How to hide the navigation bar as scroll down, when constraints are programmatically?

Question

I'm doing all my UI programatically, to avoid a massive view controller I have a class of type UIView where I'm declaring all my UI elements.

I'm declaring my scrollView like this:

class RegisterUIView: UIView {

    lazy var scrollView: UIScrollView = {
        let scroll: UIScrollView = UIScrollView()
        scroll.contentSize = CGSize(width: self.frame.size.width, height: self.frame.size.height)
        scroll.translatesAutoresizingMaskIntoConstraints = false
        return scroll
    }()

    override init(frame: CGRect) {
        super.init(frame: frame)
        addSubview(scrollView)
        NSLayoutConstraint.activate([
            scrollView.topAnchor.constraint(equalTo: self.topAnchor),
            scrollView.leadingAnchor.constraint(equalTo: self.leadingAnchor),
            scrollView.bottomAnchor.constraint(equalTo: self.bottomAnchor),
            scrollView.trailingAnchor.constraint(equalTo: self.trailingAnchor)
        ])      
   }
}

After the declaration. I create an instance of RegisterUIView in my ViewController. And in the viewWillAppear and viewWillDisappear I used the variable hidesBarsOnSwipe, to hide the navigation bar. When I scroll down the bar hides, but when I scroll up the bar is not unhiding.

I read in other question here that I need to set the top constraint to the superview. How can is set the constraints to the superview?, when I try to set it the app crashes, and is obviously because there is no superview.

class RegisterViewController: UIViewController {

    private let registerView: RegisterUIView = {
        let view: RegisterUIView = RegisterUIView(frame: .zero)
        view.translatesAutoresizingMaskIntoConstraints = false
        return view
    }()

    override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)
        self.navigationController?.hidesBarsOnSwipe = true
    }
    
    override func viewWillDisappear(_ animated: Bool) {
        super.viewWillDisappear(animated)
        self.navigationController?.hidesBarsOnSwipe = false
    }

    override func viewDidLoad() {
        super.viewDidLoad()
        setupLayout()  
    }

    func setupLayout() {
        view.addSubview(registerView)
        NSLayoutConstraint.activate([
            registerView.topAnchor.constraint(equalTo: self.view.safeAreaLayoutGuide.topAnchor),
            registerView.leadingAnchor.constraint(equalTo: self.view.safeAreaLayoutGuide.leadingAnchor),
            registerView.bottomAnchor.constraint(equalTo: self.view.safeAreaLayoutGuide.bottomAnchor),
            registerView.trailingAnchor.constraint(equalTo: self.view.safeAreaLayoutGuide.trailingAnchor)
        ])
    }

Answer 1

Add the scroll view to the subview before adding constraints. Your app crashes because you're adding constraints to an object, not in the subview.

view.addSubview(scrollView)

add that line of code to the top of your setupLayout() function before your start adding constraints.