Custom UICollectionViewFlowLayout refer to http://stackoverflow.com/questions/13492037/targetcontentoffsetforproposedcontentoffsetwithscrollingvelocity-without-subcla?nah=1#28813308

EMCollectionViewFlowLayout.swift

import UIKit

class SleepLogCollectionViewFlowLayout: UICollectionViewFlowLayout {

    override func targetContentOffset(
        forProposedContentOffset proposedContentOffset: CGPoint,
        withScrollingVelocity velocity: CGPoint
    ) -> CGPoint {
        var _proposedContentOffset = CGPoint(
            x: proposedContentOffset.x, y: proposedContentOffset.y
        )
        var offSetAdjustment: CGFloat = CGFloat.greatestFiniteMagnitude
        let horizontalCenter: CGFloat = CGFloat(
            proposedContentOffset.x + (self.collectionView!.bounds.size.width / 2.0)
        )

        let targetRect = CGRect(
            x: proposedContentOffset.x,
            y: 0.0,
            width: self.collectionView!.bounds.size.width,
            height: self.collectionView!.bounds.size.height
        )

        let array: [UICollectionViewLayoutAttributes] =
            self.layoutAttributesForElements(in: targetRect)!
            as [UICollectionViewLayoutAttributes]
        for layoutAttributes: UICollectionViewLayoutAttributes in array {
            if layoutAttributes.representedElementCategory == UICollectionElementCategory.cell {
                let itemHorizontalCenter: CGFloat = layoutAttributes.center.x
                if abs(itemHorizontalCenter - horizontalCenter) < abs(offSetAdjustment) {
                    offSetAdjustment = itemHorizontalCenter - horizontalCenter
                }
            }
        }

        var nextOffset: CGFloat = proposedContentOffset.x + offSetAdjustment

        repeat {
            _proposedContentOffset.x = nextOffset
            let deltaX = proposedContentOffset.x - self.collectionView!.contentOffset.x
            let velX = velocity.x

            if
                deltaX == 0.0 || velX == 0 || (velX > 0.0 && deltaX > 0.0) ||
                (velX < 0.0 && deltaX < 0.0)
            {
                break
            }

            if velocity.x > 0.0 {
                nextOffset = nextOffset + self.snapStep()
            } else if velocity.x < 0.0 {
                nextOffset = nextOffset - self.snapStep()
            }
        } while self.isValidOffset(offset: nextOffset)

        _proposedContentOffset.y = 0.0

        return _proposedContentOffset
    }

    func isValidOffset(offset: CGFloat) -> Bool {
        return (offset >= CGFloat(self.minContentOffset()) &&
            offset <= CGFloat(self.maxContentOffset()))
    }

    func minContentOffset() -> CGFloat {
        return -CGFloat(self.collectionView!.contentInset.left)
    }

    func maxContentOffset() -> CGFloat {
        return CGFloat(
            self.minContentOffset() + self.collectionView!.contentSize.width - self.itemSize.width
        )
    }

    func snapStep() -> CGFloat {
        return self.itemSize.width + self.minimumLineSpacing
    }
}

Updated for Swift 3

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