MainActor for UI work in Swift - Mini Project: Build & Apply

Using MainActor for UI Work in Swift

📖 Scenario: You are building a simple Swift app that updates a label on the screen. UI updates must happen on the main thread to keep the app smooth and avoid crashes.

🎯 Goal: Learn how to use @MainActor to ensure UI updates happen safely on the main thread.

📋 What You'll Learn

Create a class called ViewModel with a @MainActor annotation

Add a text property of type String inside ViewModel

Write an async function updateText() inside ViewModel that changes text

Call updateText() from outside the class using await

Print the updated text property

💡 Why This Matters

🌍 Real World

UI updates in iOS apps must happen on the main thread to avoid glitches and crashes. Using <code>@MainActor</code> helps keep UI code safe and organized.

💼 Career

Understanding <code>@MainActor</code> is important for Swift developers working on iOS apps, especially when dealing with concurrency and UI updates.

1

Create ViewModel class with @MainActor and text property

Create a class called ViewModel and annotate it with @MainActor. Inside it, create a variable text of type String initialized to "Hello".

Swift

@MainActor
class ViewModel {
    var text: String = "Hello"
    // Your code here
}

Need a hint?

Use @MainActor before the class declaration to mark it for UI work.

2

Add async function updateText() to change text

Inside the ViewModel class, add an async function called updateText() that sets text to "Updated Text".

Swift

@MainActor
class ViewModel {
    var text: String = "Hello"
    
    // Add async function updateText() here
    // Your code here
}

Need a hint?

Define updateText() as an async function that changes the text property.

3

Create ViewModel instance and call updateText() with await

Outside the ViewModel class, create an instance called viewModel. Then write an async context (like Task) and call await viewModel.updateText() inside it.

Swift

@MainActor
class ViewModel {
    var text: String = "Hello"
    
    func updateText() async {
        text = "Updated Text"
    }
}

// Create ViewModel instance and call updateText() inside Task
// Your code here

Need a hint?

Use Task { await viewModel.updateText() } to call the async function.

4

Print the updated text property

Inside the same Task block, after calling await viewModel.updateText(), add a print(viewModel.text) statement to show the updated text.

Swift

@MainActor
class ViewModel {
    var text: String = "Hello"
    
    func updateText() async {
        text = "Updated Text"
    }
}

let viewModel = ViewModel()

Task {
    await viewModel.updateText()
    // Print the updated text here
    // Your code here
}

Need a hint?

Use print(viewModel.text) to display the updated text.