Animate constraints change UIViewController - ios

Question

I have "error" view, that is placed above navigation bar and hidden. When error occured, i want that view to smoothly show from top. I tried:
class AuthViewController: UIViewController {
let error: ErrorView = {
let error = ErrorView()
error.setup()
return error
}()
var topAnchor: NSLayoutConstraint!
var botAnchor: NSLayoutConstraint!
override func viewDidLoad() {
setupErrorView()
}
private func setupErrorView(){
view.addSubview(error)
botAnchor = error.bottomAnchor.constraint(equalTo: view.topAnchor)
botAnchor.isActive = true
topAnchor = error.topAnchor.constraint(equalTo: view.topAnchor, constant: CGFloat(Offsets.navigationAndStatusBarHeight))
topAnchor.isActive = false
error.leftAnchor.constraint(equalTo: view.leftAnchor).isActive = true
error.rightAnchor.constraint(equalTo: view.rightAnchor).isActive = true
}
func showError(_ text: String){
UIView.animate(withDuration: 2.0) {[weak self] in
guard let weakSelf = self else { return }
print("attempt to animate")
weakSelf.error.show(text)
weakSelf.botAnchor.isActive = false
weakSelf.topAnchor.isActive = true
weakSelf.view.setNeedsLayout()
}
}
}
class ErrorView: UIView {
private var label: UILabel = {
return LabelSL.regular()
}()
fileprivate func setup(){
translatesAutoresizingMaskIntoConstraints = false
backgroundColor = Theme.Color.orange.value
addSubview(label)
}
fileprivate func show(_ text: String){
let sideOffset: CGFloat = 10
let verticalOffset: CGFloat = 10
label.text = text
label.topAnchor.constraint(equalTo: topAnchor, constant: verticalOffset).isActive = true
label.leftAnchor.constraint(equalTo: leftAnchor, constant: sideOffset).isActive = true
label.rightAnchor.constraint(equalTo: rightAnchor, constant: -sideOffset).isActive = true
label.bottomAnchor.constraint(equalTo: bottomAnchor, constant: -verticalOffset).isActive = true
}
}
Animation should be done when func showError(_ text: String){ method called, but it's not. View just appear instantly.

Answer 1

You're trying to animate constraints in wrong way. You should set constraints outside of animation block and only layoutIfNeeded in animation:
func showError(_ text: String){
botAnchor.isActive = false
topAnchor.isActive = true
error.show(text)
UIView.animate(withDuration: 2.0) {
self.view.layoutIfNeeded()
}
}