UIScrollView behaving weird when updating constraints subview - ios

Question

I made an UIScrollView in a XIB for my onboarding. The UIScrollView has 3 onboarding views. Long story short:
This works perfect. However I want the top left and right buttons (Overslaan - Volgende) to animate up / off the screen when the third/last page is on screen. My UIScrollView starts behaving weird when I animate the buttons off:
This is the code im using:
func scrollViewWillEndDragging(_ scrollView: UIScrollView, withVelocity velocity: CGPoint, targetContentOffset: UnsafeMutablePointer<CGPoint>) {
let pageIndex = Int(targetContentOffset.pointee.x / self.frame.width)
pageControl.currentPage = pageIndex
if stepViews[pageIndex] is OnboardingLoginView {
moveControlConstraintsOffScreen()
} else {
moveControlConstraintsOnScreen()
}
UIView.animate(withDuration: 0.5, delay: 0, usingSpringWithDamping: 1, initialSpringVelocity: 1, options: .curveEaseOut, animations: {
self.layoutIfNeeded()
}, completion: nil)
}
I debugged the code and it turns out that setting a new constant for the constraints causes the issue, regardless of the animation block. How do I make the buttons move up/off the screen without my scrollView behaving weird?

Answer 1

It looks like triggering a layout pass is interfering with your scroll view positioning. You could implement func scrollViewDidScroll(_ scrollView: UIScrollView) and try to update the button view on a per-frame basis, without changing Auto Layout constraints. I usually use the transform property for frame changes outside of Auto Layout, since any changes to frame or bounds are overwritten during the next layout pass.
func scrollViewDidScroll(_ scrollView: UIScrollView) {
guard mustBeOnLastPage else {
buttonOne.tranform = .identity
buttonTwo.tranform = .identity
return
}
let offset = scrollView.contentOffset.x
buttonOne.tranform = .init(translationX: 0, y: offset)
buttonTwo.tranform = .init(translationX: 0, y: offset)
}
Old answer
This interpretation is a bit of tangent of what you're asking for.
Since it looks like you're using the scroll view in a paging context, I would approach this problem by using UIPageViewController. Since UIPageViewController uses UIScrollView internally, you can observe the contentOffset of the last view in the scroll view to determine how far along the page has scrolled.
Yes, this involves looking inside the view hierarchy, but Apple hasn't changed it for half a decade so you should be safe. Coincidentally, I actually implemented this approach last week and it works like a charm.
If you're interested, I can further expand on this topic. The post that pointed me in the right direction can be found here.

Answer 2

Here is what i do
I can't post image，you can look at this http://i.imgur.com/U7FHoMu.gif
and this is the code
var centerYConstraint: Constraint!
func setupConstraint() {
fadeView.snp.makeConstraints { make in
make.centerX.equalToSuperview()
centerYConstraint = make.centerY.equalToSuperview().constraint
make.size.equalTo(CGSize(width: 50, height: 50))
}
}
func scrollViewWillEndDragging(_ scrollView: UIScrollView, withVelocity velocity: CGPoint,targetContentOffset: UnsafeMutablePointer<CGPoint>) {
let pageIndex = Int(targetContentOffset.pointee.x / self.view.frame.width)
if pageIndex != 1 {
centerYConstraint.update(offset: self.view.frame.height)
} else {
centerYConstraint.update(offset: 0)
}
UIView.animate(withDuration: 0.5, delay: 0, usingSpringWithDamping: 1, initialSpringVelocity: 1, options: .curveEaseOut, animations: {
self.view.layoutIfNeeded()
}, completion: nil)
}

Answer 3

Based on #CloakedEddy's answer I made 2 changes:
1: It seems layoutSubviews is responsible for the weird behaviour. To fix this I prevent the scrollView from calling layoutSubviews all the time:
override func layoutSubviews() {
super.layoutSubviews()
if !didLayoutSubviews {
for index in 0..<stepViews.count {
let page: UIView = stepViews[index]
let xPosition = scrollView.frame.width * CGFloat(index)
page.frame = CGRect(x: xPosition, y: 0, width: scrollView.bounds.width, height: scrollView.frame.height)
scrollView.contentSize.width = scrollView.frame.width * CGFloat(index + 1)
}
didLayoutSubviews = true
}
}
2: If you want to update your views for device orientations:
override func viewWillTransition(to size: CGSize, with coordinator: UIViewControllerTransitionCoordinator) {
super.viewWillTransition(to: size, with: coordinator)
onboardingView.didLayoutSubviews = false
onboardingView.setNeedsLayout()
}