Alert Controller + TextField + Validation

AlertPresenter.swift

import UIKit

class AlertPresenter {
    
    static let codeValidator = VerificationCodeValidator()
    
    static func showConfirmationCodeAlert(at vc: UIViewController, title: String, message: String,
                                          sendCodeAction: @escaping (String) -> ()) {
        
        let alert = UIAlertController(title: title, message: message, preferredStyle: .alert)
        
        let sendAction = UIAlertAction(title: "Send code", style: .default) { (_) in
            guard let textField = alert.textFields?.first, let text = textField.text else {
                return
            }
            sendCodeAction(text)
        }
        sendAction.isEnabled = false
        
        let cancel = UIAlertAction(title: "Cancel", style: .cancel) { (alertAction) in
            NotificationCenter.default.removeObserver(vc, name: UITextField.textDidChangeNotification, object: nil)
        }
        
        alert.addTextField { (textField) in
            
            textField.placeholder = "Enter code here"
            textField.textColor = .black
            textField.keyboardType = .numberPad
            textField.delegate = codeValidator
            
            NotificationCenter.default.addObserver(forName: UITextField.textDidChangeNotification, object: textField, queue: OperationQueue.main, using:
                {_ in
                    
                    let textCount = textField.text?.trimmingCharacters(in: .whitespacesAndNewlines).count ?? 0
                    let textIsNotEmpty = textCount > 3
                    
                    sendAction.isEnabled = textIsNotEmpty
            })
        }
        alert.addAction(sendAction)
        alert.addAction(cancel)
        
        vc.present(alert, animated: true, completion: nil)
    }
    
}

class VerificationCodeValidator: NSObject, UITextFieldDelegate {
    
    func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {
        let allowedCharacters = CharacterSet.decimalDigits
        let characterSet = CharacterSet(charactersIn: string)
        return allowedCharacters.isSuperset(of: characterSet)
    }
    
}

// Usage:
// AlertPresenter.showConfirmationCodeAlert(at: self, title: "Email verification code",
//                                          message: "Verification code was sent to your email") { (code) in
//                 debugPrint(code)
// }