Task groups for parallel execution in Swift - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a task group in Swift concurrency?

A task group lets you run multiple tasks in parallel and wait for all of them to finish. It's like organizing a team to do jobs at the same time and then gathering their results.

beginner

How do you add a new task to a Swift task group?

Inside the task group, you use the addTask method to start a new child task that runs concurrently with others.

beginner

What does the await keyword do when used with a task group?

It waits for all tasks in the group to finish and collects their results before moving on, ensuring you have all the data ready.

beginner

Why use task groups instead of running tasks one by one?

Task groups let tasks run at the same time, which can make your program faster, especially when tasks don't depend on each other.

intermediate

What happens if one task in a task group throws an error?

The error is thrown to the caller, and the task group cancels remaining tasks. You can handle errors using try and catch.

What is the main benefit of using task groups in Swift concurrency?

A Run tasks strictly one after another

B Automatically fix errors in tasks

C Prevent any task from running concurrently

D Run multiple tasks in parallel and wait for all to finish

Task groups allow multiple tasks to run at the same time and wait for all to complete.

Which method do you use to add a new task inside a Swift task group?

A runTask

B startTask

C addTask

D createTask

The addTask method is used to add new child tasks to a task group.

What keyword do you use to wait for all tasks in a task group to finish?

A await

B sync

C pause

D wait

The await keyword pauses execution until all tasks in the group complete.

If one task in a task group throws an error, what happens?

A The error is ignored

B The task group cancels remaining tasks and throws the error

C All tasks continue running without interruption

D The program crashes immediately

When a task throws an error, the task group cancels other tasks and propagates the error.

Why might you choose task groups over manually creating multiple tasks?

A Task groups automatically handle task cancellation and error propagation

B Task groups run tasks sequentially

C Task groups prevent concurrency

D Task groups slow down execution

Task groups help manage tasks better by handling cancellation and errors automatically.

Explain how task groups help run tasks in parallel in Swift concurrency.

Think about how a group of friends can work on different parts of a project at the same time.

Describe what happens when a task inside a task group throws an error.

Consider what happens if one team member finds a problem during a group project.

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