Structured concurrency in iOS Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ ui_behavior

intermediate

Understanding async let behavior in Swift concurrency

What will be printed when this Swift code runs?

iOS Swift

func fetchData() async {
  async let first = fetchFirst()
  async let second = fetchSecond()
  let result1 = await first
  let result2 = await second
  print("Results: \(result1), \(result2)")
}

func fetchFirst() async -> String {
  return "First"
}

func fetchSecond() async -> String {
  return "Second"
}

Task {
  await fetchData()
}

A Results: Second, First

B Results: First, Second

C Results: nil, nil

D Compilation error due to missing await

💡 Hint

Remember that async let starts tasks concurrently and await waits for their results.

The async let declarations start both fetchFirst() and fetchSecond() concurrently. Awaiting first and second gets their results in the order awaited, so the print shows "First" then "Second".

❓ lifecycle

intermediate

Task cancellation in structured concurrency

What happens when the child task is cancelled before it completes in this Swift code?

iOS Swift

func parentTask() async throws {
  let child = Task {
    try await Task.sleep(nanoseconds: 2_000_000_000)
    print("Child finished")
  }
  try await Task.sleep(nanoseconds: 1_000_000_000)
  print("Parent cancelling child")
  child.cancel()
  print("Parent finished")
}

Task {
  try await parentTask()
}

A Prints "Parent cancelling child" then "Parent finished"; "Child finished" is never printed

B Prints "Child finished" then "Parent cancelling child" then "Parent finished"

C Prints "Parent cancelling child" then "Child finished" then "Parent finished"

D Compilation error due to incorrect Task usage

💡 Hint

Consider what happens when a Task is cancelled before it finishes sleeping.

The child task is cancelled before its sleep finishes, so it never prints "Child finished". The parent prints its messages and finishes normally.

🧠 Conceptual

advanced

Why use TaskGroup in Swift concurrency?

Which statement best explains the main advantage of using TaskGroup in Swift structured concurrency?

A It automatically retries failed tasks without developer intervention.

B It serializes tasks to run one after another to save memory.

C It allows running multiple child tasks concurrently and waits for all to complete, collecting their results safely.

D It replaces async let for single asynchronous calls.

💡 Hint

Think about how TaskGroup manages multiple tasks and their results.

TaskGroup lets you create many child tasks that run concurrently and then await all their results safely, making it easier to manage dynamic numbers of tasks.

📝 Syntax

advanced

Identify the syntax error in this Swift concurrency code

Which option correctly identifies the syntax error in the following Swift code snippet?

iOS Swift

func loadData() async {
  async let data1 = fetchData1()
  let data2 = await fetchData2()
  print(await data1, data2)
}

func fetchData1() async -> String { "Data1" }
func fetchData2() async -> String { "Data2" }

A Missing await before data1 in print statement

B async let cannot be used with fetchData1()

C fetchData2() must be called without await

D print statement must be inside a Task block

💡 Hint

Remember how to get the value from an async let binding.

async let creates a concurrent task and returns a value that must be awaited before use. The print statement uses data1 without await, causing a compile error.

🔧 Debug

expert

Diagnose the cause of a deadlock in Swift structured concurrency

Given this Swift code, why does the program deadlock and never print "Done"?

iOS Swift

func deadlockExample() async {
  let task = Task {
    await deadlockExample()
  }
  await task.value
  print("Done")
}

Task {
  await deadlockExample()
}

A The print statement is unreachable due to syntax error

B Task cannot be created inside an async function

C Missing await before task creation causes deadlock

D The task recursively awaits itself causing infinite recursion and deadlock

💡 Hint

Look at the recursive call inside the Task and what it awaits.

The Task runs deadlockExample() which creates another Task that awaits deadlockExample() again, causing infinite recursion and deadlock before printing "Done".