UICollection View Flow Layout Vertical Align

Question

By default, when you are using the flow layout in a collection view, cells are centered vertically. Is there a way to change this alignment ?

Answer 1

Swift 3 Version in case someone just wants to Copy & Paste:

class TopAlignedCollectionViewFlowLayout: UICollectionViewFlowLayout {
    override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
        if let attrs = super.layoutAttributesForElements(in: rect) {
            var baseline: CGFloat = -2
            var sameLineElements = [UICollectionViewLayoutAttributes]()
            for element in attrs {
                if element.representedElementCategory == .cell {
                    let frame = element.frame
                    let centerY = frame.midY
                    if abs(centerY - baseline) > 1 {
                        baseline = centerY
                        alignToTopForSameLineElements(sameLineElements: sameLineElements)
                        sameLineElements.removeAll()
                    }
                    sameLineElements.append(element)
                }
            }
            alignToTopForSameLineElements(sameLineElements: sameLineElements) // align one more time for the last line
            return attrs
        }
        return nil
    }

    private func alignToTopForSameLineElements(sameLineElements: [UICollectionViewLayoutAttributes]) {
        if sameLineElements.count < 1 { return }
        let sorted = sameLineElements.sorted { (obj1: UICollectionViewLayoutAttributes, obj2: UICollectionViewLayoutAttributes) -> Bool in
            let height1 = obj1.frame.size.height
            let height2 = obj2.frame.size.height
            let delta = height1 - height2
            return delta <= 0
        }
        if let tallest = sorted.last {
            for obj in sameLineElements {
                obj.frame = obj.frame.offsetBy(dx: 0, dy: tallest.frame.origin.y - obj.frame.origin.y)
            }
        }
    }
}