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

XCTestExpectation pauses a test until an async condition is fulfilled or a timeout is reached.
Use a computed property for a cheap, side-effect-free value derived from existing state; use a method for expensive work, side effects, or anything that needs arguments.
Encapsulation means bundling data with the operations that act on it, and restricting direct access to that data so internal invariants can't be broken from outside.
@MainActor is a global actor that ensures code runs on the main thread.

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"
    }
}