Delete items in collection view with custom layout

I edited with <> to make it more readable

import UIKit

class testVCViewController: UIViewController, UICollectionViewDelegate, UICollectionViewDataSource, UICollectionViewDelegateFlowLayout, ImagesCVLayoutDelegate {

   @IBOutlet weak var cv: UICollectionView!
   @IBOutlet weak var btn: UIButton!

   @IBAction func btn(_ sender: UIButton) {
      items.remove(at: 0)
      cv.deleteItems(at: [IndexPath(row: 0, section: 0)]) // THIS IS WHERE THE WARNINGS AND ERROR OCCUR.
      cv.reloadData()
   }

   var items = [1,2,3,4,5,6,7,8,9] // Data source.

   func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
      return items.count
   }

   func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
      let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "cell", for: indexPath) as! zCollectionViewCell
      cell.lbl.text = "\(items[indexPath.row])"
      return cell
   }

   func numberOfSections(in collectionView: UICollectionView) -> Int {
      return 1
   }

   func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
   
      let h = cv.frame.height
      var w = cv.frame.width
   
      if UIDevice.current.orientation.isLandscape == true {
         w -= 56 // I.e. 7 gaps between 8 images.
         w = w / 8
      } else if UIDevice.current.orientation.isPortrait == true || UIDevice.current.orientation.isFlat == true {
         w -= 24 // I.e. 3 gaps between 4 images.
         w = w / 4
      }
   
      return CGSize(width: w, height: h)
   }

   func collectionView(_ collectionView:UICollectionView, heightForPhotoAtIndexPath indexPath:IndexPath) -> CGFloat{
      return 68
   }

   override func viewDidLoad() {
     super.viewDidLoad()

      cv.delegate = self
      cv.dataSource = self
   }
}

// LAYOUT CODE:


import UIKit

protocol ImagesCVLayoutDelegate: class {
   // 1. Method to ask the delegate for the height of the image
   func collectionView(_ collectionView:UICollectionView, heightForPhotoAtIndexPath indexPath:IndexPath) -> CGFloat
 
}

class ImagesCVLayout: UICollectionViewLayout {
   //1.  Layout Delegate
   weak var delegate: ImagesCVLayoutDelegate!
    
      //2. Configurable properties
      fileprivate var numberOfColumns = 4
      fileprivate var cellPadding: CGFloat = 2 // 6
    
      //3. Array to keep a cache of attributes.
      fileprivate var cache = [UICollectionViewLayoutAttributes]()
    
      //4. Content height and size
      fileprivate var contentHeight: CGFloat = 0
    
      fileprivate var contentWidth: CGFloat {
         guard let collectionView = collectionView else {
            return 0
         }
         let insets = collectionView.contentInset
         return collectionView.bounds.width - (insets.left + insets.right)
      }
    
      override var collectionViewContentSize: CGSize {
         return CGSize(width: contentWidth, height: contentHeight)
      }
    
      override func prepare() {
         // 1. Only calculate once
         guard cache.isEmpty == true, let collectionView = collectionView else {
            return
         }
         // 2. Pre-Calculates the X Offset for every column and adds an array to increment the currently max Y Offset for each column
         let columnWidth = contentWidth / CGFloat(numberOfColumns)
         var xOffset = [CGFloat]()
         for column in 0 ..< numberOfColumns {
            xOffset.append(CGFloat(column) * columnWidth)
         }
         var column = 0
         var yOffset = [CGFloat](repeating: 0, count: numberOfColumns)
       
         // 3. Iterates through the list of items in the first section
         for item in 0 ..< collectionView.numberOfItems(inSection: 0) {
          
            let indexPath = IndexPath(item: item, section: 0)
          
            // 4. Asks the delegate for the height of the picture and the annotation and calculates the cell frame.
            //let photoHeight = delegate.collectionView(collectionView, heightForPhotoAtIndexPath: indexPath)
            let photoHeight = CGFloat(100) // BODGED THIS AS I'M JUST USING TEXT IN THIS EXAMPLE.
            let height = cellPadding * 2 + photoHeight
            let frame = CGRect(x: xOffset[column], y: yOffset[column], width: columnWidth, height: height)
            let insetFrame = frame.insetBy(dx: cellPadding, dy: cellPadding)
          
            // 5. Creates an UICollectionViewLayoutItem with the frame and add it to the cache
            let attributes = UICollectionViewLayoutAttributes(forCellWith: indexPath)
            attributes.frame = insetFrame
            cache.append(attributes)
          
            // 6. Updates the collection view content height
            contentHeight = max(contentHeight, frame.maxY)
            yOffset[column] = yOffset[column] + height
          
            column = column < (numberOfColumns - 1) ? (column + 1) : 0
         }
      }
    
      override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
       
         var visibleLayoutAttributes = [UICollectionViewLayoutAttributes]()
       
         // Loop through the cache and look for items in the rect
         for attributes in cache {
            if attributes.frame.intersects(rect) {
               visibleLayoutAttributes.append(attributes)
            }
         }
         return visibleLayoutAttributes
      }
    
      override func layoutAttributesForItem(at indexPath: IndexPath) -> UICollectionViewLayoutAttributes? {
         return cache[indexPath.item]
}

How is cache updated when you remove an item ?

With line 97, if I understand well, as soon as cache is not empty, you exit.

It could be worth adding some log:

line 137,

print(#function, cache)

Before line 148

print(#function, indexPath.item, cache[indexPath.item])

Hence, you may have too many items in cache, or not consistent items, which could confuse layout somewhere.