UIKit Dynamics in Collection View always ends in chaos - ios

Question

When scrolling with my "springy flow layout" (meant to replicate the scrolling effect in Messages), the initial scroll works well, but repeated scrolling ends with all of the cells constantly bouncing all over the screen, and off the vertical axis.
I don't understand why the cells are moving off the vertical axis, since there is no movement applied in the horizontal axis. I'm simply applying this flow layout to my collection view which is currently only setup to create a bunch of dummy cells.
How do I prevent movement in the horizontal axis and how do I ensure that the cells always come to a rest eventually
class SpringyColumnLayout: UICollectionViewFlowLayout {
private lazy var dynamicAnimator = UIDynamicAnimator(collectionViewLayout: self)
override func prepare() {
super.prepare()
guard let cv = collectionView else { return }
let availableWidth = cv.bounds.inset(by: cv.layoutMargins).width
let minColumnWidth: CGFloat = 300
let maxNumberOfColumns = Int(availableWidth / minColumnWidth)
let cellWidth = (availableWidth / CGFloat(maxNumberOfColumns))
.rounded(.down)
self.itemSize = CGSize(width: cellWidth, height: 70)
self.sectionInset = UIEdgeInsets(
top: minimumInteritemSpacing,
left: 0,
bottom: 0,
right: 0
)
self.sectionInsetReference = .fromSafeArea
if dynamicAnimator.behaviors.isEmpty {
let contentSize = collectionViewContentSize
let contentBounds = CGRect(origin: .zero, size: contentSize)
guard let items = super.layoutAttributesForElements(in: contentBounds)
else { return }
for item in items {
let spring = UIAttachmentBehavior(
item: item,
attachedToAnchor: item.center
)
spring.length = 0
spring.damping = 0.8
spring.frequency = 1
self.dynamicAnimator.addBehavior(spring)
}
}
}
override func layoutAttributesForElements(
in rect: CGRect
) -> [UICollectionViewLayoutAttributes]? {
return dynamicAnimator.items(in: rect) as? [UICollectionViewLayoutAttributes]
}
override func layoutAttributesForItem(
at indexPath: IndexPath
) -> UICollectionViewLayoutAttributes? {
return dynamicAnimator.layoutAttributesForCell(at: indexPath)
}
override func shouldInvalidateLayout(
forBoundsChange newBounds: CGRect
) -> Bool {
let scrollView = self.collectionView!
let scrollDelta = newBounds.origin.y - scrollView.bounds.origin.y
let touchLocation = scrollView.panGestureRecognizer
.location(in: scrollView)
for case let spring as UIAttachmentBehavior in dynamicAnimator.behaviors {
let anchorPoint = spring.anchorPoint
let yDistanceFromTouch = abs(touchLocation.y - anchorPoint.y)
let xDistanceFromTouch = abs(touchLocation.x - anchorPoint.x)
let scrollResistance = (yDistanceFromTouch + xDistanceFromTouch) / 1500
let item = spring.items.first!
var center = item.center
if scrollDelta < 0 {
center.y += max(scrollDelta, scrollDelta * scrollResistance)
} else {
center.y += min(scrollDelta, scrollDelta * scrollResistance)
}
item.center = center
dynamicAnimator.updateItem(usingCurrentState: item)
}
return false
}
}