Example of TransformViewController

TransformViewController.swift

import UIKit

class TransformViewController : UIViewController {
    lazy var transformModel: TransformModel = { TransformModel() }()
    lazy var transformView: TransformView = { TransformView().make() }()

    var transformButton: UIButton { transformView.transformButton }
    var inputTextField: UITextField { transformView.inputTextField }

    private var transformModelObservation: NSKeyValueObservation?

    @objc func textDidChange(textField: UITextField) {
        transformModel.text = textField.text
    }

    @objc func transformTouched() {
        transformModel.reverse()
    }

    private func bindView() {
        inputTextField.delegate = self
        inputTextField.addTarget(self,
                                 action: #selector(textDidChange),
                                 for: .editingChanged)
        transformButton
            .addTarget(self,
                       action: #selector(transformTouched),
                       for: .touchUpInside)
    }

    func transformModelUpdated(model: TransformModel, change: NSKeyValueObservedChange<String?>) {
        self.inputTextField.text = model.text
    }

    private func bindModel() {

        transformModelObservation = transformModel.observe(\.text,
                                                           options: .new,
                                                           changeHandler: transformModelUpdated)
    }

    override func loadView() {
        super.loadView()
        view = transformView
        bindView()
        bindModel()

    }

    private func showOnlyLettersAlert() {
        let alert = UIAlertController(title: "Oops 🤦‍♂️",
                                      message: "Only letters please!",
                                      preferredStyle: .alert)

        alert.addAction(.init(title: "Ok",
                              style: .destructive,
                              handler: nil))

        self.present(alert,
                     animated: true,
                     completion: nil)
    }
}

extension TransformViewController: UITextFieldDelegate {
    func textField(_ textField: UITextField, shouldChangeCharactersIn range: NSRange, replacementString string: String) -> Bool {

        guard transformModel.hasOnlyLetters(string) else {
            showOnlyLettersAlert()
            return false
        }
        return true
    }
}