You should activate the constraints array using NSLayoutConstraint.activate().
func configureContents() { title.translatesAutoresizingMaskIntoConstraints = false
contentView.addSubview(title)
NSLayoutConstraint.activate([ title.heightAnchor.constraint(equalToConstant: 30), title.leadingAnchor.constraint(equalTo: contentView.layoutMarginsGuide.leadingAnchor), title.trailingAnchor.constraint(equalTo: contentView.layoutMarginsGuide.trailingAnchor), title.centerYAnchor.constraint(equalTo: contentView.centerYAnchor) ]) }