MVP-Example

Amount.swift

import Foundation

class Amount: NSObject {
    @objc dynamic var number: Int
    
    override init() {
        self.number = 0
    }
    
    func increase() {
        self.number += 1
    }
    
    func decrease() {
        self.number -= 1
    }
}

CountPresenter.swift

import Foundation

class CountPresenter: CountViewPresenter {
    private unowned var view: CountView
    
    private var amount: Amount!
    private var observation: NSKeyValueObservation?
    
    required init(view: CountView) {
        self.view = view
    }
    
    func viewDidLoad() {
        self.amount = Amount()
        self.view.onValueChange(number: 0)
        self.observation = self.amount.observe(\.number, options: .new) { object, change in
            guard let number = change.newValue else { return }
            self.view.onValueChange(number: number)
        }
    }
    
    func increaseButtonTouched() {
        self.amount.increase()
    }
    
    func decreaseButtonTouched() {
        self.amount.decrease()
    }
}

CountView.swift

import Foundation

protocol CountView: AnyObject {
    func onValueChange(number: Int)
}

CountViewPresenter.swift

import Foundation

protocol CountViewPresenter {
    init(view: CountView)
    func viewDidLoad()
    func increaseButtonTouched()
    func decreaseButtonTouched()
}

SceneDelegate.swift

import UIKit

class SceneDelegate: UIResponder, UIWindowSceneDelegate {

    var window: UIWindow?


    func scene(_ scene: UIScene, willConnectTo session: UISceneSession, options connectionOptions: UIScene.ConnectionOptions) {
        guard let scene = (scene as? UIWindowScene) else { return }
        
        let view = ViewController()
        let presenter = CountPresenter(view: view)
        view.presenter = presenter
        
        self.window = UIWindow(windowScene: scene)
        self.window?.rootViewController = view
        self.window?.makeKeyAndVisible()
    }

ViewController.swift

import UIKit

class ViewController: UIViewController {
    
    private lazy var increaseButton: UIButton = {
        let button = UIButton()
        button.setImage(UIImage(systemName: "plus"), for: .normal)
        button.addTarget(self, action: #selector(increaseButtonTouched(_sender:)), for: .touchUpInside)
        return button
    }()
    
    private lazy var decreaseButton: UIButton = {
        let button = UIButton()
        button.setImage(UIImage(systemName: "minus"), for: .normal)
        button.addTarget(self, action: #selector(decreaseButtonTouched(_sender:)), for: .touchUpInside)
        return button
    }()
    
    private lazy var amountLabel: UILabel = {
        let label = UILabel()
        label.font = label.font.withSize(32)
        label.text = String(0)
        return label
    }()
    
    var presenter: CountViewPresenter?
    
    override func viewDidLoad() {
        super.viewDidLoad()
        self.configureUI()
        self.presenter?.viewDidLoad()
    }

    
    func configureUI() {
        self.view.backgroundColor = .white
        self.view.addSubview(self.amountLabel)
        self.amountLabel.translatesAutoresizingMaskIntoConstraints = false
        self.amountLabel.centerXAnchor.constraint(equalTo: self.view.safeAreaLayoutGuide.centerXAnchor).isActive = true
        self.amountLabel.centerYAnchor.constraint(equalTo: self.view.safeAreaLayoutGuide.centerYAnchor).isActive = true
        
        self.view.addSubview(self.increaseButton)
        self.increaseButton.translatesAutoresizingMaskIntoConstraints = false
        self.increaseButton.centerYAnchor.constraint(equalTo: self.view.safeAreaLayoutGuide.centerYAnchor).isActive = true
        self.increaseButton.heightAnchor.constraint(equalToConstant: 50).isActive = true
        self.increaseButton.widthAnchor.constraint(equalToConstant: 50).isActive = true
        self.increaseButton.leadingAnchor.constraint(equalTo: self.amountLabel.trailingAnchor, constant: 20).isActive = true
        
        self.view.addSubview(self.decreaseButton)
        self.decreaseButton.translatesAutoresizingMaskIntoConstraints = false
        self.decreaseButton.centerYAnchor.constraint(equalTo: self.view.safeAreaLayoutGuide.centerYAnchor).isActive = true
        self.decreaseButton.heightAnchor.constraint(equalToConstant: 50).isActive = true
        self.decreaseButton.widthAnchor.constraint(equalToConstant: 50).isActive = true
        self.decreaseButton.trailingAnchor.constraint(equalTo: self.amountLabel.leadingAnchor, constant: -20).isActive = true
    }
    
    @objc func increaseButtonTouched(_sender: UIButton) {
        self.presenter?.increaseButtonTouched()
    }
    
    @objc func decreaseButtonTouched(_sender: UIButton) {
        self.presenter?.decreaseButtonTouched()
    }
}

extension ViewController: CountView {
    func onValueChange(number: Int) {
        self.amountLabel.text = String(number)
    }
}