Concurrency

Leaving a loading screen

When the user enters the detail screen, it calls an API like this and shows the data.

final class DetailViewController: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()
        api.fetchDetail(id: id) { result in
            self.render(result)
        }
    }
}

The interviewer asks: "What happens if the user taps back before the response arrives?" Explain the problems this code can cause, and how you'd keep the request for a screen the user left from going to waste.

Your answer

Interviewer mode

See the short answer

A response that arrives after the user goes back updates a deallocated (or different) screen, or the strong capture keeps the screen alive, and the request itself keeps running and wastes resources. Fix: keep the request as a Task and cancel it when the screen closes (SwiftUI .task does it automatically), use [weak self], and branch on CancellationError.

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Leaving a loading screen

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