MainActor for UI updates in iOS Swift - Mini App: Build & Ship

Build: MainActor UI Update Demo

This screen shows a label and a button. When the button is tapped, it simulates a background task that updates the label text safely on the main thread using @MainActor.

Target UI

-------------------------
| MainActor UI Update Demo |
|-----------------------|
|                       |
|  Label: Waiting...     |
|                       |
|  [ Update Label ]      |
|                       |
-------------------------

Display a label with initial text 'Waiting...'

Add a button labeled 'Update Label'

When button is tapped, simulate a 2-second background task

After task completes, update label text to 'Updated on MainActor!' safely on main thread using @MainActor

Starter Code

iOS Swift

import SwiftUI

struct MainActorDemoView: View {
    @State private var labelText = "Waiting..."

    var body: some View {
        VStack(spacing: 20) {
            Text(labelText)
                .font(.title)
            Button("Update Label") {
                // TODO: Call function to update label
            }
        }
        .padding()
    }

    // TODO: Add function to simulate background task and update label
}

struct MainActorDemoView_Previews: PreviewProvider {
    static var previews: some View {
        MainActorDemoView()
    }
}

Task 1

Task 2

Task 3

Solution

iOS Swift

import SwiftUI

struct MainActorDemoView: View {
    @State private var labelText = "Waiting..."

    var body: some View {
        VStack(spacing: 20) {
            Text(labelText)
                .font(.title)
            Button("Update Label") {
                updateLabel()
            }
        }
        .padding()
    }

    func updateLabel() {
        Task {
            try? await Task.sleep(nanoseconds: 2_000_000_000) // 2 seconds
            await MainActor.run {
                labelText = "Updated on MainActor!"
            }
        }
    }
}

struct MainActorDemoView_Previews: PreviewProvider {
    static var previews: some View {
        MainActorDemoView()
    }
}

We use Task to run an asynchronous background task that simulates a 2-second delay using Task.sleep. After the delay, we update the labelText state variable on the main thread by calling await MainActor.run. This ensures the UI update happens safely on the main thread, which is required for SwiftUI views.

The button calls updateLabel() when tapped, triggering the background wait and then the UI update.

Final Result

Completed Screen

-------------------------
| MainActor UI Update Demo |
|-----------------------|
|                       |
|  Label: Waiting...     |
|                       |
|  [ Update Label ]      |
|                       |
-------------------------

User taps 'Update Label' button

Label text stays 'Waiting...' for 2 seconds

After 2 seconds, label text changes to 'Updated on MainActor!'

Stretch Goal

Add a loading spinner that shows while the background task runs and hides after update

💡 Hint

Use a @State Boolean to track loading state and show ProgressView conditionally in the UI