Auto Layout constraint breaks when set active - swift

Question

I have a set of AL constraints positioning a child vc that has two positions, expanded and collapsed.
I found that when I add the collapsed constraint, a top anchor to bottom anchor constraint with a constant, when the vc is first created, there seems to be additional spacing when I activate it. Seemingly because the actual height isn't available at the time.
When I add the constraint in viewDidLayoutSubviews there additional spacing is gone and the constraint behaves properly. Except the issue that now when I switch between the constraints in an animation, I cannot deactivate the collapsed constraint as I switch to the expanded constraint and the constraint breaks. Possibly because viewDidLayoutSubviews is called throughout the transition animation.
Here's an abstract of vc setup.
var foregroundExpandedConstraint: NSLayoutConstraint!
var foregroundCollapsedConstraint: NSLayoutConstraint!
var foregroundViewController: UIViewController? {
didSet {
setupforegroundViewController(foregroundViewController: foregroundViewController!)
}
}
func setupforegroundViewController(foregroundViewController: UIViewController) {
addChildViewController(foregroundViewController)
foregroundViewController.didMove(toParentViewController: self)
guard let foregroundView = foregroundViewController.view else { return }
foregroundView.translatesAutoresizingMaskIntoConstraints = false
view.addSubview(foregroundView)
foregroundExpandedConstraint = foregroundView.topAnchor.constraint(equalTo: view.safeAreaLayoutGuide.topAnchor, constant: 15)
let height = view.safeAreaLayoutGuide.layoutFrame.height - 50 - 15
let cellHeight = ((height) / 6)
foregroundCollapsedConstraint = NSLayoutConstraint(item: foregroundView, attribute: .top, relatedBy: .equal, toItem: view.safeAreaLayoutGuide, attribute: .bottom, multiplier: 1, constant: (-cellHeight) * 2 - 50)
let foregroundViewControllerViewConstraints = [
foregroundView.leadingAnchor.constraint(equalTo: view.leadingAnchor),
foregroundView.trailingAnchor.constraint(equalTo: view.trailingAnchor),
foregroundView.heightAnchor.constraint(equalTo: view.safeAreaLayoutGuide.heightAnchor, constant: -50 - 15),
foregroundExpandedConstraint!
]
NSLayoutConstraint.activate(foregroundViewControllerViewConstraints)
}
And here the animations are preformed using UIViewPropertyAnimator.
func animateTransitionIfNeeded(state: ForegroundState, duration: TimeInterval) {
let containerFrameAnimator = UIViewPropertyAnimator(duration: duration, dampingRatio: 1) {
[unowned self] in
switch state {
case .expanded:
self.foregroundCollapsedConstraint?.isActive = false
self.foregroundExpandedConstraint?.isActive = true
self.view.layoutIfNeeded()
case .collapsed:
self.foregroundExpandedConstraint?.isActive = false
self.foregroundCollapsedConstraint?.isActive = true
self.view.layoutIfNeeded()
}
}
containerFrameAnimator.addCompletion { [weak self] (position) in
if position == .start {
switch state {
case .collapsed:
self?.foregroundCollapsedConstraint?.isActive = false
self?.foregroundExpandedConstraint?.isActive = true
self?.foregroundIsExpanded = true
self?.view.layoutIfNeeded()
case .expanded:
self?.foregroundExpandedConstraint?.isActive = false
self?.foregroundCollapsedConstraint?.isActive = true
self?.foregroundIsExpanded = false
self?.view.layoutIfNeeded()
}
} else if position == .end {
switch state {
case .collapsed:
self?.foregroundExpandedConstraint?.isActive = false
self?.foregroundCollapsedConstraint?.isActive = true
self?.foregroundIsExpanded = false
case .expanded:
self?.foregroundExpandedConstraint?.isActive = false
self?.foregroundCollapsedConstraint?.isActive = true
self?.foregroundIsExpanded = true
}
}
self?.runningAnimations.removeAll()
}
Again to reiterate, when I use the following code, setting the constraint as the vc is added to the view hierarchy, it doesn't layout properly. Checking the constraints I see they change after view did layout subviews is called. Each constraint changes appropriately except for the collapsed constraint.
When I add the collapsed constraint in view did layout subviews it behaves properly however I am unable to deactivate it going forwards and the constraint breaks.
override func viewDidLayoutSubviews() {
super.viewDidLayoutSubviews()
let height = view.safeAreaLayoutGuide.layoutFrame.height - 50 - 15
let cellHeight = ((height) / 6)
if let v = foregroundViewController?.view {
foregroundCollapsedConstraint = NSLayoutConstraint(item: v, attribute: .top, relatedBy: .equal, toItem: view.safeAreaLayoutGuide, attribute: .bottom, multiplier: 1, constant: (-cellHeight) * 2 - 50)
}
}
Edit: I've created a repo demonstrating the issue: https://github.com/louiss98/UIViewPropertyAnimator-Layout-Test
Any suggestions?

Answer 1

You can eliminate the "broken" constraint by changing the constant instead of creating a new constraint.
In your viewDidLayoutSubviews() func,
change:
override func viewDidLayoutSubviews() {
super.viewDidLayoutSubviews()
let height = view.safeAreaLayoutGuide.layoutFrame.height - 50 - 15
let cellHeight = ((height) / 6)
foregroundCollapsedConstraint = NSLayoutConstraint(item: testViewController.view, attribute: .top, relatedBy: .equal, toItem: view.safeAreaLayoutGuide, attribute: .bottom, multiplier: 1, constant: (-cellHeight) * 2 - 50)
}
to:
override func viewDidLayoutSubviews() {
super.viewDidLayoutSubviews()
let height = view.safeAreaLayoutGuide.layoutFrame.height - 50 - 15
let cellHeight = ((height) / 6)
foregroundCollapsedConstraint.constant = (-cellHeight) * 2 - 50
}