Await keyword in iOS Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ ui_behavior

intermediate

Understanding async UI update with await

What will be the visible text on the label after the following Swift async function runs?

iOS Swift

func updateLabel() async {
  label.text = "Loading..."
  let result = await fetchData()
  label.text = result
}

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

A "Data Loaded" is shown after fetchData completes

B "Loading..." remains visible

C The label text is empty

D The app crashes due to missing await

💡 Hint

Remember that await pauses the function until the async call finishes.

The await keyword pauses updateLabel until fetchData returns. So the label first shows "Loading..." then updates to "Data Loaded" after await completes.

❓ lifecycle

intermediate

Await in viewDidLoad lifecycle method

What happens if you call an async function with await inside viewDidLoad in a UIViewController?

iOS Swift

override func viewDidLoad() {
  super.viewDidLoad()
  Task {
    await loadData()
  }
}

func loadData() async {
  // fetch data asynchronously
}

A loadData runs asynchronously without blocking UI

B The UI blocks until loadData finishes

C The app crashes because viewDidLoad cannot be async

D loadData runs synchronously

💡 Hint

Using Task inside viewDidLoad allows async code without blocking.

viewDidLoad cannot be async, but wrapping await calls inside Task runs them asynchronously without blocking the UI.

📝 Syntax

advanced

Identify the syntax error with await usage

Which option contains a syntax error when using await in Swift?

iOS Swift

func fetchUser() async -> String {
  return "User"
}

func showUser() {
  let user = await fetchUser()
  print(user)
}

A

func showUser() async {
  let user = await fetchUser()
  print(user)
}

B

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C

func showUser() async {
  let user = fetchUser()
  print(user)
}

D

func showUser() {
  let user = await fetchUser()
  print(user)
}

💡 Hint

Await can only be used inside async functions.

Option D uses await inside a non-async function, causing a syntax error.

🔧 Debug

advanced

Debugging missing await causing runtime issue

What issue arises if you forget to use await when calling an async function?

iOS Swift

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

func process() async {
  let data = fetchData()
  print(data)
}

A data is a String and prints "Data"

B The code causes a compile-time error

C data is a Task<String> and prints the task description

D The app crashes at runtime

💡 Hint

Calling an async function inside an async context requires 'await', otherwise compile error.

Without 'await', the code causes a compile-time error because async functions must be called with 'await' in async contexts.

🧠 Conceptual

expert

Effect of await on concurrency and thread blocking

Which statement best describes what the await keyword does in Swift concurrency?

A await blocks the current thread until the async function finishes

B await runs the async function on a background thread automatically

C await suspends the current async task, allowing other tasks to run without blocking the thread

D await converts an async function into a synchronous one

💡 Hint

Think about how await helps keep the app responsive.

Await suspends the current async task, freeing the thread to run other tasks. It does not block the thread.

Practice

1. What does the await keyword do in Swift's async functions?

easy

A. It converts a synchronous function to asynchronous.

B. It makes the function run faster.

C. It pauses the function until the async task completes.

D. It cancels the async task immediately.

Solution

1. Step 1: Understand the role of await

   The await keyword pauses the execution of an async function until the awaited task finishes.

2. Step 2: Compare other options

   Options B, C, and D describe behaviors that await does not perform.

3. Final Answer:

   It pauses the function until the async task completes. -> Option C

4. Quick Check:

   await pauses async tasks = A [OK]

Hint: Remember: await waits for task completion before continuing [OK]

Common Mistakes:

• Thinking await speeds up the function
• Using await outside async functions
• Confusing await with task cancellation

2. Which of the following is the correct way to declare an async function that uses await inside it?

easy

A. func fetchData() async { let data = await loadData() }

B. func fetchData() { let data = await loadData() }

C. async func fetchData() { let data = await loadData() }

D. func fetchData() async await { let data = loadData() }

Solution

1. Step 1: Check async function declaration syntax

   In Swift, the correct syntax is func name() async {} to declare an async function.

2. Step 2: Verify usage of await inside async function

   func fetchData() async { let data = await loadData() } correctly uses await inside an async function. Options B, C, and D have syntax errors.

3. Final Answer:

   func fetchData() async { let data = await loadData() } -> Option A

4. Quick Check:

   Async function syntax = A [OK]

Hint: Async functions must have 'async' keyword before braces [OK]

Common Mistakes:

• Omitting async keyword in function declaration
• Placing async before func keyword
• Adding await in function signature

3. What will be printed by this Swift code?

func fetchNumber() async -> Int {
  return 5
}

func printNumber() async {
  let number = await fetchNumber()
  print(number)
}

Task {
  await printNumber()
}

medium

A. No output, code has syntax error

B. 5

C. 0

D. The program crashes

Solution

1. Step 1: Understand async function behavior

   The function fetchNumber() returns 5 asynchronously. Using await waits for this result.

2. Step 2: Trace the print statement

   The printNumber() function awaits fetchNumber() and prints the returned value 5.

3. Final Answer:

   5 -> Option B

4. Quick Check:

   Await returns value before print = C [OK]

Hint: Await pauses until value returns, then prints it [OK]

Common Mistakes:

• Expecting 0 because of async
• Thinking code crashes without main thread
• Confusing async function call without await

4. Identify the error in this Swift code snippet:

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

func showData() {
  let result = await loadData()
  print(result)
}

medium

A. Missing async keyword in showData function declaration.

B. print statement is not allowed inside async functions.

C. loadData function should not return String.

D. Cannot use await inside a non-async function.

Solution

1. Step 1: Check function declarations and await usage

   The function showData() is not marked async but uses await, which is invalid.

2. Step 2: Understand await usage rules

   In Swift, await can only be used inside async functions. So showData() must be async to use await.

3. Final Answer:

   Cannot use await inside a non-async function. -> Option D

4. Quick Check:

   Await only in async functions = D [OK]

Hint: Await requires async function context [OK]

Common Mistakes:

• Forgetting to mark function async when using await
• Thinking await works in normal functions
• Misunderstanding return types with async

5. You want to fetch user data and then fetch user posts, both are async functions. Which code correctly waits for both tasks in order using await?

hard

A. func loadUserData() async {
let user = await fetchUser()
let posts = await fetchPosts(user.id)
print(posts)
}

B. func loadUserData() async {
let user = fetchUser()
let posts = fetchPosts(user.id)
print(posts)
}

C. func loadUserData() {
let user = await fetchUser()
let posts = await fetchPosts(user.id)
print(posts)
}

D. func loadUserData() async {
let user = await fetchUser()
let posts = fetchPosts(user.id)
print(posts)
}

Solution

1. Step 1: Confirm async function declaration

   The function must be marked async to use await inside it, so options A and D are candidates.

2. Step 2: Check await usage for both async calls

   func loadUserData() async { let user = await fetchUser() let posts = await fetchPosts(user.id) print(posts) } awaits both fetchUser() and fetchPosts(), ensuring sequential completion. func loadUserData() async { let user = await fetchUser() let posts = fetchPosts(user.id) print(posts) } misses await on fetchPosts().

3. Final Answer:

   func loadUserData() async { let user = await fetchUser() let posts = await fetchPosts(user.id) print(posts) } -> Option A

4. Quick Check:

   Await both async calls in async function = B [OK]

Hint: Await each async call to run in order inside async function [OK]

Common Mistakes:

• Forgetting await on second async call
• Not marking function async
• Calling async functions without await