prepare() not called in my custom collectionView layout

Question

I am trying to setup a custom collectionView layout and feel like I may be missing a step.

I created the collectionView, defined constraints, changed the layout from flow to custom and picked my custom class.

Now its not working and nothing is showing up so I am trying to debug the class method but I can't get any print statements to work because no function seems to be called. What step am I missing in setting this up? Here is the class:

import UIKit

class CustomCollectionViewLayout: UICollectionViewLayout {

    private var cache = [IndexPath: UICollectionViewLayoutAttributes]()
    private var contentWidth = CGFloat()
    private var visibleLayoutAttributes = [UICollectionViewLayoutAttributes]()
    private var oldBounds = CGRect.zero
    private var collectionViewStartY: CGFloat {
        guard let collectionView = collectionView else {
            return 0
        }
        return collectionView.bounds.minY
    }
    private var collectionViewHeight: CGFloat {
        return collectionView!.frame.height
    }
    override public var collectionViewContentSize: CGSize {
        return CGSize(width: contentWidth, height: collectionViewHeight)
    }

    override public func prepare() {
        print("calling prepare") //This is what's not printing
        guard let collectionView = collectionView,
            cache.isEmpty else {
                return
        }

        updateInsets()
        collectionView.decelerationRate = .fast
        cache.removeAll(keepingCapacity: true)
        cache = [IndexPath: UICollectionViewLayoutAttributes]()
        oldBounds = collectionView.bounds
        var xOffset: CGFloat = 0
        var cellWidth: CGFloat = 5

        for item in 0 ..< collectionView.numberOfItems(inSection: 0) {
            let cellIndexPath = IndexPath(item: item, section: 0)
            let cellattributes = UICollectionViewLayoutAttributes(forCellWith: cellIndexPath)
            cellattributes.frame = CGRect(x: xOffset, y: 0, width: cellWidth, height: collectionViewHeight)
            xOffset = xOffset + cellWidth
            contentWidth = max(contentWidth,xOffset)
            cache[cellIndexPath] = cellattributes
        }
    }

    override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
        visibleLayoutAttributes.removeAll(keepingCapacity: true)
        print("layoutattributes called")

        for (_, attributes) in cache {
            visibleLayoutAttributes.append(self.shiftedAttributes(from: attributes))
        }
        return visibleLayoutAttributes
    }

    override func layoutAttributesForItem(at indexPath: IndexPath) -> UICollectionViewLayoutAttributes? {
        guard let attributes = cache[indexPath] else { fatalError("No attributes cached") }
        print("layoutattributes called")
        return shiftedAttributes(from: attributes)
    }
    override public func shouldInvalidateLayout(forBoundsChange newBounds: CGRect) -> Bool {
        if oldBounds.size != newBounds.size {
            cache.removeAll(keepingCapacity: true)
        }
        return true
    }

    override func invalidateLayout(with context: UICollectionViewLayoutInvalidationContext) {
        if context.invalidateDataSourceCounts { cache.removeAll(keepingCapacity: true) }
        super.invalidateLayout(with: context)
    }
}
extension CustomCollectionViewLayout {

    func updateInsets() {
        guard let collectionView = collectionView else { return }
        collectionView.contentInset.left = (collectionView.bounds.size.width - 2.5) / 2
        collectionView.contentInset.right = (collectionView.bounds.size.width - 2.5) / 2
    }
    override func targetContentOffset(forProposedContentOffset proposedContentOffset: CGPoint, withScrollingVelocity velocity: CGPoint) -> CGPoint {
        guard let collectionView = collectionView else { return super.targetContentOffset(forProposedContentOffset: proposedContentOffset) }
        let midX: CGFloat = collectionView.bounds.size.width / 2
        guard let closestAttribute = findClosestAttributes(toXPosition: proposedContentOffset.x + midX) else { return super.targetContentOffset(forProposedContentOffset: proposedContentOffset) }
        return CGPoint(x: closestAttribute.center.x - midX, y: proposedContentOffset.y)
    }

    private func findClosestAttributes(toXPosition xPosition: CGFloat) -> UICollectionViewLayoutAttributes? {
        guard let collectionView = collectionView else { return nil }
        let searchRect = CGRect(
            x: xPosition - collectionView.bounds.width, y: collectionView.bounds.minY,
            width: collectionView.bounds.width * 2, height: collectionView.bounds.height
        )
        let closestAttributes = layoutAttributesForElements(in: searchRect)?.min(by: { abs($0.center.x - xPosition) < abs($1.center.x - xPosition) })
        return closestAttributes
    }
    private var continuousFocusedIndex: CGFloat {
        guard let collectionView = collectionView else { return 0 }
        let offset = collectionView.bounds.width / 2 + collectionView.contentOffset.x - 2.5 / 2
        return offset / 2.5
    }
    private func shiftedAttributes(from attributes: UICollectionViewLayoutAttributes) -> UICollectionViewLayoutAttributes {
        guard let attributes = attributes.copy() as? UICollectionViewLayoutAttributes else { fatalError("Couldn't copy attributes") }
        let roundedFocusedIndex = round(continuousFocusedIndex)
        let focusedItemWidth = CGFloat(20)
        if attributes.indexPath.item == Int(roundedFocusedIndex){
            attributes.transform = CGAffineTransform(scaleX: 10, y: 1)
        } else {
            let translationDirection: CGFloat = attributes.indexPath.item < Int(roundedFocusedIndex) ? -1 : 1
            attributes.transform = CGAffineTransform(translationX: translationDirection * 20, y: 0)
        }
        return attributes
    }
}

If the class is invalid somehow would that prevent its methods from being called? What is it that auto-executes the prepare() method on layout?

And here is the view Controller where its used:

class ViewController: UIViewController, UICollectionViewDelegate,UICollectionViewDataSource {

    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return 10
    }

    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let cell = customCollectionView.dequeueReusableCell(withReuseIdentifier: "singleCell", for: indexPath)
        cell.backgroundColor = UIColor.random()
        return cell
    }

    @IBOutlet weak var customCollectionView: UICollectionView!

    override func viewDidLoad() {
        super.viewDidLoad()
        customCollectionView.delegate = self

    }
}

Answer 1

You should provide the number of sections for collection view. Otherwise, it is assumed to be 0 and, therefore, the implementation of the layout does not begin.

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

Also try to debug the values of collectionView and collectionViewLayout in your view controller