MasonaryFlowLayout.swift

import UIKit

final class MasonaryFlowLayout: UICollectionViewFlowLayout {

    override func layoutAttributesForItemAtIndexPath(indexPath: NSIndexPath) -> UICollectionViewLayoutAttributes? {
        guard let attrs = super.layoutAttributesForItemAtIndexPath(indexPath) else { return nil }

        if indexPath.item == 0 {
            return attrs
        }

        if attrs.frame.origin.x - 1 <= sectionInset.left {
            return attrs
        }

        let indexPathForPreviousItem = NSIndexPath(forItem:indexPath.item - 1, inSection:indexPath.section)
        guard let layoutAttributesForPreviousItem = layoutAttributesForItemAtIndexPath(indexPathForPreviousItem) else {
            return attrs
        }
        let frameForPreviousItem = layoutAttributesForPreviousItem.frame
        let xOffset = frameForPreviousItem.origin.x + frameForPreviousItem.width + minimumInteritemSpacing

        guard let newAttrs = attrs.copy() as? UICollectionViewLayoutAttributes else {
            return attrs
        }

        newAttrs.frame.origin.x = xOffset
        return newAttrs
    }

    override func layoutAttributesForElementsInRect(rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
        return super.layoutAttributesForElementsInRect(rect)?.flatMap { attr in
            if attr.representedElementKind == nil {
                return self.layoutAttributesForItemAtIndexPath(attr.indexPath)
            }
            return attr
        }
    }
}