Get updated IndexPath for visible item after UICollectionView has scrolled - ios

Question

I have a collectionView with different periods (day to year) for user to choose. Each item's size is equal to the size of collectionView itself.
#IBOutlet weak var periodCollectionView: UICollectionView!
#IBOutlet weak var itemLabel: UILabel!
let collectionValues = ["Day", "Week", "Month", "Year"]
var currentVisibleItem: Int?
override func viewDidLoad() {
super.viewDidLoad()
}
func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
return 4
}
func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
guard let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "PeriodName", for: indexPath) as? DisplayPeriodCollectionViewCell
else {
fatalError("Unable to cast collection view cell as DisplayPeriodCollectionViewCell")
}
cell.periodName.text = collectionValues[indexPath.item]
return cell
}
func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
let height = collectionView.frame.height
let width = collectionView.frame.width
return CGSize(width: width, height: height)
}
What i am trying to do is get an info label to display the index of current visible item after collectionView has scrolled. Her is the code:
func scrollViewDidEndDecelerating(_ scrollView: UIScrollView) {
let centerPoint = CGPoint(x: periodCollectionView.center.x + periodCollectionView.contentOffset.x, y: 1.0)
print("centerPoint: \(centerPoint)")
if let currentItemIndexPath = periodCollectionView.indexPathForItem(at: centerPoint) {
currentVisibleItem = currentItemIndexPath.item
print("index of current item: \(currentVisibleItem!)")
itemLabel.text = "\(currentVisibleItem!)"
} else {
print("could not get index of current item...")
}
}
I've added print statements to check if values are defined correctly at runtime, and they show everything is fine. The problem is that label does not always update its text after scrolling ends, but instead waits for new scrolling to occur:
I can't figure out why this is happening - setting a breakpoint at scrollViewDidEndDecelerating shows that all code gets executed, so what's wrong with the label text?
UPD. Not-updating label text appears to be a Simulator issue - when running on device, label updates correctly.

Answer 1

Try this
override func scrollViewDidEndDecelerating(_ scrollView: UIScrollView) {
let currentIndex = scrollView.contentOffset.x / CGFloat((itemWidth + interitemSpacing / 2))
print(currentIndex)
}

Answer 2

Please try the following code.
I used this many times in my real projects and this works fine.
func scrollViewWillEndDragging(_ scrollView: UIScrollView, withVelocity velocity: CGPoint, targetContentOffset: UnsafeMutablePointer<CGPoint>) {
let visibleIndex = Int(targetContentOffset.pointee.x / collectionView.frame.width)
print(visibleIndex)
}

Answer 3

In your case you can use:
func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell
because each item is at the same size as your collection view.
if you still want to use scroll view delegate you should write your page calculation in the following delegate functions:
func scrollViewDidEndDragging(UIScrollView, willDecelerate: Bool)
func scrollViewDidEndDecelerating(UIScrollView)
func scrollViewDidEndScrollingAnimation(UIScrollView)