MainActor for UI updates in iOS Swift - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to ensure the UI update runs on the main thread using MainActor.

iOS Swift

func updateLabel() {
  Task {
    await [1] {
      label.text = "Hello, world!"
    }
  }
}

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

AMainActor.run

BDispatchQueue.global().async

CTask.sleep

DDispatchQueue.main.asyncAfter

Attempts:

3 left

💡 Hint

Common Mistakes

Using background queues like DispatchQueue.global() causes UI update errors.

Forgetting to use await with MainActor.run.

Using MainActor.run ensures the code inside runs on the main thread, which is required for UI updates.

2fill in blank

medium

Complete the function declaration to mark it as running on the main actor.

iOS Swift

@[1]
func refreshUI() {
  label.text = "Updated"
}

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

AGlobalActor

BUIActor

CMainActor

DBackgroundActor

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect actor names like @GlobalActor or @BackgroundActor.

Omitting the @ symbol.

Marking the function with @MainActor ensures it always runs on the main thread for UI safety.

3fill in blank

hard

Fix the error in this async function to update UI safely on the main thread.

iOS Swift

func loadData() async {
  let data = await fetchData()
  await [1] {
    label.text = data
  }
}

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

ADispatchQueue.global().async

BDispatchQueue.main.sync

CTask.detached

DMainActor.run

Attempts:

3 left

💡 Hint

Common Mistakes

Using background queues for UI updates.

Using synchronous dispatch on main queue inside async code.

Using MainActor.run inside async functions ensures UI updates happen on the main thread safely.

4fill in blank

hard

Fill both blanks to create a dictionary comprehension that filters keys with length greater than 3 and converts keys to uppercase.

iOS Swift

let filteredDict = Dictionary(uniqueKeysWithValues: data.compactMap { key, value in
  guard key.count [1] 3 else { return nil }
  return (key.[2](), value)
})

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

A>

Blowercased

Cuppercased

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using < instead of > in the guard condition.

Using lowercased() instead of uppercased() for keys.

The guard checks keys longer than 3 characters using >. The keys are converted to uppercase with uppercased().

5fill in blank

hard

Fill all three blanks to create a dictionary comprehension that filters values greater than 10, converts keys to uppercase, and uses the value as is.

iOS Swift

let result = Dictionary(uniqueKeysWithValues: items.compactMap { key, value in
  guard value [1] 10 else { return nil }
  return (key.[2](), [3])
})

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

A<

B>

Cvalue

Duppercased

Attempts:

3 left

💡 Hint

Common Mistakes

Using < instead of > in the guard.

Swapping key and value in the return tuple.

The guard filters values greater than 10 using >. Keys are converted to uppercase with uppercased(). The value is returned as is.

Practice

1. What is the main purpose of using @MainActor in Swift UI code?

easy

A. To ensure UI updates happen on the main thread

B. To run background tasks faster

C. To prevent memory leaks

D. To encrypt user data

Solution

1. Step 1: Understand UI thread requirements

   UI updates must happen on the main thread to avoid crashes and keep the app smooth.

2. Step 2: Role of @MainActor

   @MainActor ensures the annotated code runs on the main thread, making UI updates safe.

3. Final Answer:

   To ensure UI updates happen on the main thread -> Option A

4. Quick Check:

   MainActor = UI updates on main thread [OK]

Hint: MainActor always means UI code runs on main thread [OK]

Common Mistakes:

• Thinking @MainActor speeds up background tasks
• Confusing @MainActor with memory management
• Assuming @MainActor encrypts data

2. Which of the following is the correct way to declare a function that updates UI using MainActor in Swift?

easy

A. func updateUI() { @MainActor /* code */ }

B. func updateUI() @MainActor { /* code */ }

C. func @MainActor updateUI() { /* code */ }

D. @MainActor func updateUI() { /* code */ }

Solution

1. Step 1: Recall correct syntax for @MainActor

   @MainActor is an attribute placed before the function declaration.

2. Step 2: Check each option

   @MainActor func updateUI() { /* code */ } correctly places @MainActor before the function name.

3. Final Answer:

   @MainActor func updateUI() { /* code */ } -> Option D

4. Quick Check:

   @MainActor before func = correct syntax [OK]

Hint: @MainActor goes before func keyword [OK]

Common Mistakes:

• Placing @MainActor after func name
• Putting @MainActor inside function body
• Using @MainActor as a suffix

3. Consider this Swift code snippet:

@MainActor
class ViewModel {
  var count = 0

  func increment() {
    count += 1
  }
}

let vm = ViewModel()
Task {
  await vm.increment()
  print(vm.count)
}

What will be printed?

medium

A. Compilation error

B. 0

C. 1

D. Runtime crash

Solution

1. Step 1: Understand @MainActor on class

   The class ViewModel is marked @MainActor, so its methods and properties are isolated to the main actor.

2. Step 2: Analyze the code

   await vm.increment() correctly increments count. Accessing vm.count inside the Task closure is safe because it's running on the main actor context.

3. Final Answer:

   1 -> Option C

4. Quick Check:

   Accessing @MainActor properties within the actor context is allowed [OK]

Hint: @MainActor properties can be accessed safely within actor context without await [OK]

Common Mistakes:

• Thinking it prints 1 and forgetting await on print(vm.count)
• Expecting runtime crash instead of compile error
• Believing count stays 0 due to async context

4. You have this Swift function:

func updateLabel() {
  label.text = "Hello"
}

When called from a background thread, the app crashes. How can you fix it using MainActor?

medium

A. Add @MainActor before the function declaration

B. Call updateLabel() inside DispatchQueue.global()

C. Remove the function and update label directly

D. Wrap label update inside DispatchQueue.main.async without @MainActor

Solution

1. Step 1: Identify cause of crash

   UI updates must happen on main thread; background thread causes crash.

2. Step 2: Use @MainActor to fix

   Adding @MainActor before function ensures it runs on main thread, preventing crash.

3. Final Answer:

   Add @MainActor before the function declaration -> Option A

4. Quick Check:

   @MainActor fixes UI thread crash [OK]

Hint: Use @MainActor to run UI code on main thread automatically [OK]

Common Mistakes:

• Using global queue instead of main queue
• Updating UI directly from background thread
• Ignoring thread safety causing crashes

5. You want to update a SwiftUI view's text label from a background task safely. Which approach correctly uses @MainActor and async/await to update the UI?

hard

A. Use DispatchQueue.global().async to update the label

B. Declare the update function with @MainActor and call it with await inside the background task

C. Call the update function directly from the background thread without @MainActor

D. Update the label inside the background task without any special handling

Solution

1. Step 1: Understand UI update rules

   UI updates must happen on main thread to avoid crashes and glitches.

2. Step 2: Use @MainActor with async/await

   Marking the update function with @MainActor ensures it runs on main thread. Calling it with await from background task switches context safely.

3. Step 3: Evaluate other options

   Options A, B, and D update UI from background thread causing errors or crashes.

4. Final Answer:

   Declare the update function with @MainActor and call it with await inside the background task -> Option B

5. Quick Check:

   @MainActor + await = safe UI update [OK]

Hint: Use @MainActor function called with await for UI updates [OK]

Common Mistakes:

• Updating UI directly from background thread
• Ignoring await when calling @MainActor function
• Using global queue for UI updates