Async functions in iOS Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ ui_behavior

intermediate

What will this SwiftUI view display after 2 seconds?

Consider this SwiftUI view using an async function to fetch data:

struct ContentView: View {
  @State private var message = "Loading..."

  var body: some View {
    Text(message)
      .task {
        message = await fetchMessage()
      }
  }

  func fetchMessage() async -> String {
    try? await Task.sleep(nanoseconds: 2_000_000_000)
    return "Hello, async!"
  }
}

What text will be shown on screen after 2 seconds?

iOS Swift

struct ContentView: View {
  @State private var message = "Loading..."

  var body: some View {
    Text(message)
      .task {
        message = await fetchMessage()
      }
  }

  func fetchMessage() async -> String {
    try? await Task.sleep(nanoseconds: 2_000_000_000)
    return "Hello, async!"
  }
}

A The app crashes due to missing await

B "Hello, async!" replaces "Loading..." after 2 seconds

C "Loading..." remains forever

D "Loading..." changes to empty string after 2 seconds

💡 Hint

Think about what the .task modifier does and how await pauses the function.

The .task modifier runs the async code when the view appears. The await fetchMessage() waits 2 seconds, then updates message. This triggers the view to show "Hello, async!".

❓ lifecycle

intermediate

What happens if you call an async function without await in Swift?

Given this code snippet:

func loadData() async {
  print("Start loading")
  try? await Task.sleep(nanoseconds: 1_000_000_000)
  print("Finished loading")
}

func example() {
  loadData()
  print("After loadData call")
}

What is the order of printed lines when example() is called?

iOS Swift

func loadData() async {
  print("Start loading")
  try? await Task.sleep(nanoseconds: 1_000_000_000)
  print("Finished loading")
}

func example() {
  loadData()
  print("After loadData call")
}

A Start loading\nFinished loading\nAfter loadData call

B Compilation error: missing await

C Start loading\nAfter loadData call\nFinished loading

D After loadData call\nStart loading\nFinished loading

💡 Hint

Remember that calling an async function without await starts it but does not wait for it to finish.

Calling loadData() without await starts the async function but does not pause example(). So "Start loading" prints first, then "After loadData call" immediately, and finally "Finished loading" after 1 second.

📝 Syntax

advanced

Which option correctly defines an async function that returns an Int after delay?

Choose the correct Swift function definition that waits 1 second and returns 42 asynchronously.

A

func getNumber() async -> Int {
  try? await Task.sleep(nanoseconds: 1_000_000_000)
  return 42
}

B

async func getNumber() -> Int {
  try? await Task.sleep(nanoseconds: 1_000_000_000)
  return 42
}

C

func getNumber() -> async Int {
  try? await Task.sleep(nanoseconds: 1_000_000_000)
  return 42
}

D

func async getNumber() -> Int {
  try? await Task.sleep(nanoseconds: 1_000_000_000)
  return 42
}

💡 Hint

In Swift, the async keyword goes after the function parentheses.

The correct syntax is func name() async -> ReturnType. Options A, B, and C misuse the placement of async, causing syntax errors.

🔧 Debug

advanced

Why does this async function cause a runtime error?

Analyze this code snippet:

func fetchData() async -> String {
  DispatchQueue.global().async {
    print("Fetching data")
  }
  return "Done"
}

When calling await fetchData(), why might this cause unexpected behavior?

iOS Swift

func fetchData() async -> String {
  DispatchQueue.global().async {
    print("Fetching data")
  }
  return "Done"
}

A The function causes a compile-time error because DispatchQueue cannot be used in async functions

B The function throws a runtime exception due to missing await

C The function blocks the main thread indefinitely

D The function returns immediately before the async block finishes, causing race conditions

💡 Hint

Think about how DispatchQueue.global().async works compared to Swift concurrency.

The DispatchQueue.global().async runs code asynchronously but does not suspend the async function. So fetchData() returns "Done" immediately, while the print happens later. This can cause race conditions if the caller expects the work to be done.

🧠 Conceptual

expert

What is the main advantage of Swift async/await over completion handlers?

Choose the best explanation for why Swift introduced async/await instead of using completion handlers for asynchronous code.

A Async/await makes asynchronous code look and behave like synchronous code, improving readability and reducing callback nesting

B Async/await runs all code on the main thread, improving UI responsiveness

C Async/await eliminates the need for error handling in asynchronous code

D Async/await automatically parallelizes all asynchronous tasks without developer control

💡 Hint

Think about how code looks and feels when using async/await compared to nested callbacks.

