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import UIKit

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

    private class func alignToTopForSameLineElements(sameLineElements: [UICollectionViewLayoutAttributes])
    {
        if sameLineElements.count < 1
        {
            return
        }
        let sorted = sameLineElements.sort { (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 = CGRectOffset(obj.frame, 0, tallest.frame.origin.y - obj.frame.origin.y)
            }
        }
    }
}