Main Thread UI

All UI updates must be dispatched to the main thread. AppKit and UIKit will crash or behave unpredictably if views are mutated from background threads.

// From a background callback
URLSession.shared.dataTask(with: url) { data, _, _ in
    DispatchQueue.main.async {
        self.label.stringValue = "Done"
        self.progressBar.isHidden = true
    }
}.resume()

In async/await code, use @MainActor on methods that touch UI:

@MainActor
func updateProgress(_ value: Double) {
    progressBar.doubleValue = value
}