Snapping, centering and automatically sizing flow layout

HomeCollectionViewFlowLayout.swift

import UIKit

class HomeCollectionViewFlowLayout: UICollectionViewFlowLayout {
    
    var isSetUp = false
    
    override func prepare() {
        super.prepare()
        
        setUpIfNeeded()
    }
    
    override func targetContentOffset(forProposedContentOffset proposedContentOffset: CGPoint, withScrollingVelocity velocity: CGPoint) -> CGPoint {
        let targetContentOffset = super.targetContentOffset(forProposedContentOffset: proposedContentOffset, withScrollingVelocity: velocity)
        
        guard  let collectionView = collectionView, let layoutAttributes = layoutAttributesForElements(in: collectionView.bounds) else {
            return targetContentOffset
        }
        
        let centerX = collectionView.bounds.width / 2.0
        let z = proposedContentOffset.x + centerX
        
        guard let first = layoutAttributes.sorted(by: { abs($0.center.x - z) < abs($1.center.x - z) }).first else {
            return targetContentOffset
        }
        
        return CGPoint(x: first.center.x - centerX, y: proposedContentOffset.y)
    }
}

extension HomeCollectionViewFlowLayout {
    
    func setUpIfNeeded() {
        guard let collectionView = collectionView, !isSetUp else {
            return
        }
        
        collectionView.decelerationRate = UIScrollViewDecelerationRateFast
        collectionView.showsHorizontalScrollIndicator = false
        collectionView.showsVerticalScrollIndicator = false
        
        scrollDirection = .horizontal
        
        minimumLineSpacing = 25.0
        minimumInteritemSpacing = 0.0
        
        if layoutAttributesForElements(in: collectionView.bounds)?.count == 1 {
            itemSize = collectionView.bounds.insetBy(dx: 25.0, dy: 25.0).size
        } else {
            itemSize = collectionView.bounds.insetBy(dx: 50.0, dy: 25.0).size
        }
        
        let leftRight = (collectionView.bounds.width - itemSize.width) / 2.0
        sectionInset = UIEdgeInsets(top: 0.0, left: leftRight, bottom: 0.0, right: leftRight)
        
        invalidateLayout()
        
        isSetUp = true
    }
}