class ViewController: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()
        let mainView = MainView()

        view.addSubview(mainView.root)
        mainView.buildConstraints()
        view.leadingAnchor.constraint(equalTo: mainView.root.leadingAnchor).isActive = true
    }
}

struct BuildableView<T> where T: UIView {
    var view: T
    init(_ build: (_ initialView: T) -> T) {
        self.view = build(T())
    }
    init(_ build: () -> T) {
        self.view = build()
    }
}

struct MainView {
    private let label = BuildableView<UILabel> { label in
        label.text = "hey"
        label.textColor = .blue
        return label
    }
    private let button = BuildableView<UIButton> { button in
        button.setTitle("button title", for: .normal)
        button.backgroundColor = .cyan
        return button
    }
    let root = UIView()

    init() {
        root.addSubview(label.view)
        root.addSubview(button.view)
    }

    func buildConstraints() {
        label.view.trailingAnchor.constraint(equalTo: button.view.leadingAnchor).isActive = true
    }

}