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)

        /// Observes a UITextField for various events and reports them via callbacks.
        /// Sets itself as the text field's delegate and target-action target.
        class TextFieldObserver: NSObject, UITextFieldDelegate {
            let textFieldValueChanged: (UITextField) -> Void
            let textFieldShouldReturn: (UITextField) -> Bool

            init(textField: UITextField, valueChanged: @escaping (UITextField) -> Void, shouldReturn: @escaping (UITextField) -> Bool) {
                self.textFieldValueChanged = valueChanged
                self.textFieldShouldReturn = shouldReturn
                super.init()
                textField.delegate = self
                textField.autocorrectionType = .no
                textField.addTarget(self, action: #selector(TextFieldObserver.textFieldValueChanged(sender:)), for: .editingChanged)
            }

            @objc func textFieldValueChanged(sender: UITextField) {
                textFieldValueChanged(sender)
            }

            // MARK: UITextFieldDelegate
            func textFieldShouldReturn(_ textField: UITextField) -> Bool {
                return textFieldShouldReturn(textField)
            }
        }

        var textFieldObserver: TextFieldObserver?

        // Every `UIAlertAction` handler must eventually call this
        func finish(result: TextInputResult) {
            // Capture the observer to keep it alive while the alert is on screen
            textFieldObserver = nil
            onCompletion(result)
        }

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

        addTextField(configurationHandler: { textField in
            textFieldConfiguration?(textField)
            textFieldObserver = TextFieldObserver(textField: textField,
                                                  valueChanged: { [weak self] textField in
                                                    let isValid = validationRule.isValid(textField.text ?? "")
                                                    self?.preferredAction = isValid ? okAction : nil
                                                    okAction.isEnabled = isValid
            },
                                                  shouldReturn: { textField in
                                                    validationRule.isValid(textField.text ?? "")
            })
        })
        // Start with a disabled OK button if necessary
        okAction.isEnabled = validationRule.isValid(textFields?.first?.text ?? "")
    }
}