Async functions in iOS Swift - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an async function named fetchData.

iOS Swift

func fetchData() [1] {
    // function body
}

Drag options to blanks, or click blank then click option'

Async

Bawait

Cthrows

Dasync

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'await' instead of 'async' to declare the function.

Forgetting to mark the function as async.

The async keyword declares that the function runs asynchronously.

2fill in blank

medium

Complete the code to call an async function fetchData and wait for its result.

iOS Swift

Task {
    let data = try? await [1]()
    print(data ?? "No data")
}

Drag options to blanks, or click blank then click option'

AfetchData

Bfetch

CgetData

DloadData

Attempts:

3 left

💡 Hint

Common Mistakes

Calling the function without 'await'.

Using a wrong function name.

You must call the async function by its exact name and use await to wait for its result.

3fill in blank

hard

Fix the error in the async function declaration by filling the blank.

iOS Swift

func loadData() [1] throws {
    // function body
}

Drag options to blanks, or click blank then click option'

Aawait

Bsync

Casync

Dthrows async

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'await' instead of 'async' in the declaration.

Combining 'throws' and 'async' incorrectly.

The async keyword must be used to mark the function as asynchronous. It can be combined with throws but must be separate.

4fill in blank

hard

Fill both blanks to declare and call an async function correctly.

iOS Swift

func fetchUser() [1] {
    // fetch user data
}

Task {
    let user = try? await [2]()
    print(user ?? "No user")
}

Drag options to blanks, or click blank then click option'

Aasync

Bawait

CfetchUser

DloadUser

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to mark the function as async.

Calling the function without 'await'.

Using the wrong function name.

The function must be declared with async. When calling it inside a Task, use await before the function name. Here, the function name is fetchUser.

5fill in blank

hard

Fill all three blanks to create an async function that throws and call it correctly.

iOS Swift

func fetchData() [1] [2] {
    // fetch data
}

Task {
    do {
        let data = try [3] fetchData()
        print(data)
    } catch {
        print("Error")
    }
}

Drag options to blanks, or click blank then click option'

Aasync

Bthrows

Cawait

Dtry

Attempts:

3 left

💡 Hint

Common Mistakes

Omitting 'async' or 'throws' in the function declaration.

Calling the function without 'await'.

Forgetting to use 'try' when calling a throwing function.

The function must be declared with async and throws. When calling it inside a Task, use try and await to handle errors and wait for the result.

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