MainActor for UI work in Swift - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to mark the function to run on the main thread.

Swift

@[1]
func updateUI() {
    print("UI updated")
}

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

AUIActor

BBackgroundActor

CMainActor

DAsyncActor

Attempts:

3 left

💡 Hint

Common Mistakes

Using a wrong actor attribute that does not guarantee main thread execution.

The @MainActor attribute ensures the function runs on the main thread, which is required for UI updates.

2fill in blank

medium

Complete the code to ensure the async function runs on the main thread.

Swift

func fetchData() async {
    await [1] {
        print("Data fetched and UI updated")
    }
}

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

ATask.detached

BDispatchQueue.global().async

CDispatchQueue.main.async

DMainActor.run

Attempts:

3 left

💡 Hint

Common Mistakes

Using DispatchQueue.global() which runs on a background thread.

Using Task.detached which runs detached from any actor.

MainActor.run ensures the closure runs on the main thread asynchronously.

3fill in blank

hard

Fix the error in the code to update the UI safely on the main thread.

Swift

class ViewModel {
    func update() async {
        await [1] {
            print("UI updated")
        }
    }
}

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

AMainActor.run

BDispatchQueue.global().async

CTask.detached

DDispatchQueue.main.async

Attempts:

3 left

💡 Hint

Common Mistakes

Using DispatchQueue.global() which runs on a background thread.

Using Task.detached which runs detached from any actor.

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

4fill in blank

hard

Fill both blanks to create a MainActor isolated class with a UI update method.

Swift

@[1]
class UIHandler {
    func refresh() [2] {
        print("Refreshing UI")
    }
}

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

AMainActor

Basync

Cnonisolated

Dsync

Attempts:

3 left

💡 Hint

Common Mistakes

Marking the method as nonisolated which breaks main thread isolation.

Using sync keyword which is invalid in Swift.

Marking the class with @MainActor isolates it to the main thread, and the async method allows asynchronous UI updates.

5fill in blank

hard

Fill all three blanks to define a MainActor isolated class with an async UI update method and call it safely.

Swift

import Foundation

@[1]
class Controller {
    func updateUI() [2] {
        print("UI Updated")
    }
}

func performUpdate() async {
    let controller = Controller()
    await controller.[3]()
}

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

AMainActor

Basync

CupdateUI

Dsync

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to mark the method async.

Calling the async method without await.

Using sync keyword which is invalid.

The class is marked @MainActor to isolate it to the main thread, the method is async to allow awaiting, and the method is called with await.