Horizontal center scroll UICollectionView layout.

HorizontalCenterScrollFlowLayout.swift

import UIKit

class HorizontalCenterScrollFlowLayout: UICollectionViewFlowLayout {
    override func targetContentOffset(forProposedContentOffset proposedContentOffset: CGPoint, withScrollingVelocity velocity: CGPoint) -> CGPoint {
        var offsetAdjustment = CGFloat.greatestFiniteMagnitude
        let horizontalCenter = proposedContentOffset.x + collectionView!.bounds.width / 2.0
        let targetRect = CGRect(x: proposedContentOffset.x, y: 0, width: collectionView!.bounds.width, height: collectionView!.bounds.height)

        if let attributes = super.layoutAttributesForElements(in: targetRect) {
            for attribute in attributes {
                let itemHorizontalCenter = attribute.center.x
                if abs(itemHorizontalCenter - horizontalCenter) < abs(offsetAdjustment) {
                    offsetAdjustment = itemHorizontalCenter - horizontalCenter
                }
            }
        }
        
        return CGPoint(x: proposedContentOffset.x + offsetAdjustment, y: proposedContentOffset.y)
    }
}