每次滚动视图结束时将collectionView的单元格居中

class ViewController: UIViewController, UICollectionViewDelegate {
    // ...
    collectionView.delegate = self
    // ...
}

func scrollViewDidEndDecelerating(_ scrollView: UIScrollView) {
    let visibleRect = CGRect(origin: collectionView.contentOffset, size: collectionView.bounds.size)
    let visiblePoint = CGPoint(x: visibleRect.midX, y: visibleRect.midY)
    
    if let indexPath = collectionView.indexPathForItem(at: visiblePoint) {
        collectionView.scrollToItem(at: indexPath, at: .centeredHorizontally, animated: true)
    }
}

let layout = UICollectionViewFlowLayout()
layout.scrollDirection = .horizontal
collectionView.collectionViewLayout = layout