Async/await allows writing asynchronous code in a linear, easy-to-read style, avoiding deeply nested completion handlers (callback hell). It does not force main thread execution, remove error handling, or automatically parallelize tasks.

Practice

1. What does marking a function with async in Swift mean?

easy

A. The function cannot call other functions.

B. The function runs only once and then stops.

C. The function must return an integer value.

D. The function can pause and resume without blocking the app's main thread.

Solution

1. Step 1: Understand async function behavior

   Async functions allow pausing and resuming work without freezing the app's main thread.

2. Step 2: Compare options with async meaning

   Only The function can pause and resume without blocking the app's main thread. correctly describes this behavior; others are unrelated or incorrect.

3. Final Answer:

   The function can pause and resume without blocking the app's main thread. -> Option D

4. Quick Check:

   Async means non-blocking pause/resume = A [OK]

Hint: Async means work happens without freezing UI [OK]

Common Mistakes:

• Thinking async means the function runs only once
• Confusing async with return types
• Believing async functions cannot call others

2. Which of the following is the correct way to declare an async function in Swift?

easy

A. func fetchData() async throws -> String {}

B. async func fetchData() -> String {}

C. func async fetchData() throws -> String {}

D. func fetchData() throws async String {}

Solution

1. Step 1: Recall Swift async function syntax

   In Swift, async comes after the parameter list and before the return type.

2. Step 2: Match options to correct syntax

   Only func fetchData() async throws -> String {} matches the correct syntax: func name() async throws -> Type {}.

3. Final Answer:

   func fetchData() async throws -> String {} -> Option A

4. Quick Check:

   Async keyword after () = C [OK]

Hint: Async keyword goes after function parameters () [OK]

Common Mistakes:

• Placing async before func keyword
• Mixing async and throws order
• Putting async after return type

3. What will be printed by this Swift code?

func fetchNumber() async -> Int {
  return 42
}

Task {
  let number = await fetchNumber()
  print(number)
}

medium

A. 42

B. nil

C. Task

D. Compilation error

Solution

1. Step 1: Understand async function return

   The async function fetchNumber() returns 42 as an Int.

2. Step 2: Await call inside Task and print

   The await keyword waits for the async function to finish, then prints 42.

3. Final Answer:

   42 -> Option A

4. Quick Check:

   Await fetch returns 42 printed = D [OK]

Hint: Await pauses until async returns value [OK]

Common Mistakes:

• Expecting nil because of async
• Thinking Task prints itself
• Assuming compilation error without await

4. Identify the error in this Swift async code:

func loadData() async -> String {
  return "Data"
}

let result = loadData()
print(result)

medium

A. Async functions cannot return String

B. Missing await keyword when calling async function

C. Function must be marked throws

D. Cannot print async function result

Solution

1. Step 1: Check async function call

   Async functions must be called with await to get their result.

2. Step 2: Identify missing await

   The code calls loadData() without await, causing a compile error.

3. Final Answer:

   Missing await keyword when calling async function -> Option B

4. Quick Check:

   Call async with await = B [OK]

Hint: Always use await when calling async functions [OK]

Common Mistakes:

• Calling async functions without await
• Thinking async functions can't return String
• Assuming print can't show async results

5. You want to fetch user data and then fetch user image asynchronously in Swift. Which code correctly waits for both tasks in order?

hard

A. async func loadUser() {
let data = fetchUserData()
let image = fetchUserImage(data.id)
print(image)
}

B. func loadUser() {
let data = fetchUserData()
let image = fetchUserImage(data.id)
print(image)
}

C. async func loadUser() {
let data = await fetchUserData()
let image = await fetchUserImage(data.id)
print(image)
}

D. async func loadUser() {
let data = await fetchUserData()
let image = fetchUserImage(data.id)
print(image)
}

Solution

1. Step 1: Understand async calls order

   Both fetchUserData() and fetchUserImage() are async and must be awaited to complete before next step.

2. Step 2: Check which code awaits both calls

   Only async func loadUser() { let data = await fetchUserData() let image = await fetchUserImage(data.id) print(image) } awaits both calls in order, ensuring data is ready before fetching image.

3. Final Answer:

   async func loadUser() { let data = await fetchUserData() let image = await fetchUserImage(data.id) print(image) } -> Option C

4. Quick Check:

   Await each async call in order = A [OK]

Hint: Await each async call to run sequentially [OK]

Common Mistakes:

• Not awaiting async calls
• Calling async functions without async keyword
• Mixing sync and async calls incorrectly