Swift - constraint animation - incorrect subviews animating - ios

Question

I have extended UIView:
class A: UIView {
private var _height: NSLayoutConstraint!
private var _centerLabel: UILabel = {
let lb = UILabel()
lb.text = "Some text"
lb.adjustsFontSizeToFitWidth = true
lb.textAlignment = .center
lb.translatesAutoresizingMaskIntoConstraints = false
return lb
}()
func open() {
self._height.constant = 50
self.superview?.layoutIfNeeded()
}
func close() {
UIView.animate(withDuration: 1, delay: 1, options: .curveLinear, animations: {
self._height.constant = 0
self.superview?.layoutIfNeeded()
}, completion: nil)
}
override init(frame: CGRect) {
super.init(frame: frame)
self.addSubview(_centerLabel)
_height = self.heightAnchor.constraint(equalToConstant: 0)
NSLayoutConstraint.activate([
_centerLabel.leadingAnchor.constraint(equalTo: self.leadingAnchor, constant: 16),
_centerLabel.trailingAnchor.constraint(equalTo: self.trailingAnchor, constant: -16),
_centerLabel.topAnchor.constraint(lessThanOrEqualTo: self.topAnchor, constant: 16),
_centerLabel.bottomAnchor.constraint(greaterThanOrEqualTo: self.bottomAnchor, constant: -16),
_height
])
}
}
MainViewController:
class MainViewController: UIViewController {
#IBOutlet weak var a: A!
override func viewDidLoad() {
super.viewDidLoad()
  }
#IBAction func open(_ sender: Any) {
a.open()
}
#IBAction func close(_ sender: Any) {
a.close()
}
}
If I call close(), instance of UIView A animates correctly, all other constraint attached to it as well. However, content of A (_centerLabel) disappears immediatelly, its height is not animated at all. Why?