I’m working on a login screen in my iOS app using Swift. The user needs to enter their phone number and press a “Send Code” button to receive a verification code. How can I implement the following features into my send code button?

1. The button should be disabled and greyed out immediately after the user presses it, regardless of any errors.
2. If there is an error (e.g., the user didn’t enter enough digits), the button should be re-enabled after the user dismisses the error alert. The countdown timer should not start in this case.
3. If the user successfully receives a verification code, the button should remain disabled for 1 minute, during which a countdown timer is displayed.

The point of this is to prevent users from spamming the “Send Code” button. Here is the relevant code:

import UIKit
import FirebaseAuth
import FirebaseFunctions

class LoginViewController: BaseViewController, UITextFieldDelegate {

    let phoneTextField: UITextField = {
        let textField = UITextField()
        textField.placeholder = "Phone number"
        textField.borderStyle = .roundedRect
        textField.keyboardType = .numberPad
        textField.translatesAutoresizingMaskIntoConstraints = false
        return textField
    }()
    
    let sendCodeButton: UIButton = {
        let button = UIButton(type: .system)
        button.setTitle("Send code", for: .normal)
        button.layer.cornerRadius = 10
        button.titleLabel?.font = UIFont.boldSystemFont(ofSize: 18)
        button.isEnabled = false
        button.addTarget(self, action: #selector(sendCodeButtonTapped), for: .touchUpInside)
        button.translatesAutoresizingMaskIntoConstraints = false
        return button
    }()
    
    let countdownLabel: UILabel = {
        let label = UILabel()
        label.text = ""
        label.font = UIFont.systemFont(ofSize: 14)
        label.textAlignment = .left
        label.textColor = .gray
        label.translatesAutoresizingMaskIntoConstraints = false
        label.isHidden = true
        return label
    }()
    
    lazy var functions = Functions.functions()
    var isCountdownActive = false
    var countdownEndTime: Date?

    override func viewDidLoad() {
        super.viewDidLoad()
        setupUI()
        updateButtonStyles()
        phoneTextField.addTarget(self, action: #selector(textFieldDidChange), for: .editingChanged)
        loadCountdownState()
    }

    override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)
        updateSendCodeButtonState()
    }

    @objc private func sendCodeButtonTapped() {
        guard let phoneNumber = phoneTextField.text, phoneNumber.count == 10 else {
            showAlert(title: "Error", message: "Phone number must be exactly 10 digits.")
            sendCodeButton.isEnabled = true
            return
        }
        
        let formattedPhoneNumber = "+1(phoneNumber)"
        
        sendCodeButton.isEnabled = false
        updateSendCodeButtonState()
        
        activityIndicator.startAnimating()
        
        functions.httpsCallable("checkPhoneNumberExists").call(["phoneNumber": formattedPhoneNumber]) { [weak self] result, error in
            self?.activityIndicator.stopAnimating()
            
            if let error = error {
                self?.showAlert(title: "Error", message: "Error checking phone number: (error.localizedDescription)")
                self?.sendCodeButton.isEnabled = true
                return
            }
            
            guard let data = result?.data as? [String: Any], let exists = data["exists"] as? Bool else {
                self?.showAlert(title: "Error", message: "Invalid response from server.")
                self?.sendCodeButton.isEnabled = true
                return
            }
            
            if exists {
                self?.sendVerificationCode(to: formattedPhoneNumber)
            } else {
                self?.showAlert(title: "Error", message: "Phone number does not exist. Please sign up first.")
                self?.sendCodeButton.isEnabled = true
            }
        }
    }

    private func sendVerificationCode(to phoneNumber: String) {
        PhoneAuthProvider.provider().verifyPhoneNumber(phoneNumber, uiDelegate: nil) { [weak self] verificationID, error in
            if let error = error {
                self?.showAlert(title: "Error", message: "Unable to send code. Please try again.")
                self?.sendCodeButton.isEnabled = true
                return
            }
            
            UserDefaults.standard.set(verificationID, forKey: "authVerificationID")
            UserDefaults.standard.set(phoneNumber, forKey: "phoneNumber")
            
            self?.countdownLabel.isHidden = false
            self?.startCountdown(seconds: 60)
            
            let verificationVC = LoginPhoneVerificationViewController(phoneNumber: phoneNumber)
            self?.navigationController?.pushViewController(verificationVC, animated: true)
        }
    }

    private func startCountdown(seconds: Int) {
        var remainingSeconds = seconds
        isCountdownActive = true
        countdownEndTime = Date().addingTimeInterval(TimeInterval(seconds))
        UserDefaults.standard.set(countdownEndTime?.timeIntervalSince1970, forKey: "countdownEndTime")
        countdownLabel.text = "Resend code in: (remainingSeconds)s"
        
        Timer.scheduledTimer(withTimeInterval: 1.0, repeats: true) { [weak self] timer in
            remainingSeconds -= 1
            if remainingSeconds > 0 {
                self?.countdownLabel.text = "Resend code in: (remainingSeconds)s"
            } else {
                timer.invalidate()
                self?.isCountdownActive = false
                self?.countdownEndTime = nil
                UserDefaults.standard.removeObject(forKey: "countdownEndTime")
                self?.updateSendCodeButtonState()
                self?.countdownLabel.isHidden = true
            }