方法一：

方法一效果

//滚动自然
//注意：isPagingEnabled = fasle
    func scrollViewWillEndDragging(_ scrollView: UIScrollView, withVelocity velocity: CGPoint, targetContentOffset: UnsafeMutablePointer<CGPoint>) {
        let targetOffset: CGPoint = getNearestContentOffset(targetContentOffset.pointee)
        print(self.bannerCollectView.contentOffset.x)
        targetContentOffset.pointee = targetOffset
    }
    func getNearestContentOffset(_ offset: CGPoint) -> CGPoint {
        let pageSize = itemWidth + miniSpace
        let newPage: Int = Int(roundf(Float(offset.x/pageSize)))
        return CGPoint(x: CGFloat(newPage) * pageSize, y: offset.y)
    }

方法二：

方法二效果

//注意：isPagingEnabled = true
//每次只滚动一个单元格距离 
func scrollViewWillBeginDragging(_ scrollView: UIScrollView) {
        stopOffsetX = scrollView.contentOffset.x
    }
    func scrollViewWillEndDragging(_ scrollView: UIScrollView, withVelocity velocity: CGPoint, targetContentOffset: UnsafeMutablePointer<CGPoint>) {
        //如果是向右滑或者滑动距离大于item的一半，则像右移动一个item+space的距离，反之向左
        let currentPointX = scrollView.contentOffset.x
        let moveLen: CGFloat = currentPointX - stopOffsetX
        let shouldPage: Int = Int(moveLen/(itemWidth/2 + miniSpace/2)) //一次只能滚动一格
        if velocity.x > 0 || shouldPage > 0 {
            dragLen = 1
        } else if velocity.x < 0 || shouldPage < 0 {
            dragLen = -1
        } else {
            dragLen = 0
        }
    }
    func scrollViewWillBeginDecelerating(_ scrollView: UIScrollView) {
        let currentndIndex: UInt = UInt(stopOffsetX + CGFloat(itemWidth + miniSpace)*0.5) / UInt(itemWidth + miniSpace)
        bannerCollectView.scrollToItem(at: NSIndexPath.init(row: Int(currentndIndex) + dragLen, section: 0) as IndexPath, at: UICollectionView.ScrollPosition.centeredHorizontally, animated: true)
    }