Thanks to this challenge, I will never forget what Paul's been repeating for a while now: "you never manipulate user interfaces anywhere but the main thread, right? Right"

My app was crashing because I was trying to present an UIAlertViewController outside the main thread.

Hope this helps in case someone is stuck on this.

KB

Yes. Running code on the background thread should be reserved for time consuming tasks like decoding or maintenance 😊

By the way to prevent this issue in your future projects, or have a peace of mind with reusable code, consider having an extension on UIViewController, something like this

// UIViewController+Ext.swift

import UIKit

extension UIViewController {

    func presentAlertOnMainThread(title: String, message: String, buttonTitle: String, otherActions: [UIAlertAction]? = nil) {
        DispatchQueue.main.async {
            let alertVC         = UIAlertController(title: title, message: message, preferredStyle: .alert)
            let defaultAction   = UIAlertAction(title: buttonTitle, style: .default)
            alertVC.addAction(defaultAction)

            if otherActions != nil { otherActions?.forEach { alertVC.addActions($0) } }
            self.present(alertVC, animated: true)
        }
    }
}

Apart from the defaultAction e.g. an "OK" button, for the otherActions parameter, we could declare any other UIAlertAction we desire then pass it to this function above as an array.