swift CollectioinView 居中放大 无限循环滚动

居中放大 参考：https://blog.csdn.net/u013282507/article/details/54136812

自白
以前弄过无限循环滚动的，好长时间没弄了，想从网上直接找一个来用， 找到的确实把数据条目重复发大做到的，cell数据多的时候会在内存中占着一大块数据，而且我是在首页用，内存根本释放不掉，感觉不好。 还是花时间实现了自己以前的做法。（一直没管理自己的代码库， 管理自己的代码库是多么重要）

无限滚动原理
假设有 a、b、c 三条数据，在collectionview中的数据是 c、a、b、c、a 共5条数据，当向左滚动，滚动到第4条数据c，滚动结束后，无动画滚动到第1条数据c，就可以继续向左滚动了；当向右滚动，滚动到第2条数据a，滚动结束后，无动画滚动到第5条数据a，就可以继续向右滚动了

本来代码也是基于前辈的这个XLCardSwitch改的, 修改后的代码贴出来，以后用
懒到一定程度了

import UIKit

//滚动切换回调方法
typealias XLCardScollIndexChangeBlock = (Int) -> Void

//布局类
class XLCardSwitchFlowLayout: UICollectionViewFlowLayout {
    //卡片和父视图宽度比例
    let cardWidthScale: CGFloat = 1.0 // 0.7
    //卡片和父视图高度比例
    let cardHeightScale: CGFloat = 1.0 // 0.8
    //滚动到中间的调方法
    var indexChangeBlock: XLCardScollIndexChangeBlock?
    
    override func prepare() {
        self.scrollDirection = UICollectionView.ScrollDirection.horizontal
        self.sectionInset = UIEdgeInsets(top: self.insetY(), left: self.insetX(), bottom: self.insetY(), right: self.insetX())
        self.itemSize = CGSize(width: self.itemWidth(), height: self.itemHeight())
        self.minimumLineSpacing = 5
    }
    
    override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
        //获取cell的布局
        let originalAttributesArr = super.layoutAttributesForElements(in: rect)
        //复制布局,以下操作，在复制布局中处理
        var attributesArr: Array = Array.init()
        for attr: UICollectionViewLayoutAttributes in originalAttributesArr! {
            attributesArr.append(attr.copy() as! UICollectionViewLayoutAttributes)
        }
        
        //屏幕中线
        let centerX: CGFloat =  (self.collectionView?.contentOffset.x)! + (self.collectionView?.bounds.size.width)!/2.0
        
        //最大移动距离，计算范围是移动出屏幕前的距离
        let maxApart: CGFloat  = ((self.collectionView?.bounds.size.width)! + self.itemWidth())/2.0
        
        //刷新cell缩放   现在不缩放了 只判断是否滚动到中间
        for attributes: UICollectionViewLayoutAttributes in attributesArr {
            //获取cell中心和屏幕中心的距离
            let apart: CGFloat = abs(attributes.center.x - centerX)
// 放到的注释了， 不用了， 只用居中的效果
//            //移动进度 -1~0~1
//            let progress: CGFloat = apart/maxApart
//            //在屏幕外的cell不处理
//            if (abs(progress) > 1) {continue}
//            //根据余弦函数，弧度在 -π/4 到 π/4,即 scale在 √2/2~1~√2/2 间变化
//            let scale: CGFloat = abs(cos(progress * CGFloat(Double.pi/4)))
//            //缩放大小
//            attributes.transform = CGAffineTransform.init(scaleX: scale, y: scale)
            //更新中间位
            if (apart <= self.itemWidth()/2.0) {
                self.indexChangeBlock?(attributes.indexPath.row)
            }
        }
        return attributesArr
    }
    
    //MARK -
    //MARK 配置方法
    //卡片宽度
    func itemWidth() -> CGFloat {
//        return (self.collectionView?.bounds.size.width)! * cardWidthScale
        return (self.collectionView?.bounds.size.width)! - 50
    }
    
    //卡片高度
    func itemHeight() -> CGFloat {
        return (self.collectionView?.bounds.size.height)! * cardHeightScale
    }
    
    //设置左右缩进
    func insetX() -> CGFloat {
        let insetX: CGFloat = ((self.collectionView?.bounds.size.width)! - self.itemWidth())/2.0
        return insetX
    }
    
