Await keyword in iOS Swift - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the await keyword do in Swift?

The await keyword pauses the current function until an asynchronous task finishes, allowing other code to run meanwhile.

beginner

Can you use await outside an asynchronous function in Swift?

No, await can only be used inside functions marked with async.

intermediate

How does await improve app responsiveness?

await lets the app do other work while waiting for slow tasks, like network calls, so the app stays smooth and doesn’t freeze.

intermediate

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

The async function returns a task immediately without waiting for the result, which can cause bugs if you expect the result right away.

beginner

Example: What does this code do?
<pre>func fetchData() async {
  let data = await download()
  print(data)
}</pre>

This function waits for download() to finish before printing the data. The await pauses fetchData() until download() completes.

Where can you use the await keyword in Swift?

A Only inside functions marked with <code>async</code>

B Inside any function

C Only in the main thread

D Only in classes

await must be inside async functions to pause and wait for asynchronous tasks.

What does await do when calling an async function?

A Pauses execution until the async function finishes

B Ignores the function result

C Runs the function synchronously

D Runs the function twice

await pauses the current function until the async function completes and returns a result.

What happens if you call an async function without await?

A You get a compile error

B You get a task object immediately without waiting

C The function runs synchronously

D The app crashes

Calling an async function without await returns a task immediately, not the final result.

Why is using await good for app performance?

A It blocks the main thread

B It makes code run faster synchronously

C It lets other code run while waiting

D It disables multitasking

await allows the app to stay responsive by letting other tasks run while waiting for slow operations.

Which keyword must a function have to use await inside it?

A defer

B sync

C await

D async

Only functions marked with async can use await to wait for asynchronous tasks.

Explain in your own words how the await keyword works in Swift and why it is useful.

Think about how waiting for slow tasks can freeze an app and how <code>await</code> helps avoid that.

Describe a simple example where you would use await in a Swift app.

Imagine fetching data from the internet and needing to wait for it before showing it.

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