Why modern concurrency matters in Swift - Challenge Your Understanding

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of async task execution order

What is the output of this Swift code using async/await and Task groups?

Swift

import Foundation

func fetchNumber(_ n: Int) async -> Int {
    try? await Task.sleep(n * 100_000_000) // sleep n*0.1 seconds
    return n
}

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

Task {
    await test()
}

RunLoop.main.run()

A 1\n2\n3

B Compilation error due to missing await

C The numbers 1, 2, 3 printed in any order

D 3\n2\n1

💡 Hint

Think about how Task groups run tasks concurrently and how results arrive.

Task groups run child tasks concurrently, so the order of completion depends on the sleep duration. Since sleep times differ, results can come in any order.

🧠 Conceptual

intermediate

Why use structured concurrency?

Which of the following best explains why structured concurrency is important in modern Swift concurrency?

A It helps manage the lifecycle of concurrent tasks to avoid leaks and unexpected behavior.

B It replaces all synchronous code with asynchronous code automatically.

C It allows launching infinite background threads without limits.

D It makes all code run faster by parallelizing everything.

💡 Hint

Think about how tasks are organized and cleaned up.

Structured concurrency ensures tasks are grouped and their lifetimes managed, preventing resource leaks and making code easier to reason about.

🔧 Debug

advanced

Identify the concurrency bug

What error or problem will this Swift code cause when run?

Swift

var sharedCounter = 0

func increment() async {
    for _ in 1...1000 {
        sharedCounter += 1
    }
}

Task {
    await increment()
}
Task {
    await increment()
}

Task.sleep(1_000_000_000) // wait 1 second
print(sharedCounter)

A Prints 2000 as expected

B Prints a number less than 2000 due to race condition

C Compilation error because sharedCounter is not @MainActor

D Runtime crash due to data race detected by Swift

💡 Hint

Think about what happens when two tasks modify the same variable without synchronization.

The sharedCounter is accessed concurrently without protection, causing a race condition and lost increments, so the final value is less than 2000.

📝 Syntax

advanced

Which code compiles without error?

Which of these Swift concurrency snippets compiles without error?

A func foo() async -> Int { return await bar() } func bar() async -> Int { 42 }

B func foo() async -> Int { return 42 }

C func foo() async -> Int { return await bar() } func bar() -> Int { 42 }

D func foo() -> Int { return await bar() } func bar() async -> Int { 42 }

💡 Hint

Remember async functions can only await other async functions.

Option A is correct because foo is async and awaits bar which is also async. Other options have mismatched async/await usage causing compile errors.

🚀 Application

expert

How many tasks run concurrently?

Given this Swift code, how many tasks run concurrently at the peak?

Swift

import Foundation

func work(id: Int) async {
    print("Start \(id)")
    try? await Task.sleep(500_000_000) // 0.5 seconds
    print("End \(id)")
}

func main() async {
    await withTaskGroup(of: Void.self) { group in
        for i in 1...5 {
            group.addTask {
                await work(id: i)
            }
            if i == 3 {
                try? await Task.sleep(600_000_000) // 0.6 seconds
            }
        }
    }
}

Task {
    await main()
}

RunLoop.main.run()

A Only 1 task runs at a time due to the sleep inside the loop

B 2 tasks run concurrently because the sleep pauses task addition

C 3 tasks run concurrently at peak because of the sleep after adding 3 tasks

D 5 tasks run concurrently at the same time

💡 Hint

Task groups start tasks immediately; sleeps inside the loop do not block running tasks.

All 5 tasks are added quickly and run concurrently. The sleep inside the loop pauses only the loop, not the already started tasks.