    //上下缩进
    func insetY() -> CGFloat {
        let insetY: CGFloat = ((self.collectionView?.bounds.size.height)! - self.itemHeight())/2.0
        return insetY
    }
    
    //是否实时刷新布局
    override func shouldInvalidateLayout(forBoundsChange newBounds: CGRect) -> Bool {
        return true
    }
}

//代理
@objc protocol XLCardSwitchDelegate: NSObjectProtocol {
    //滑动切换到新的位置回调
    @objc optional func cardSwitchDidScrollToIndex(index: Int) -> ()
    //手动点击了
    @objc optional func cardSwitchDidSelectedAtIndex(index: Int) -> ()
}

//数据源
@objc protocol XLCardSwitchDataSource: NSObjectProtocol {
    //卡片的个数
    func cardSwitchNumberOfCard() -> (Int)
    //卡片cell
    func cardSwitchCellForItemAtIndex(index: Int) -> (UICollectionViewCell)
}

//展示类
class XLCardSwitch: UIView ,UICollectionViewDelegate,UICollectionViewDataSource {
    //公有属性
    weak var delegate: XLCardSwitchDelegate?
    weak var dataSource: XLCardSwitchDataSource?
    var selectedIndex: Int = 0
    var pagingEnabled: Bool = false
    //私有属性
    private var _dragStartX: CGFloat = 0
    private var _dragEndX: CGFloat = 0
    private var _dragAtIndex: Int = 0
    
    private let flowlayout = XLCardSwitchFlowLayout()
    
    private var isInfinite = false
    private var isAutoScroll = false
    private var scrollTimeInterval: TimeInterval = 3.0
    private var timer: Timer?
    
    private lazy var _collectionView: UICollectionView = {
        let view = UICollectionView(frame: .zero, collectionViewLayout: flowlayout)
        view.delegate = self
        view.dataSource = self
        view.backgroundColor = UIColor.clear
        view.showsHorizontalScrollIndicator = false
        return view
    }()
    
   
    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
    }
    
    override init(frame: CGRect) {
        super.init(frame: frame)
        self.buildUI()
    }
    
    func buildUI() {
        self.addSubview(_collectionView)
        
        //添加回调方法
        flowlayout.indexChangeBlock = { (index) -> () in
            if self.selectedIndex != index {
                self.selectedIndex = index
                // 3 0 1 2 3 0
                if self.isInfinite {
                    let num = self.dataSource?.cardSwitchNumberOfCard() ?? 0  // 4
                    if index == 0 {
                        self.delegateUpdateScrollIndex(index: num - 1)
                    }else if index == num + 1 {
                        self.delegateUpdateScrollIndex(index: 0)
                    } else {
                        self.delegateUpdateScrollIndex(index: index-1)
                    }
                } else {
                    self.delegateUpdateScrollIndex(index: index)
                }
//                self.delegateUpdateScrollIndex(index: index)
            }
        }
        
    }
    
    func reloadData() {
        _collectionView.reloadData()
        if isInfinite {
            self.autoScrollFixToPosition(index: 1)
        }
        if isAutoScroll {
            removeTimer()
            addTimer()
        }
    }
    
    func setScrollViewTag(tag: Int) {
        _collectionView.tag = tag
    }
    
    
    //MARK:自动滚动
    func isAutoScroll(autoScroll: Bool) {
        isAutoScroll = autoScroll
        if isAutoScroll {
            isInfinite(infinite: true)
        }
    }
    func isInfinite(infinite: Bool) {
        isInfinite = infinite
    }
    func scrollTimeIntrval(timeInterval: TimeInterval) {
        scrollTimeInterval = timeInterval
    }
    func addTimer() {
        let timer = Timer.scheduledTimer(timeInterval: scrollTimeInterval, target: self, selector: #selector(nextpage), userInfo: nil, repeats: true)
        RunLoop.main.add(timer, forMode: .commonModes)
        self.timer = timer
    }
    func removeTimer() {
        self.timer?.invalidate()
        self.timer = nil
    }
    
    @objc private func nextpage() {
        switchToIndex(index: self.selectedIndex + 1)
    }
    
    //MARK:自动布局
    override func layoutSubviews() {
        super.layoutSubviews()
        _collectionView.frame = self.bounds
    }

