Determining when a view created programmatically did appear on screen - swift

Question

I have a textField which, when tapped, pops up a vertical stackView created programatically. The stackView is a child of the textField. textField.addSubview(stackView) places the stackView's origin at the textField's origin. I wish to move the stackView's origin up vertically by an amount equal to the height of the stackView. If I do:
func textFieldDidBeginEditing(_ textField: UITextField) {
stackView = UIStackView() // var stackView: UIStackView!
stackView.axis = .vertical
stackView.alignment = .leading
stackView.distribution = .fillEqually
stackView.translatesAutoresizingMaskIntoConstraints = false
stackView.spacing = 0
stackView.backgroundColor = .white
stackView.layer.borderWidth = 1
stackView.layer.borderColor = UIColor.gray.cgColor
for title in titleList {
stackView.addArrangedSubview(createButton(title))
}
textField.addSubview(stackView)
stackView.transform = CGAffineTransform(translationX: 0, y: -stackView.frame.height)
}
It doesn't work. The stackView's origin remains at the textField's origin. But if I use a constant:
textField.addSubview(stackView)
stackView.transform = CGAffineTransform(translationX: 0, y: -144)
it works. I have confirmed through debugging that stackView.frame.height is 0 at the time the CGAffineTransform is performed. If I check its value after it has exited the function, I do see that its height is 144. That means the stackView hasn't actually appeared yet within the function.
Is there a way to catch when the programmatically-created stackView appears (sort of like the way viewDidAppear(_:) works) so that I can then set its position on the screen?

Answer 1

I don't think there is a canonical way to know when an individual view has appeared on screen. You might be able to hook into the draw method (which is hacky), but a stack view doesn't draw anything itself, so I don't think that works.
You could probably use UIViewController.viewDidLayoutSubviews, but you might need to add the stack view to the root view for that to work.
In any case, adding the UIStackView as a subview of the UITextField is bad practice. You probably want to add it to the parent view instead. And yes, using AutoLayout is the way to go here. Let the framework handle the layout, don't try to do it using transforms.