A UIAlertController with a text field and the ability to perform validation on the text the user has entered while the alert is on screen. The OK button is only enabled when the entered text passes validation. More info: https://oleb.net/2018/uialertcontroller-textfield/

UIAlertController+TextField.swift

import UIKit

/// A validation rule for text input.
public enum TextValidationRule {
    /// Any input is valid, including an empty string.
    case noRestriction
    /// The input must not be empty.
    case nonEmpty
    /// The enitre input must match a regular expression. A matching substring is not enough.
    case regularExpression(NSRegularExpression)
    /// The input is valid if the predicate function returns `true`.
    case predicate((String) -> Bool)

    public func isValid(_ input: String) -> Bool {
        switch self {
        case .noRestriction:
            return true
        case .nonEmpty:
            return !input.isEmpty
        case .regularExpression(let regex):
            let fullNSRange = NSRange(input.startIndex..., in: input)
            return regex.rangeOfFirstMatch(in: input, options: .anchored, range: fullNSRange) == fullNSRange
        case .predicate(let p):
            return p(input)
        }
    }
}

extension UIAlertController {
    public enum TextInputResult {
        /// The user tapped Cancel.
        case cancel
        /// The user tapped the OK button. The payload is the text they entered in the text field.
        case ok(String)
    }

    /// Creates a fully configured alert controller with one text field for text input, a Cancel and
    /// and an OK button.
    ///
    /// - Parameters:
    ///   - title: The title of the alert view.
    ///   - message: The message of the alert view.
    ///   - cancelButtonTitle: The title of the Cancel button.
    ///   - okButtonTitle: The title of the OK button.
    ///   - validationRule: The OK button will be disabled as long as the entered text doesn't pass
    ///     the validation. The default value is `.noRestriction` (any input is valid, including
    ///     an empty string).
    ///   - textFieldConfiguration: Use this to configure the text field (e.g. set placeholder text).
    ///   - onCompletion: Called when the user closes the alert view. The argument tells you whether
    ///     the user tapped the Close or the OK button (in which case this delivers the entered text).
    public convenience init(title: String, message: String? = nil,
                            cancelButtonTitle: String, okButtonTitle: String,
                            validate validationRule: TextValidationRule = .noRestriction,
                            textFieldConfiguration: ((UITextField) -> Void)? = nil,
                            onCompletion: @escaping (TextInputResult) -> Void) {
        self.init(title: title, message: message, preferredStyle: .alert)

        /// Required for running validation when the user presses the Return key
        class TextFieldDelegate: NSObject, UITextFieldDelegate {
            let validationRule: TextValidationRule
            init(validationRule: TextValidationRule) {
                self.validationRule = validationRule
                super.init()
            }

            func textFieldShouldReturn(_ textField: UITextField) -> Bool {
                // Only allow to dismiss the alert view if the input passes validation
                return validationRule.isValid(textField.text ?? "")
            }
        }

        var textFieldDelegate: TextFieldDelegate?
        var textFieldDidChangeObserver: Any?

        // Every `UIAlertAction` handler must eventually call this
        func finish(result: TextInputResult) {
            // Capture the delegate and observer to keep them alive while the alert is on screen
            textFieldDelegate = nil
            if let observer = textFieldDidChangeObserver {
                NotificationCenter.default.removeObserver(observer)
            }
            onCompletion(result)
        }

        let cancelAction = UIAlertAction(title: cancelButtonTitle, style: .cancel, handler: { _ in
            finish(result: .cancel)
        })
        let createAction = UIAlertAction(title: okButtonTitle, style: .default, handler: { [unowned self] _ in
            finish(result: .ok(self.textFields?.first?.text ?? ""))
        })
        addAction(cancelAction)
        addAction(createAction)
        preferredAction = createAction

        addTextField(configurationHandler: { textField in
        textFieldConfiguration?(textField)
            textFieldDelegate = TextFieldDelegate(validationRule: validationRule)
            textField.delegate = textFieldDelegate
            // Monitor the text field to disable the OK button for invalid inputs
            textFieldDidChangeObserver = NotificationCenter.default.addObserver(forName: UITextField.textDidChangeNotification, object: textField, queue: .main) { _ in
                createAction.isEnabled = validationRule.isValid(textField.text ?? "")
            }
        })
        // Start with a disabled OK button if necessary
        createAction.isEnabled = validationRule.isValid(textFields?.first?.text ?? "")
    }
}