    //MARK:-
    //MARK:CollectionView方法
    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        if let num = self.dataSource?.cardSwitchNumberOfCard(), num > 0 {
            if num == 1 {
                isInfinite = false
                isAutoScroll = false
                return num
            }
            if isInfinite {
                return  num + 2    // 3 0 1 2 3 0 布局cell顺序
            } else {
                return num
            }
        } else {
            return 0
        }
//        return (self.dataSource?.cardSwitchNumberOfCard()) ?? 0
    }
    
    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        if isInfinite {
            let num = self.dataSource?.cardSwitchNumberOfCard() ?? 0
            if indexPath.row == 0 {
                return (self.dataSource?.cardSwitchCellForItemAtIndex(index: num - 1))!
            }else if indexPath.row == num + 1 {
                return (self.dataSource?.cardSwitchCellForItemAtIndex(index: 0))!
            } else {
                return (self.dataSource?.cardSwitchCellForItemAtIndex(index: indexPath.row - 1))!
            }
        } else {
            return (self.dataSource?.cardSwitchCellForItemAtIndex(index: indexPath.row))!
        }
//        return (self.dataSource?.cardSwitchCellForItemAtIndex(index: indexPath.row))!
    }
    
    func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
        //执行代理方法
        selectedIndex = indexPath.row
        self.scrollToCenterAnimated(animated: true)
        if isInfinite {
            let num = self.dataSource?.cardSwitchNumberOfCard() ?? 0
            if indexPath.row == 0 {
                self.delegateSelectedAtIndex(index: num-1)
            }else if indexPath.row == num + 1 {
                self.delegateSelectedAtIndex(index: 0)
            } else {
                self.delegateSelectedAtIndex(index: indexPath.row - 1)
            }
        } else {
            self.delegateSelectedAtIndex(index: indexPath.row)
        }
//        self.delegateSelectedAtIndex(index: indexPath.row)
    }
    
    //MARK:-
    //MARK:ScrollViewDelegate
    @objc func fixCellToCenter() -> () {
        if self.selectedIndex != _dragAtIndex {
            self.scrollToCenterAnimated(animated: true)
            return
        }
        //最小滚动距离
        let dragMiniDistance: CGFloat = self.bounds.size.width/20.0
        if _dragStartX - _dragEndX >= dragMiniDistance {
            self.selectedIndex -= 1//向右
        }else if _dragEndX - _dragStartX >= dragMiniDistance {
            self.selectedIndex += 1 //向左
        }
        
        let maxIndex: Int  = (_collectionView.numberOfItems(inSection: 0)) - 1
        self.selectedIndex = max(self.selectedIndex, 0)
        self.selectedIndex = min(self.selectedIndex, maxIndex)
        self.scrollToCenterAnimated(animated: true)
    }
    
    //滚动到中间
    func scrollToCenterAnimated(animated: Bool) -> () {
//        _collectionView.scrollToItem(at: IndexPath.init(row:self.selectedIndex, section: 0), at: UICollectionView.ScrollPosition.centeredHorizontally, animated: true)
        _collectionView.scrollToItem(at: IndexPath.init(row: self.selectedIndex, section: 0), at: UICollectionViewScrollPosition.centeredHorizontally, animated: animated)
    }
    
    //手指拖动开始
    func scrollViewWillBeginDragging(_ scrollView: UIScrollView) {
        if isAutoScroll {
            removeTimer()
        }
        if (!self.pagingEnabled) { return }
        _dragStartX = scrollView.contentOffset.x
        _dragAtIndex = self.selectedIndex
    }
    
    //手指拖动停止
    func scrollViewDidEndDragging(_ scrollView: UIScrollView, willDecelerate decelerate: Bool) {
        if isAutoScroll {
            addTimer()
        }
        if (!self.pagingEnabled) { return }
        _dragEndX = scrollView.contentOffset.x
        //在主线程执行居中方法
        DispatchQueue.main.async {
            self.fixCellToCenter()
        }
    }
    
    func scrollViewDidEndScrollingAnimation(_ scrollView: UIScrollView) {
        // 3 0 1 2 3 0
        if self.isInfinite {
            let num = self.dataSource?.cardSwitchNumberOfCard() ?? 0  // 4
            let index = self.selectedIndex
            if index == 0 {
                self.autoScrollFixToPosition(index: num)
            }else if index == num + 1 {
                self.autoScrollFixToPosition(index: 1)
            } else {
            }
        }
    }
    
