iOS 实现三张图片左右循环滚动的效果，大家有没有什么思路？ 需求是：屏幕中总共要显示三张图片，中间是图片的完整部分，左右两边分别是图片的一部分内容，每次滑动都相当于那种分页的效果

考虑下直接写动画？

FSPagerView

无限滚动已经出现那么多年了，思路无非就是你说的这种。如果是定时器自动滚动的话，每次都滚动中间那组就行了。如果是手动触发的话，向左向右滚动到尽头应该还是比较累的。

讲道理，当你不同滚动的时候，你无法偷偷换到中间的那组，一般会选择滚动结束的时候偷偷把 offset 设置回去

怎么弄这么麻烦，scrollViewWillEndDragging 的第三个参数是个指针，可以修改的

@iOCZ 能详细讲讲不佬，刚学 iOS😭

@tudoutiaoya 我说的这个是 SDCycleScrollView 的实现思路，不过他似乎没处理你这种手工滚到头的情况，你可以看看源码。。。我看你这个没定时器

根据 https://juejin.cn/post/6940140043042291748 的缩放效果改了一个，你试试效果

```Swift

import UIKit

class ViewController: UIViewController {

private let margin: CGFloat = 20
private var itemW: CGFloat = .zero
private let cellID = "baseCellID"
private let cellCount = 100
private var collectionView: UICollectionView!

override func viewDidLoad() {
super.viewDidLoad()
setUpView()
}

override func viewDidLayoutSubviews() {
super.viewDidLayoutSubviews()
}

func setUpView() {
let layout = ZGFlowLayout()
let collH: CGFloat = 200
let itemH = collH - margin * 2
itemW = view.bounds.width - margin * 2 - 100
layout.itemSize = CGSize(width: itemW, height: itemH)
layout.minimumLineSpacing = margin
layout.scrollDirection = .horizontal

collectionView = UICollectionView(frame: CGRect(x: 0, y: 180, width: view.bounds.width, height: collH), collectionViewLayout: layout)
collectionView.backgroundColor = .black
collectionView.showsHorizontalScrollIndicator = false
collectionView.dataSource = self
collectionView.delegate = self

collectionView.register(UICollectionViewCell.self, forCellWithReuseIdentifier: cellID)
view.addSubview(collectionView)
scrollTo(index: cellCount/2)
}
}

extension ViewController: UICollectionViewDelegate, UICollectionViewDataSource{

func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
return cellCount
}

func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
let cell = collectionView.dequeueReusableCell(withReuseIdentifier: cellID, for: indexPath)
let colors: [UIColor] = [.red, .yellow, .blue]
cell.backgroundColor = colors[indexPath.item % 3]
return cell
}

func scrollViewDidEndDecelerating(_ scrollView: UIScrollView) {
let scrollStop = !scrollView.isTracking && !scrollView.isDragging && !scrollView.isDecelerating
guard scrollStop else { return }
ZGScrollViewDidEndScroll(scrollView: scrollView)
}

func scrollViewDidScroll(_ scrollView: UIScrollView) {
//防止滚动到最后或者最前，如果觉得改变时突兀，可以增加 cellCount
let page = getCurrentPage(scrollView: scrollView)
if page >= 95 || page <= 5 {
scrollTo(index: page%3 + cellCount+1)
}
}

private func ZGScrollViewDidEndScroll(scrollView: UIScrollView) {
let page = getCurrentPage(scrollView: scrollView)
scrollTo(index: page%3 + cellCount+1)
}

private func scrollTo(index: Int){
collectionView.scrollToItem(at: IndexPath(item: index, section: 0), at: .centeredHorizontally, animated: false)
}

private func getCurrentPage(scrollView: UIScrollView) -> Int{
//第一个 page 偏移量会少了多显示出来的一半，不使用之后的计算，直接判定为 0
var page: CGFloat = 0
if scrollView.contentOffset.x > 0 {
//计算单次滑动的偏移量
let scrollW = scrollView.frame.width
//显示的多出一半的宽度
let half = (scrollW - itemW)/2
//除第一个外其余每次滑动的偏移量
let eachOffset = (itemW+margin)
//第一个 cell 的偏移量
let firstOffset = eachOffset-half
page = (scrollView.contentOffset.x-firstOffset)/(eachOffset) + 1
}
return Int(page)
}
}

class ZGFlowLayout: UICollectionViewFlowLayout {

override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
let attributes = super.layoutAttributesForElements(in: rect)
let centerX = collectionView!.contentOffset.x + collectionView!.bounds.width / 2
attributes?.forEach({ (attr) in
let pad = abs(centerX - attr.center.x)
let factor = 0.0009
let scale = 1 / (1 + pad * CGFloat(factor))
attr.transform = CGAffineTransform(scaleX: scale, y: scale)
})
return attributes
}

override func targetContentOffset(forProposedContentOffset proposedContentOffset: CGPoint, withScrollingVelocity velocity: CGPoint) -> CGPoint {
var targetPoint = proposedContentOffset
let centerX = proposedContentOffset.x + collectionView!.bounds.width / 2
let attrs = self.layoutAttributesForElements(in: CGRect(x: proposedContentOffset.x, y: proposedContentOffset.y, width: collectionView!.bounds.size.width, height: collectionView!.bounds.size.height))
var moveDistance: CGFloat = CGFloat(MAXFLOAT)
attrs!.forEach { (attr) in
if abs(attr.center.x - centerX) < abs(moveDistance) {
moveDistance = attr.center.x - centerX
}
}
if targetPoint.x > 0 && targetPoint.x < collectionViewContentSize.width - collectionView!.bounds.width {
targetPoint.x += moveDistance
}
return targetPoint
}

override func shouldInvalidateLayout(forBoundsChange newBounds: CGRect) -> Bool {
return true
}

override var collectionViewContentSize: CGSize {
return CGSize(width: sectionInset.left + sectionInset.right + (CGFloat(collectionView!.numberOfItems(inSection: 0)) * (itemSize.width + minimumLineSpacing)) - minimumLineSpacing, height: 0)
}
}


```

@V2SuperUser 为啥我的代码不能像 OP 一样有格式

@V2SuperUser 修正：两处 scrollTo(index: page%3 + cellCount+1)改为 scrollTo(index: page%3 + cellCount/2+1)

效果图： https://imgur.com/a/37Kk50T