How to position a subView below a subView with dynamic height? - ios

Question

I have the following UIView:
It is a UIView which contains three subviews. A regular UILabel (Hello World) at the top, a custom UIViewController (CategoryList) which contains a CollectionView with buttons (alpha,beta, ...) and another custom UIViewController with just a label (SALUT).
I do auto-layout programmatically and position SALUT (var sCtrl) below the CategoryList (var catList) with
sCtrl.view.topAnchor.constraint(equalTo: catList.view.bottomAnchor).isActive = true
This results in the picture above, where SALUT is not positioned below the category list as I would like it to be. I sort of understand why since when I set the constraint the buttons are not yet laid out properly in the CollectionView and thus the bottom anchor of the catList is not set.
In the CategoryList:UIVIewController I have the following in order to get the collection view to get the correct height.
override public func viewDidLayoutSubviews() {
super.viewDidLayoutSubviews()
heightConstraint = collectionView.heightAnchor.constraint(equalToConstant: collectionView.collectionViewLayout.collectionViewContentSize.height)
self.view.addConstraint(heightConstraint)
}
This works but since viewDidLayoutSubviews() is called after the constraint is set on SALUT the SALUT position is wrong. How can I get it right in an easy way? (I guess I could make a controller for my main view and check there when subviews are laid out and then update subview positions but this seems overly complicated to me).
The full code for the main view is below so you can see the layout positioning code. If needed I can also provide the subviews code but I suppose it shouldn't be needed since they should be considered black boxes...
class MyView: UIView {
let label = UILabel()
var sCtrl : SubController
var catList : CategoryList
var topConstraint = NSLayoutConstraint()
override init(frame: CGRect) {
sCtrl = SubController()
catList = CategoryList()
super.init(frame: frame)
setup()
}
private func setup() {
self.backgroundColor = UIColor.green
label.translatesAutoresizingMaskIntoConstraints = false
label.backgroundColor = UIColor.yellow
label.text = "Hello world"
label.textAlignment = .center
self.addSubview(label)
catList.view.translatesAutoresizingMaskIntoConstraints = false
self.addSubview(catList.view)
sCtrl.view.translatesAutoresizingMaskIntoConstraints = false
self.addSubview(sCtrl.view)
let margins = self.layoutMarginsGuide
label.leadingAnchor.constraint(equalTo: margins.leadingAnchor, constant: 0).isActive = true
label.trailingAnchor.constraint(equalTo: margins.trailingAnchor, constant: 0).isActive = true
label.topAnchor.constraint(equalTo: margins.topAnchor, constant: 0).isActive = true
label.heightAnchor.constraint(equalToConstant: 100.0).isActive=true
catList.view.leadingAnchor.constraint(equalTo: margins.leadingAnchor, constant: 0).isActive = true
catList.view.topAnchor.constraint(equalTo: label.bottomAnchor, constant: 1.0).isActive = true
catList.view.widthAnchor.constraint(equalTo: margins.widthAnchor, multiplier: 1.0).isActive = true
catList.view.trailingAnchor.constraint(equalTo: margins.trailingAnchor, constant:0).isActive = true
sCtrl.view.topAnchor.constraint(equalTo: catList.view.bottomAnchor).isActive = true
}
required init?(coder aDecoder: NSCoder) {
fatalError("init(coder:) has not been implemented")
}
}
let v = MyView(frame: CGRect(x: 0, y: 0, width: 400, height: 600))

Answer 1

Ok, I found the issue. I had missed to add the height constraint to the CategoryList's own view. Adding this it works just fine.
override public func viewDidLayoutSubviews() {
super.viewDidLayoutSubviews()
heightConstraint = collectionView.heightAnchor.constraint(equalToConstant: collectionView.collectionViewLayout.collectionViewContentSize.height)
self.view.addConstraint(heightConstraint)
//THE MISSING LINE!!
self.view.heightAnchor.constraint(equalTo: self.collectionView.heightAnchor, constant: 0).isActive = true
}