent Child Tasks”?

Running independent async work concurrently with async let bindings. 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 “async let and Concurrent Child Tasks” 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

async/await Syntax and Calling Async Functions
Task and Task Cancellation
async let and Concurrent Child Tasks
Actors: Protecting Shared Mutable State

← Back to Swift Academy