CenterCellCollectionViewFlowLayout.swift

/// Source: https://bitbucket.org/mmick66/centercellpagingcollectionview
class CenterCellCollectionViewFlowLayout: UICollectionViewFlowLayout {
    
    var mostRecentOffset = CGPoint()
    
    override func targetContentOffset(forProposedContentOffset proposedContentOffset: CGPoint, withScrollingVelocity velocity: CGPoint) -> CGPoint {
        
        guard let collectionView = self.collectionView else {
            self.mostRecentOffset = super.targetContentOffset(forProposedContentOffset: proposedContentOffset)
            return self.mostRecentOffset
        }
        
        guard velocity.x != 0 else {
            return self.mostRecentOffset
        }
        
        if(proposedContentOffset.x == -(collectionView.contentInset.left)) {
            return proposedContentOffset
        }
        
        let cvBounds = collectionView.bounds
        let halfWidth = cvBounds.size.width * 0.5
        guard let attributesForVisibleCells = self.layoutAttributesForElements(in: cvBounds) else {
            self.mostRecentOffset = super.targetContentOffset(forProposedContentOffset: proposedContentOffset)
            return self.mostRecentOffset
        }
        
        var candidateAttributes: UICollectionViewLayoutAttributes?
        for attributes in attributesForVisibleCells where attributes.representedElementCategory == .cell {
            if (attributes.center.x == 0) || (attributes.center.x > (collectionView.contentOffset.x + halfWidth) && velocity.x < 0) {
                continue
            }
            candidateAttributes = attributes
        }
        
        guard let validAttributes = candidateAttributes else {
            return self.mostRecentOffset
        }
        
        self.mostRecentOffset = CGPoint(x: floor(validAttributes.center.x - halfWidth), y: proposedContentOffset.y)
        return self.mostRecentOffset
    }
}