Structured concurrency model in Swift

Introduction

Structured concurrency helps you write clear and safe code that runs tasks at the same time. It keeps tasks organized so you don't lose track of them.

When you want to run multiple tasks at once but keep them easy to manage.

When you need to wait for several tasks to finish before moving on.

When you want to avoid bugs from tasks running without control.

When you want your program to stop all related tasks if one fails.

When you want your code to be easier to read and understand.

Syntax

Swift

func example() async {
    await withTaskGroup(of: Void.self) { group in
        group.addTask {
            // task 1 work
        }
        group.addTask {
            // task 2 work
        }
        // wait for all tasks to finish
    }
}

withTaskGroup creates a group to run tasks together.

Tasks inside the group run concurrently but are managed as one unit.

Examples

This example runs two tasks that return strings and prints each result as it arrives.

Swift

func fetchImages() async {
    await withTaskGroup(of: String.self) { group in
        group.addTask { "Image 1" }
        group.addTask { "Image 2" }

        for await image in group {
            print("Got: \(image)")
        }
    }
}

This example shows how to handle tasks that can throw errors inside a throwing task group.

Swift

func downloadFiles() async throws {
    try await withThrowingTaskGroup(of: Data.self) { group in
        group.addTask { try downloadFile(url: url1) }
        group.addTask { try downloadFile(url: url2) }

        for try await fileData in group {
            print("Downloaded file with size: \(fileData.count)")
        }
    }
}

Sample Program

This program runs two tasks concurrently to fetch user data and prints each piece as it arrives.

Swift

import Foundation

func fetchUserData() async {
    await withTaskGroup(of: String.self) { group in
        group.addTask {
            // Simulate fetching user name
            "Alice"
        }
        group.addTask {
            // Simulate fetching user city
            "Wonderland"
        }

        for await data in group {
            print("Received: \(data)")
        }
    }
}

@main
struct Main {
    static func main() async {
        await fetchUserData()
    }
}

Important Notes

Structured concurrency makes sure all tasks finish or cancel together.

Use withTaskGroup for simple tasks and withThrowingTaskGroup if tasks can fail.

Tasks inside groups run concurrently but are easier to control and debug.

Summary

Structured concurrency groups tasks to run together safely.

It helps keep your code clean and avoids lost or runaway tasks.

Use task groups to run and wait for multiple tasks easily.

Practice

1. What is the main purpose of structured concurrency in Swift?

easy

A. To group tasks so they run safely and complete before continuing

B. To allow tasks to run forever without waiting

C. To make tasks run only on the main thread

D. To avoid using async and await keywords

Solution

1. Step 1: Understand structured concurrency concept

   Structured concurrency groups related tasks so they run safely and finish before the program moves on.

2. Step 2: Compare options with concept

   Only To group tasks so they run safely and complete before continuing correctly describes grouping tasks to run safely and complete before continuing.

3. Final Answer:

   To group tasks so they run safely and complete before continuing -> Option A

4. Quick Check:

   Structured concurrency = safe grouped tasks [OK]

Hint: Structured concurrency means tasks finish before moving on [OK]

Common Mistakes:

• Thinking tasks run forever without waiting
• Believing tasks only run on main thread
• Confusing structured concurrency with avoiding async/await

2. Which of the following is the correct syntax to create a task group in Swift?

easy

A. withTaskGroup(of: Int.self) { group in ... }

B. asyncGroup { ... }

C. taskGroup.run { ... }

D. TaskGroup { await ... }

Solution

1. Step 1: Recall correct task group syntax

   Swift uses withTaskGroup(of: Type.self) { group in ... } to create a task group.

2. Step 2: Check options for exact syntax

   Only withTaskGroup(of: Int.self) { group in ... } matches the correct syntax; others are invalid or made-up.

3. Final Answer:

   withTaskGroup(of: Int.self) { group in ... } -> Option A

4. Quick Check:

   Task group syntax = withTaskGroup(of:) [OK]

Hint: Task groups start with withTaskGroup(of:) [OK]

Common Mistakes:

• Using TaskGroup without 'with'
• Trying taskGroup.run which doesn't exist
• Using asyncGroup which is not a Swift keyword

3. What will be the output of this Swift code using a task group?

func fetchValues() async -> [Int] {
  await withTaskGroup(of: Int.self) { group in
    group.addTask { 3 }
    group.addTask { 7 }
    group.addTask { 2 }

    var results = [Int]()
    for await value in group {
      results.append(value)
    }
    return results.sorted()
  }
}

Task {
  let values = await fetchValues()
  print(values)
}

medium

A. [3, 7, 2]

B. [2, 3, 7]

C. [7, 3, 2]

D. Compilation error

Solution

1. Step 1: Understand task group execution

   The tasks return 3, 7, and 2 asynchronously. The for-await loop collects them in any order.

2. Step 2: Analyze the return statement

   The results array is sorted before returning, so output is sorted ascending: [2, 3, 7].

3. Final Answer:

   [2, 3, 7] -> Option B

4. Quick Check:

   Sorted results = [2, 3, 7] [OK]

Hint: Remember results are sorted before printing [OK]

Common Mistakes:

• Assuming tasks run in order added
• Forgetting the sorting step
• Thinking code causes compile error

4. Identify the error in this Swift code using a task group:

func calculateSum() async -> Int {
  await withTaskGroup(of: Int.self) { group in
    group.addTask { 10 }
    group.addTask { 20 }
    var total = 0
    for value in group {
      total += value
    }
    return total
  }
}

medium

A. Return type should be Void

B. Cannot add tasks inside withTaskGroup

C. Missing 'await' before 'for' loop

D. Variable 'total' should be declared outside the group

Solution

1. Step 1: Check for correct async iteration

   When iterating over a task group, 'for await' must be used to await each task's result.

2. Step 2: Identify missing 'await'

   The code uses 'for value in group' instead of 'for await value in group', causing a syntax error.

3. Final Answer:

   Missing 'await' before 'for' loop -> Option C

4. Quick Check:

   Async iteration requires 'for await' [OK]

Hint: Use 'for await' to loop over task groups [OK]

Common Mistakes:

• Using 'for' instead of 'for await' on task groups
• Thinking tasks can't be added inside withTaskGroup
• Misunderstanding return type requirements

5. You want to fetch user data and user posts concurrently using structured concurrency. Which approach correctly uses a task group to run both tasks and combine their results?

hard

A. Use DispatchQueue.global().async to run tasks and wait with sleep()

B. Run tasks sequentially without a task group to avoid concurrency issues

C. Create two separate Task blocks and return results without waiting

D. Use withTaskGroup to add two tasks fetching data and posts, then await all and return combined tuple

Solution

1. Step 1: Understand concurrency for multiple async tasks

   Structured concurrency uses task groups to run multiple async tasks concurrently and wait for all to finish.

2. Step 2: Evaluate options for correct concurrency usage

   Use withTaskGroup to add two tasks fetching data and posts, then await all and return combined tuple correctly uses withTaskGroup to add tasks, await their completion, and combine results safely.

3. Final Answer:

   Use withTaskGroup to add two tasks fetching data and posts, then await all and return combined tuple -> Option D

4. Quick Check:

   Task groups run and wait for multiple tasks [OK]

Hint: Use withTaskGroup to run and wait multiple tasks [OK]

Common Mistakes:

• Running tasks sequentially losing concurrency benefits
• Using DispatchQueue with sleep instead of structured concurrency
• Not waiting for tasks to finish before returning