What is `MainActor` and when do you use it?

Answer: @MainActor is a global actor that ensures code runs on the main thread. Use it for UI updates, ViewModels, and any code that must not run off the main thread.

Code Example:

@MainActor
class ProfileViewModel: ObservableObject {
    @Published var username: String = ""
    @Published var isLoading: Bool = false

    func loadProfile() async {
        isLoading = true
        defer { isLoading = false }
        username = try await ProfileService.fetchUsername()
        // All assignments happen on main thread automatically
    }
}

// Marking just a function (not the whole type)
func updateUI() async {
    await MainActor.run {
        label.text = "Done"
    }
}