CustomCollectionViewLayout_Prepare.swift

override func prepare() {
    super.prepare()
    guard let collectionView = collectionView, let delegate = delegate else {
        return
    }
    guard collectionView.numberOfSections > 0 else { return }

    var position: CGFloat = 0.0
    (0 ..< numberOfSections).forEach { section in
        layoutHeader(position: &position, collectionView: collectionView, delegate: delegate, section: section)
        layoutItems(position: position, collectionView: collectionView, delegate: delegate, section: section)
        layoutFooter(position: &position, collectionView: collectionView, delegate: delegate, section: section)
    }
}

private func layoutHeader(
    position: inout CGFloat,
    collectionView: UICollectionView,
    delegate: CollectionViewLayoutDelegate,
    section: Int
) {
    ....
}

private func layoutItems(
    position: CGFloat,
    collectionView: UICollectionView,
    delegate: CollectionViewLayoutDelegate,
    section: Int
) {
    ....
}

private func layoutFooter(
    position: inout CGFloat,
    collectionView: UICollectionView,
    delegate: CollectionViewLayoutDelegate,
    section: Int
) {
    ....
}