    //MARK:-
    //MARK:执行代理方法
    //回调滚动方法
    func delegateUpdateScrollIndex(index: Int) -> () {
        guard let delegate = self.delegate else { return }
        if (delegate.responds(to: #selector(delegate.cardSwitchDidScrollToIndex(index:)))) {
            delegate.cardSwitchDidScrollToIndex?(index: index)
        }
    }
    
    //回调点击方法
    func delegateSelectedAtIndex(index: Int) -> () {
        guard let delegate = self.delegate else { return }
        if (delegate.responds(to: #selector(delegate.cardSwitchDidSelectedAtIndex(index:)))) {
            delegate.cardSwitchDidSelectedAtIndex?(index: index)
        }
    }
    
    //MARK:-
    //MARK:切换位置方法
    func switchToIndex(index: Int) -> () {
        DispatchQueue.main.async {
            self.selectedIndex = index
            self.scrollToCenterAnimated(animated: true)
        }
    }
    
    func autoScrollFixToPosition(index: Int) -> () {
//        DispatchQueue.main.asyncAfter(deadline: .now()+1) {
//            self.selectedIndex = index
//            self.scrollToCenterAnimated(animated: false)
//        }
        DispatchQueue.main.async {
            self.selectedIndex = index
            self.scrollToCenterAnimated(animated: false)
        }
    }
    
    //向前切换
    func switchPrevious() -> () {
        guard let index = currentIndex() else { return }
        var targetIndex = index - 1
        if !isInfinite {
            targetIndex = max(0, targetIndex)
        }
        self.switchToIndex(index: targetIndex)
    }
    
    //向后切换
    func switchNext() -> () {
        guard let index = currentIndex() else { return }
        var targetIndex = index + 1
        if !isInfinite {
            let maxIndex = (self.dataSource?.cardSwitchNumberOfCard())! - 1
            targetIndex = min(maxIndex, targetIndex)
        }
//        var maxIndex = (self.dataSource?.cardSwitchNumberOfCard())! - 1
//        targetIndex = min(maxIndex, targetIndex)
        
        self.switchToIndex(index: targetIndex)
    }
    
    func currentIndex() -> Int? {
        let x = _collectionView.contentOffset.x + _collectionView.bounds.width/2
        let index = _collectionView.indexPathForItem(at: CGPoint(x: x, y: _collectionView.bounds.height/2))?.item
        if isInfinite {
            let num = self.dataSource?.cardSwitchNumberOfCard() ?? 0
            if index == 0 {
                return num - 1
            }else if index == num + 1 {
                return 0
            } else {
                return ((index ?? 1) - 1)
            }
        } else {
            return index
        }
    }
    
    //MARK:-
    //MARK:数据源相关方法
    open func register(cellClass: AnyClass?, forCellWithReuseIdentifier identifier: String) {
        _collectionView.register(cellClass, forCellWithReuseIdentifier: identifier)
    }
    
    open func register(nib: UINib?, forCellWithReuseIdentifier identifier: String) {
        _collectionView.register(nib, forCellWithReuseIdentifier: identifier)
    }
    
    open func dequeueReusableCell(withReuseIdentifier identifier: String, for index: Int) -> UICollectionViewCell {
        return _collectionView.dequeueReusableCell(withReuseIdentifier: identifier, for: IndexPath(row: index, section: 0))
    }
    
    deinit {
        if self.timer != nil {
            self.timer?.invalidate()
            self.timer = nil
        }
    }
}

使用
和人家的使用一样，只是多了是否无限滚动，是否自动滚动，滚动时间的方法

//滚动卡片
    lazy var cardSwitch: XLCardSwitch = {
        let temp = XLCardSwitch.init()
        temp.frame = CGRect(x: 0, y: 55, width: ScreenWidth, height: 139)
        temp.pagingEnabled = true
        temp.dataSource = self
        temp.delegate = self
        temp.setScrollViewTag(tag: 9090)  // 这个是避免父视图滚动冲突的，和本章知识无关
        temp.isAutoScroll(autoScroll: true)
        //注册cell
        temp.register(nib: UINib(nibName: "ItemCell", bundle: nil), forCellWithReuseIdentifier: "ItemCell")
        return temp
    }()