UIScrollView Content Offset on Initialization - ios

Question

We are currently working in an older codebase for our iOS application and are running into a weird bug where the UIScrollViews paging is not matching on the initialization but only once a user selects the button to change the view.
Expected Result:
The result we have:
Each ScrollView has three slides nested inside of them. We initialize the ScrollView like this:
override init(frame: CGRect) {
super.init(frame: frame)
self.commonInit()
}
required init?(coder aDecoder: NSCoder) {
super.init(coder: aDecoder)
self.commonInit()
}
private func commonInit() {
Bundle.main.loadNibNamed("DIScrollView", owner: self, options: nil)
contentView.frame = self.bounds
addSubview(contentView)
contentView.autoresizingMask = [.flexibleHeight,.flexibleWidth]
contentView.layer.borderColor = UIColor.white.cgColor
contentView.layer.borderWidth = 2.0
scrollView.delegate = self
setUpScrollViewer()
}
You can see we call to set up the ScrollView and that is done like this:
public func setUpScrollViewer() {
let slides = self.createSlides()
let defaultIndex = 1
scrollView.Initialize(slides: slides, scrollToIndex: defaultIndex)
pageControl.numberOfPages = slides.count
pageControl.currentPage = defaultIndex
}
Now that all the content is available for each slide, we want to handle the content and we do so with a ScrollView extension:
extension UIScrollView {
//this function adds slides to the scrollview and constraints to the subviews (slides)
//to ensure the subviews are properly sized
func Initialize(slides:[UIView], scrollToIndex:Int) {
//Take second slide to base size from
let frameWidth = slides[1].frame.size.width
self.contentSize = CGSize(width: frameWidth * CGFloat(slides.count), height: 1)
for i in 0 ..< slides.count {
//turn off auto contstraints. We will be setting our own
slides[i].translatesAutoresizingMaskIntoConstraints = false
self.addSubview(slides[i])
//pin the slide to the scrollviewers edges
if i == slides.startIndex {
slides[i].leadingAnchor.constraint(equalTo: self.leadingAnchor).isActive = true
} else { //pin each subsequent slides leading edge to the previous slides trailing anchor
slides[i].leadingAnchor.constraint(equalTo: slides[i - 1].trailingAnchor).isActive = true
}
slides[i].topAnchor.constraint(equalTo: self.topAnchor).isActive = true
slides[i].widthAnchor.constraint(equalTo: self.widthAnchor).isActive = true
slides[i].heightAnchor.constraint(equalTo: self.heightAnchor).isActive = true
}
//the last slides trailing needs to be pinned to the scrollviewers trailing.
slides.last?.trailingAnchor.constraint(equalTo: self.trailingAnchor).isActive = true
self.scrollRectToVisible(CGRect(x: frameWidth * CGFloat(scrollToIndex), y: 0, width: frameWidth, height: 1), animated: false)
}
}
I have tried manually setting contentOffset and nothing seems to be adjusting on the initialization. If the user selects the button it hides and then unhides it to display it properly with no logic adjusting this. Giving me the impression this issue is on the init.
Summary:
When the main view loads, the scrollView is showing me the first slide in the index when i need to be focused on the second slide. However if the user hides and then unhides the scrollView it works as intended.
How do i get the UIScrollView to actually load and initialize updating the scrollView to show the second slide and not initialize on the first slide?

Answer 1

Try explicitely running the scrollRectToVisible in the main thread using
DispatchQueue.main.async {
}

Answer 2

My guess is that all this code runs before the views are positioned by the layout system, and the first slide’s frame is the default 0 x 0 size. When the app returns to this view auto layout has figured out the size of this slide, so the calculation works.
Tap into the layout cycle to scroll to the right place after the layout. Maybe override viewDidLayoutSubviews() to check if it’s in the initial layout and then set the scroll position.

Answer 3

Use constraints for your contentView instead setting frame and autoresizingMask.
Call view.layoutIfNeeded() in the viewController before scrollRectToVisible or setContentOffset(I prefer the last)