UICollection View Flow Layout Vertical Align

Swift 4 with functional oriented approach:

class TopAlignedCollectionViewFlowLayout: UICollectionViewFlowLayout {
    override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
        let attributes = super.layoutAttributesForElements(in: rect)?
            .map { $0.copy() } as? [UICollectionViewLayoutAttributes]

        attributes?
            .reduce([CGFloat: (CGFloat, [UICollectionViewLayoutAttributes])]()) {
                guard $1.representedElementCategory == .cell else { return $0 }
                return $0.merging([ceil($1.center.y): ($1.frame.origin.y, [$1])]) {
                    ($0.0 < $1.0 ? $0.0 : $1.0, $0.1 + $1.1)
                }
            }
            .values.forEach { minY, line in
                line.forEach {
                    $0.frame = $0.frame.offsetBy(
                        dx: 0,
                        dy: minY - $0.frame.origin.y
                    )
                }
            }

        return attributes
    }
}