CenterCellCollectionViewFlowLayout

gistfile1.swift

class CenterCellCollectionViewFlowLayout: UICollectionViewFlowLayout {
    
    override func targetContentOffset(forProposedContentOffset proposedContentOffset: CGPoint, withScrollingVelocity velocity: CGPoint) -> CGPoint {
        guard let collectionView = self.collectionView else {
            return super.targetContentOffset(forProposedContentOffset: proposedContentOffset, withScrollingVelocity: velocity)
        }
        
        if collectionView.numberOfItems(inSection: 0) == 0 {
            return super.targetContentOffset(forProposedContentOffset: proposedContentOffset)
        }
        
        if let first = layoutAttributesForItem(at: IndexPath(item: 0, section: 0)),
            let last = layoutAttributesForItem(at: IndexPath(item: collectionView.numberOfItems(inSection: 0) - 1, section: 0))
        {
            let left = (collectionView.frame.width - first.bounds.size.width) * 0.5
            let right = (collectionView.frame.width - last.bounds.size.width) * 0.5
            sectionInset = UIEdgeInsets(top: 0, left: left, bottom: 0, right: right)
        }
        
        let collectionViewSize = collectionView.bounds.size
        let proposedContentOffsetCenterX = proposedContentOffset.x + collectionViewSize.width * 0.5
        
        let proposedRect = collectionView.bounds
        
        var candidateAttributes: UICollectionViewLayoutAttributes?
        if let layoutAttributes = self.layoutAttributesForElements(in: proposedRect) {
            for attributes in layoutAttributes {
                if attributes.representedElementCategory != .cell {
                    continue
                }
                
                let currentOffset = collectionView.contentOffset
                let currentPosition  = (currentOffset.x + collectionViewSize.width * 0.5)
                if (attributes.center.x <= currentPosition && velocity.x > 0) || (attributes.center.x >= currentPosition && velocity.x < 0) {
                    continue
                }
                
                if candidateAttributes == nil {
                    candidateAttributes = attributes
                    continue
                }
                
                let lastCenterOffset = candidateAttributes!.center.x - proposedContentOffsetCenterX
                let centerOffset = attributes.center.x - proposedContentOffsetCenterX
                
                if fabsf(Float(centerOffset)) < fabsf(Float(lastCenterOffset)) {
                    candidateAttributes = attributes
                }
            }
        }
        if candidateAttributes != nil {
            return CGPoint(x: candidateAttributes!.center.x - collectionViewSize.width * 0.5, y: proposedContentOffset.y)
        } else {
            return super.targetContentOffset(forProposedContentOffset: proposedContentOffset)
        }
    }
    
}