Task groups for parallel execution in Swift

Introduction

Task groups help run many tasks at the same time easily. This makes programs faster by doing work in parallel.

When you want to download many images from the internet at once.

When you need to process multiple files at the same time.

When you want to run several calculations simultaneously to save time.

When you want to fetch data from different sources in parallel.

When you want to perform many small tasks without waiting for each to finish.

Syntax

Swift

await withTaskGroup(of: ReturnType.self) { group in
    group.addTask {
        // code for one task
        return result
    }
    // add more tasks
    for await result in group {
        // handle each result
    }
}

Use await because tasks run asynchronously.

Each task returns a value of the specified type.

Examples

This runs two tasks that add numbers and prints their results as they finish.

Swift

await withTaskGroup(of: Int.self) { group in
    group.addTask { 1 + 1 }
    group.addTask { 2 + 2 }
    for await result in group {
        print(result)
    }
}

This creates three tasks that return messages and prints each message.

Swift

await withTaskGroup(of: String.self) { group in
    for i in 1...3 {
        group.addTask { "Task \(i) done" }
    }
    for await message in group {
        print(message)
    }
}

Sample Program

This program runs five tasks in parallel. Each task waits a bit and then returns the square of a number. The results print as they finish.

Swift

import Foundation

@main
struct Main {
    static func main() async {
        await withTaskGroup(of: Int.self) { group in
            for i in 1...5 {
                group.addTask {
                    // Simulate work with sleep
                    try? await Task.sleep(nanoseconds: UInt64(i) * 100_000_000)
                    return i * i
                }
            }
            for await square in group {
                print("Square: \(square)")
            }
        }
    }
}

Important Notes

Task groups run tasks concurrently, but order of results may vary.

Use for await to get results as tasks complete.

Task groups help manage many tasks without complex code.

Summary

Task groups let you run many tasks at the same time easily.

Use addTask to add work inside the group.

Collect results with for await to handle each task's output.

Practice

1. What is the main purpose of using a TaskGroup in Swift concurrency?

easy

A. To run multiple tasks in parallel and wait for all to complete

B. To create a single long-running task

C. To pause the current task indefinitely

D. To handle errors in synchronous code

Solution

1. Step 1: Understand what TaskGroup does

   A TaskGroup lets you run many tasks at the same time, managing them together.

2. Step 2: Identify the main use

   It allows parallel execution and waits until all tasks finish before continuing.

3. Final Answer:

   To run multiple tasks in parallel and wait for all to complete -> Option A

4. Quick Check:

   TaskGroup = parallel tasks + wait [OK]

Hint: TaskGroup runs tasks together and waits for all [OK]

Common Mistakes:

• Thinking TaskGroup runs only one task
• Confusing TaskGroup with error handling
• Assuming TaskGroup pauses tasks

2. Which of the following is the correct way to add a task inside a TaskGroup in Swift?

easy

A. group.runTask(fetchData())

B. group.createTask(fetchData)

C. group.addTask { await fetchData() }

D. group.startTask { fetchData() }

Solution

1. Step 1: Recall the method to add tasks

   The correct method to add a task is addTask with a closure.

2. Step 2: Check syntax correctness

   group.addTask { await fetchData() } uses group.addTask { await fetchData() }, which is the right syntax.

3. Final Answer:

   group.addTask { await fetchData() } -> Option C

4. Quick Check:

   Use addTask with closure [OK]

Hint: Use addTask with closure to add work [OK]

Common Mistakes:

• Using wrong method names like runTask or createTask
• Not using closure syntax
• Omitting await inside the task

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

func fetchNumber(_ n: Int) async -> Int {
    return n * 2
}

func runGroup() async {
    await withTaskGroup(of: Int.self) { group in
        for i in 1...3 {
            group.addTask { await fetchNumber(i) }
        }
        var results = [Int]()
        for await result in group {
            results.append(result)
        }
        print(results.sorted())
    }
}

Task {
    await runGroup()
}

medium

A. Compilation error

B. [1, 2, 3]

C. [6, 4, 2]

D. [2, 4, 6]

Solution

1. Step 1: Understand the task group adds tasks for numbers 1 to 3

   Each task returns the input number multiplied by 2: 2, 4, and 6.

2. Step 2: Collect results and sort them before printing

   Results are collected in any order but sorted before printing, so output is [2, 4, 6].

3. Final Answer:

   [2, 4, 6] -> Option D

4. Quick Check:

   Results doubled and sorted = [2,4,6] [OK]

Hint: Remember to sort results after parallel tasks [OK]

Common Mistakes:

• Assuming results print in order added
• Forgetting to await inside addTask
• Confusing output with original numbers

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

await withTaskGroup(of: Int.self) { group in
    for i in 1...3 {
        group.addTask { fetchNumber(i) }
    }
    for await result in group {
        print(result)
    }
}

Assuming fetchNumber is an async function.

medium

A. Missing await inside addTask closure

B. Cannot use for await with TaskGroup

C. TaskGroup must be declared with var

D. fetchNumber must return Void

Solution

1. Step 1: Check how fetchNumber is called inside addTask

   fetchNumber is async, so it must be awaited inside the closure.

2. Step 2: Identify missing await keyword

   The closure calls fetchNumber(i) without await, causing a compile error.

3. Final Answer:

   Missing await inside addTask closure -> Option A

4. Quick Check:

   Async calls inside addTask need await [OK]

Hint: Always await async calls inside addTask closures [OK]

Common Mistakes:

• Forgetting await inside addTask
• Thinking for await can't be used with TaskGroup
• Declaring TaskGroup incorrectly

5. You want to fetch data from multiple URLs in parallel using a TaskGroup and collect only the successful results (non-nil). Which approach correctly filters out nil results while running tasks in parallel?

hard

A. Run tasks sequentially inside the group to avoid nil results

B. Add tasks returning optional data, then filter non-nil in for await loop

C. Use addTask with throwing function and catch errors outside group

D. Use withThrowingTaskGroup but ignore errors and collect all results

Solution

1. Step 1: Understand the goal to run tasks in parallel and filter nil

   Running tasks in parallel with TaskGroup allows concurrency; filtering nil after collecting results is straightforward.

2. Step 2: Evaluate options for filtering nil results

   Add tasks returning optional data, then filter non-nil in for await loop correctly adds tasks returning optional data and filters non-nil results in the for await loop, matching the goal.

3. Final Answer:

   Add tasks returning optional data, then filter non-nil in for await loop -> Option B

4. Quick Check:

   Filter nil after parallel tasks with for await [OK]

Hint: Filter nil results after collecting parallel task outputs [OK]

Common Mistakes:

• Trying to run tasks sequentially losing parallelism
• Ignoring errors without filtering nil
• Misusing throwing task groups for simple filtering