Screen.swift

import UIKit

final class ViewController: UIViewController {
    // MARK: UI

    let label = UILabel()
    let incrementButton = UIButton()
    let decrementButton = UIButton()

    // MARK: Properties

    let viewModel = ViewModel()

    // MARK: Lifecycle

    override func viewDidLoad() {
        super.viewDidLoad()

        // MARK: Setup

        view.addSubview(label)
        view.addSubview(incrementButton)
        view.addSubview(decrementButton)

        label.translatesAutoresizingMaskIntoConstraints = false
        label.centerYAnchor.constraint(equalTo: view.centerYAnchor).isActive = true
        label.centerXAnchor.constraint(equalTo: view.centerXAnchor).isActive = true

        incrementButton.backgroundColor = .red
        incrementButton.setTitle("Increment", for: .normal)
        incrementButton.translatesAutoresizingMaskIntoConstraints = false
        incrementButton.centerYAnchor.constraint(equalTo: view.centerYAnchor, constant: 50).isActive = true
        incrementButton.leftAnchor.constraint(equalTo: view.leftAnchor, constant: 100).isActive = true
        incrementButton.rightAnchor.constraint(equalTo: view.rightAnchor, constant: -100).isActive = true
        incrementButton.addTarget(self, action: #selector(viewModel.increment), for: .touchUpInside)

        decrementButton.backgroundColor = .red
        decrementButton.setTitle("Decrement", for: .normal)
        decrementButton.translatesAutoresizingMaskIntoConstraints = false
        decrementButton.centerYAnchor.constraint(equalTo: view.centerYAnchor, constant: 100).isActive = true
        decrementButton.leftAnchor.constraint(equalTo: view.leftAnchor, constant: 100).isActive = true
        decrementButton.rightAnchor.constraint(equalTo: view.rightAnchor, constant: -100).isActive = true
        decrementButton.addTarget(self, action: #selector(viewModel.decrement), for: .touchUpInside)

        // MARK: Binding

        viewModel.countBox.bind { [weak self] in
            self?.label.text = "\($0)"
        }
    }
}