UICollectionViewControllerFlowLayout boiler plate. Just fill out the `prepare` function with you layout logic and the rest just works.

SampleFlowLayout.swift

class SampleFlowLayout: UICollectionViewLayout {

    override func prepare() {
        guard let collectionView = self.collectionView else { return }
        self.itemAttributes.removeAllObjects()

        // go through every section..
        for section in 0..<collectionView.numberOfSections {
            let numberOfItems = collectionView.numberOfItems(inSection: section)

            // ..and every item
            for item in 0..<numberOfItems {
                // create the frame attribute for the index path
                let attributes = UICollectionViewLayoutAttributes(forCellWith: IndexPath(item: item, section: section))
                attributes.frame = self.frameForItemNumber(itemNumber, isBelowRibbon: isBelowRibbon)
                attributes.zIndex = 1
                self.itemAttributes.add(attributes)
            }
        }

        // save the last frame
        let lastAtt = self.itemAttributes.lastObject as? UICollectionViewLayoutAttributes

        // calculate the total content size
        if (lastAtt != nil) {
            let maxCellY = lastAtt!.frame.maxY
            self.contentSize = CGSize(width: self.collectionView!.frame.width, height: maxCellY + bottomPadding)
        }
        else {
            self.contentSize = CGSize.zero
        }
    }

    // MARK: Flow Layout Overrides

    override var collectionViewContentSize: CGSize {
        return self.contentSize
    }

    override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
        // hand back the attributes that are in the rect
        let filteredArray = self.itemAttributes.filtered(using: NSPredicate(block: { (object, binding) -> Bool in
            let attObject = object as! UICollectionViewLayoutAttributes
            return rect.intersects(attObject.frame)
        })) as! [UICollectionViewLayoutAttributes]

        return filteredArray
    }

    override func shouldInvalidateLayout(forBoundsChange newBounds: CGRect) -> Bool {
        return newBounds.width != self.collectionView?.frame.width || newBounds.height != self.collectionView?.frame.height
    }
}