How to Track UICollectionView index - ios

Question

I want a variable in my code that keeps track of the index of my UICollectionView, but I can't get it to work. After some troubleshooting, I've boiled down the code to the following, which if pasted into an empty viewController should work since no storyboard is involved. The animated gif illustrates the problem. Initially my variable "selectedItem" is equal to the UICollectionView Cell text which reflects the data = [0,1,2,3], but then when I swipe right, it immediately becomes off by 1. Then it stays off by 1 until at the last cell where it matches again. The pattern repeats when going in reverse. Thanks for any help --
import UIKit
class CodeCollView2: UIViewController, UICollectionViewDataSource,UICollectionViewDelegate, UICollectionViewDelegateFlowLayout {
var data = [0,1,2,3] //["0", "1", "2", "3" ]
let cellId = "cellId2"
var selectedItem = 0
lazy var cView: UICollectionView = {
let layout = UICollectionViewFlowLayout()
layout.scrollDirection = .horizontal
layout.minimumLineSpacing = 0
let cv = UICollectionView(frame: .zero, collectionViewLayout: layout)
cv.isPagingEnabled = true
cv.dataSource = self
cv.delegate = self
return cv
}()
var indexLabel: UILabel = {
let label = UILabel()
label.text = ""
label.font = UIFont.systemFont(ofSize: 30)
return label
}()
override func viewDidLoad() {
super.viewDidLoad()
setupViews()
}
func setupViews() {
cView.register(CCell2.self, forCellWithReuseIdentifier: cellId)
view.addSubview(cView)
cView.translatesAutoresizingMaskIntoConstraints = false
cView.centerYAnchor.constraint(equalTo: view.centerYAnchor).isActive = true
cView.leadingAnchor.constraint(equalTo: view.leadingAnchor).isActive = true
cView.trailingAnchor.constraint(equalTo: view.trailingAnchor).isActive = true
cView.heightAnchor.constraint(equalToConstant: 200).isActive = true
view.addSubview(indexLabel)
indexLabel.translatesAutoresizingMaskIntoConstraints = false
indexLabel.bottomAnchor.constraint(equalTo: cView.topAnchor).isActive = true
indexLabel.centerXAnchor.constraint(equalTo: cView.centerXAnchor).isActive = true
}
func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
return data.count
}
func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
let cell = collectionView.dequeueReusableCell(withReuseIdentifier: cellId, for: indexPath) as! CCell2
selectedItem = indexPath.item
indexLabel.text = "seletedItem = \(selectedItem)"
cell.itemValue = data[selectedItem]
return cell
}
func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
return collectionView.frame.size
}
}
//============== CVCell ==================
class CCell2: UICollectionViewCell {
var itemValue: Int? {
didSet {
if let val = itemValue {
itemLabel.text = "\(val)"
}
}
}
var itemLabel: UILabel = {
let label = UILabel()
label.font = UIFont.systemFont(ofSize: 100)
return label
}()
override init(frame: CGRect) {
super.init(frame: frame)
backgroundColor = .lightGray
addSubview(itemLabel)
itemLabel.translatesAutoresizingMaskIntoConstraints = false
itemLabel.centerYAnchor.constraint(equalTo: centerYAnchor).isActive = true
itemLabel.centerXAnchor.constraint(equalTo: centerXAnchor).isActive = true
}
required init?(coder aDecoder: NSCoder) {
fatalError("init(coder:) has not been implemented")
}
}

Answer 1

As Nikita's answer mentions, cellForItemAt is called when a cell is going to be shown, even if you only see a bit of it and go back to the previous one, so you shouldn't use to decided what cell is at the centre.
scrollViewDidScroll is the right way of tracking which cell you have at the centre, and you can print what index you are on with something like this:
func scrollViewDidScroll(_ scrollView:UIScrollView)
{
let midX:CGFloat = scrollView.bounds.midX
let midY:CGFloat = scrollView.bounds.midY
let point:CGPoint = CGPoint(x:midX, y:midY)
guard
let indexPath:IndexPath = collectionView.indexPathForItem(at:point)
else
{
return
}
let currentPage:Int = indexPath.item
indexLabel.text = "seletedItem = \(currentPage)"
}

Answer 2

Tracking the selected item in the 'cellForItemAt' is not a good idea. I would suggest you to track it in the scrollViewDidScroll delegate method of the UIScrollViewDelegate.
Something like this should work:
func scrollViewDidScroll(_ scrollView: UIScrollView) {
let currentPage = cView.contentOffset.x / self.view.bounds.width
}