Error when adding constraints in XCode Playground

Question

I have this class in my playground, that has a UIButton in it. Everything is fine, until I try adding constraints with a second item (width/height constraints work).

class ButtonView: UIView {
    required init?(coder aDecoder: NSCoder) { ... }

    override init(frame: CGRect) {
        super.init(frame: frame)
        //ButtonView Setup
        ...
        let button = Button()
        //Button Constraints
        button.translatesAutoresizingMaskIntoConstraints = false
        button.centerXAnchor.constraint(equalTo: self.centerXAnchor).isActive = true //Error here
        self.layoutIfNeeded()
        self.addSubview(button)

    }

    var button: Button?

}

self is the right item to work with here, right? It is a UIView. So I don't see the problem.

Answer 1

Item should be added before adding the constraints

self.addSubview(button)
button.centerXAnchor.constraint(equalTo: self.centerXAnchor).isActive = true