A convenient way to create and present AlertControllers.

AlertHelper.swift

import UIKit

extension UIAlertAction {
    convenience init(title: String, handler: ((UIAlertAction?) -> Void)? = nil) {
        self.init(title: title, style: .default, handler: handler)
    }
}

/// - Tag: AlertHelper
struct AlertHelper {

    private static var closeButtonText = Constants.Alert.closeButtonText
    private static var cancelButtonText = Constants.Alert.cancelButtonText
    private static var affirmativeButtonText = Constants.Alert.affirmativeButtonText
    private static var settingsButtonText = Constants.Alert.settingsButtonText

    // MARK: Actionable Alerts

    static func showSingleButtonAlert(in viewController: UIViewController,
                                      title: String?,
                                      message: String?,
                                      dismissLabel: String = closeButtonText,
                                      completion: @escaping (() -> Void) = {}) {

        let alert = UIAlertController(title: title, message: message, preferredStyle: .alert)
        // The completion block of a UIAlertController is called when the alert is displayed, not when it is dismissed.
        // In this way, you cannot dismiss a viewController that has presented the alert until after it is dismissed.
        alert.addAction(UIAlertAction(title: dismissLabel, handler: { _ in
            // Dismiss ViewController that presented the alert here.
            completion()
        }))
        viewController.present(alert, animated: true, completion: nil)
    }

    static func showConfirmationAlert(in viewController: UIViewController,
                                      title: String?,
                                      message: String?,
                                      confirmationActionText actionText: String = affirmativeButtonText,
                                      cancelActionText: String = cancelButtonText,
                                      confirmationActionHandler handler: @escaping (UIAlertAction?) -> Void) {

        let positiveAction = UIAlertAction(title: actionText, style: .default, handler: handler)
        let cancelAction = UIAlertAction(title: cancelActionText, style: .cancel)

        let alert = UIAlertController(title: title, message: message, preferredStyle: .alert)
        alert.addAction(positiveAction)
        alert.addAction(cancelAction)
        alert.preferredAction = positiveAction

        viewController.present(alert, animated: true) {}
    }

    // MARK: Settings

    static func showSettingsAlert(in viewController: UIViewController, title: String?, message: String?, fallbackMessage: String? = nil) {
        guard let settingsUrl = URL(string: UIApplicationOpenSettingsURLString),
            UIApplication.shared.canOpenURL(settingsUrl) else {
                showSingleButtonAlert(in: viewController, title: title, message: fallbackMessage ?? message)
                return
        }
        let alert = UIAlertController(title: title, message: message, preferredStyle: .alert)
        let settingsAction = UIAlertAction(title: settingsButtonText) { _ in
            if #available(iOS 10.0, *) {
                UIApplication.shared.open(settingsUrl, completionHandler: nil)
            } else {
                UIApplication.shared.openURL(settingsUrl)
            }
        }
        alert.addAction(settingsAction)
        alert.addAction(UIAlertAction(title: closeButtonText))

        viewController.present(alert, animated: true, completion: nil)
    }

    // MARK: Error Alerts

    static func showAlert(for error: Error,
                          in viewController: UIViewController,
                          title: String? = nil,
                          dismissLabel: String = closeButtonText) {
        if let customError = error as? CustomError {
            return showAlert(for: customError, in: viewController)
        }

        let error = error as NSError
        let alertTitle: String
        let alertMessage: String

        if let errorTitle = error.userInfo["title"] as? String, let errorMessage = error.userInfo["detail"] as? String {
            alertTitle = title ?? errorTitle
            alertMessage = errorMessage
        } else {
            alertTitle = title ?? String(format: "Error %i", error.code)
            alertMessage = error.localizedDescription
        }

        let alert = UIAlertController(title: alertTitle, message: alertMessage, preferredStyle: .alert)
        alert.addAction(UIAlertAction(title: dismissLabel))
        viewController.present(alert, animated: true)
    }

    static func showAlert(for error: CustomError, in viewController: UIViewController) {
        let isUnauthorizedError = error.localizedDescription == NetworkError.unauthorized.localizedDescription

        if isUnauthorizedError {//} && isLoggedIn {
            return
        }
        let alert = UIAlertController(title: error.title, message: error.localizedDescription, preferredStyle: .alert)
        alert.addAction(UIAlertAction(title: closeButtonText))
        viewController.present(alert, animated: true)
    }

    // MARK: Private Helpers

    private static func alertTitle(for error: Error) -> String {
        let error = error as NSError
        return String(format: "Error %i", error.code)
    }
}

CustomError.swift

import Foundation

/// Used with Alert Helper to easily recognize and display custom error alerts
protocol CustomError: LocalizedError {
    var title: String { get }
    var localizedDescription: String { get }
}

extension CustomError {
    var title: String {
        return "Error"
    }
}