UICollectionView horizontal paging - can I use Flow Layout?

Question

This is related to but distinct from To use Flow Layout, or to Customize?.

Here is an illustration of what I’m trying to do:

Answer 1

The previous above implementation was not complete, buggy, and with fixed cell size. Here's a more literal translation for the code:

import UIKit

class HorizontalFlowLayout: UICollectionViewLayout {
    var itemSize = CGSizeZero {
        didSet {
            invalidateLayout()
        }
    }
    private var cellCount = 0
    private var boundsSize = CGSizeZero

    override func prepareLayout() {
        cellCount = self.collectionView!.numberOfItemsInSection(0)
        boundsSize = self.collectionView!.bounds.size
    }

    override func collectionViewContentSize() -> CGSize {
        let verticalItemsCount = Int(floor(boundsSize.height / itemSize.height))
        let horizontalItemsCount = Int(floor(boundsSize.width / itemSize.width))

        let itemsPerPage = verticalItemsCount * horizontalItemsCount
        let numberOfItems = cellCount
        let numberOfPages = Int(ceil(Double(numberOfItems) / Double(itemsPerPage)))

        var size = boundsSize
        size.width = CGFloat(numberOfPages) * boundsSize.width
        return size
    }

    override func layoutAttributesForElementsInRect(rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
        var allAttributes = [UICollectionViewLayoutAttributes]()
        for var i = 0; i < cellCount; i++ {
            let indexPath = NSIndexPath(forRow: i, inSection: 0)
            let attr = self.computeLayoutAttributesForCellAtIndexPath(indexPath)
            allAttributes.append(attr)
        }
        return allAttributes
    }

    override func layoutAttributesForItemAtIndexPath(indexPath: NSIndexPath) -> UICollectionViewLayoutAttributes? {
        return self.computeLayoutAttributesForCellAtIndexPath(indexPath)
    }

    override func shouldInvalidateLayoutForBoundsChange(newBounds: CGRect) -> Bool {
        return true
    }

    func computeLayoutAttributesForCellAtIndexPath(indexPath: NSIndexPath) -> UICollectionViewLayoutAttributes {
        let row = indexPath.row
        let bounds = self.collectionView!.bounds

        let verticalItemsCount = Int(floor(boundsSize.height / itemSize.height))
        let horizontalItemsCount = Int(floor(boundsSize.width / itemSize.width))
        let itemsPerPage = verticalItemsCount * horizontalItemsCount

        let columnPosition = row % horizontalItemsCount
        let rowPosition = (row/horizontalItemsCount)%verticalItemsCount
        let itemPage = Int(floor(Double(row)/Double(itemsPerPage)))

        let attr = UICollectionViewLayoutAttributes(forCellWithIndexPath: indexPath)

        var frame = CGRectZero
        frame.origin.x = CGFloat(itemPage) * bounds.size.width + CGFloat(columnPosition) * itemSize.width
        frame.origin.y = CGFloat(rowPosition) * itemSize.height
        frame.size = itemSize
        attr.frame = frame

        return attr
    }
}