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Last active September 13, 2022 14:20
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  • Save ole/f76630731c9a0cda90bb6bae28e82927 to your computer and use it in GitHub Desktop.
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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:
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
textField.delegate = self
textField.addTarget(self, action: #selector(TextFieldObserver.textFieldValueChanged(sender:)), for: .editingChanged)
@objc func textFieldValueChanged(sender: UITextField) {
// 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
let cancelAction = UIAlertAction(title: cancelButtonTitle, style: .cancel, handler: { _ in
finish(result: .cancel)
let okAction = UIAlertAction(title: okButtonTitle, style: .default, handler: { [unowned self] _ in
finish(result: .ok(self.textFields?.first?.text ?? ""))
preferredAction = okAction
addTextField(configurationHandler: { textField in
textFieldObserver = TextFieldObserver(textField: textField,
valueChanged: { textField in
okAction.isEnabled = validationRule.isValid(textField.text ?? "")
shouldReturn: { textField in
validationRule.isValid(textField.text ?? "")
// Start with a disabled OK button if necessary
okAction.isEnabled = validationRule.isValid(textFields?.first?.text ?? "")
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very nice code! I`ve taken a lot from this gist, thanks)

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CrystDragon commented Aug 17, 2018

Exactly! By using closures creatively in extensions, the "cannot define stored properties in extensions" problem can be bypassed often. And when working with protocol extensions, we can reduce the amount of required methods in protocols.

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awesome extension.

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This is great!

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wata commented Feb 26, 2019

Thank you for great codes!

I am using it like this:

        case .nonEmpty:
            return !input.trimmingCharacters(in: .whitespacesAndNewlines).isEmpty

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limaho commented Apr 3, 2019

Thank you for the code is working fine, but how can I get the result in string without the
.ok(TextEnteredByUser) in the onCompletion?

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dinara95 commented May 3, 2019

I think you can add get() method to TextInputResult.

public enum TextInputResult {
        case cancel
        case ok(String)

        func get() -> String {
            switch self {
            case .ok(let text):
                return text
            case .cancel:
                return ""

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