UIAlertController+Error.swift

import PromiseKit

public extension UIViewController {
    @discardableResult
    func alert(error: Error, title: String? = nil, file: StaticString = #file, line: UInt = #line) -> Guarantee<Void> {
        let (promise, seal) = Guarantee<UIAlertAction>.pending()

        let alert = UIAlertController(title: title ?? String(describing: type(of: error)), message: error.legibleDescription, preferredStyle: .alert)
        alert.addAction(.init(title: "OK", style: .default, handler: seal))

        if let transitionCoordinator = transitionCoordinator {
            let window = self.view.window
            transitionCoordinator.animate(alongsideTransition: nil, completion: { [weak self] _ in
                if let presenter = self ?? window?.rootViewController {
                    presenter.present(alert, animated: true)
                }
            })
        } else if let vc = UIApplication.shared.visibleViewController, vc.view != nil {
            vc.present(alert, animated: true)
        } else if view.window != nil {
            present(alert, animated: true)
        } else {
            print("error: Could not present UIAlertViewController")
            return Guarantee()
        }

        print("\(file):\(line)", error)  // you should log some of these to Sentry
        return promise.asVoid()
    }
}

extension UIApplication {
    var visibleViewController: UIViewController? {
        var vc = UIApplication.shared.keyWindow?.rootViewController
        while let presentedVc = vc?.presentedViewController {
            if let navVc = (presentedVc as? UINavigationController)?.viewControllers.last {
                vc = navVc
            } else if let tabVc = (presentedVc as? UITabBarController)?.selectedViewController {
                vc = tabVc
            } else {
                vc = presentedVc
            }
        }
        return vc
    }
}