Task groups for parallel execution in Swift - Mini Project: Build & Apply

Task groups for parallel execution

📖 Scenario: You are building a simple Swift program that fetches data from multiple sources at the same time. This helps your app work faster by doing tasks in parallel.

🎯 Goal: Learn how to use TaskGroup in Swift to run multiple tasks in parallel and collect their results.

📋 What You'll Learn

Create an array of URLs as strings

Create a variable to hold the number of URLs

Use a TaskGroup to fetch data from all URLs in parallel

Print the combined results after all tasks finish

💡 Why This Matters

🌍 Real World

Fetching data from multiple web services at the same time to improve app speed.

💼 Career

Understanding parallel execution with TaskGroup is important for building efficient Swift apps that handle multiple tasks concurrently.

1

Create the list of URLs

Create a constant array called urls with these exact string values: "https://site1.com", "https://site2.com", "https://site3.com".

Swift

# Create the array 'urls' with the given URLs
# Your code here

Hint

Use square brackets [] to create an array and separate strings with commas.

2

Create a count variable

Create a constant called count that stores the number of elements in the urls array using urls.count.

Swift

let urls = ["https://site1.com", "https://site2.com", "https://site3.com"]
# Create 'count' to hold the number of URLs
# Your code here

Hint

Use urls.count to get the number of items in the array.

3

Use TaskGroup to fetch data in parallel

Inside an asynchronous function called fetchAllData(), use await withTaskGroup(of: String?.self) to create a task group named group. For each url in urls, add a task to group that returns a string like "Data from \(url)". Collect all results into an array called results.

Swift

let urls = ["https://site1.com", "https://site2.com", "https://site3.com"]
let count = urls.count

func fetchAllData() async {
    // Use TaskGroup to fetch data
    // Your code here
}

Hint

Use withTaskGroup to run tasks in parallel and collect their results in an array.

4

Print the combined results

Inside the fetchAllData() function, after collecting the results, print the results array using print(results). Then, call fetchAllData() from Task to run it asynchronously.

Swift

let urls = ["https://site1.com", "https://site2.com", "https://site3.com"]
let count = urls.count

func fetchAllData() async {
    let results = await withTaskGroup(of: String?.self) { group -> [String] in
        for url in urls {
            group.addTask {
                return "Data from \(url)"
            }
        }
        var collectedResults: [String] = []
        for await result in group {
            if let data = result {
                collectedResults.append(data)
            }
        }
        return collectedResults
    }
    // Print the results array
    # Your code here
}

// Call fetchAllData asynchronously
# Your code here

Hint

Use print(results) to show the array. Use Task { await fetchAllData() } to run the async function.

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