Swift实现3D轮播图效果

//数据源数组
 let totalCount = 100
 var models: [String] = [String]() {
  didSet {
   //判断元素个数
   if models.count < 2 {
    collectionView.isScrollEnabled = false
   }
   //网上的找来的一个办法添加100组数据源对应的索引数组indexArr
   for _ in 0..<totalCount {
    for j in 0..<models.count {
     indexArr.append(j)
    }
   }
   //开始就滚动到第50组数据源
   collectionView.scrollToItem(at: IndexPath(item: totalCount/2 * models.count, section: 0), at: .centeredHorizontally, animated: false)
  }
 }

func scrollViewDidEndDecelerating(_ scrollView: UIScrollView) {
  //找到滚动停止的点对应的collectionView的indexPath
  let point = self.convert(collectionView.center, to: collectionView)
  let index = collectionView.indexPathForItem(at: point)
  let indexpath = (index?.row ?? 0) % models.count
  
  collectionView.scrollToItem(at: IndexPath(item: totalCount/2 * models.count + indexpath, section: 0), at: .centeredHorizontally, animated: false)
 }

override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
  
  //集合视图的宽高（这里默认宽高相等）
  let itemHeight = self.collectionView?.frame.height ?? 0
  //可是视图内的attributes数组
  let array = super.layoutAttributesForElements(in: rect)
  //item透明度开始变化时的item的中心点x
  let centerX = self.collectionView!.contentOffset.x + itemHeight/2
  print(self.collectionView?.contentOffset.x ?? 0)
  
  for attributes in array! {
   //开始变化时的item的中心点x 与 实际中心点相比较
   let value = attributes.center.x - centerX
   let delta = abs(value)
   //设置缩放比例，此处4*itemHeight可根据缩放效果进行修改
   let scale = 1 - delta/(4*itemHeight)
 
   //设置缩放比例
   attributes.transform = CGAffineTransform.init(scaleX: scale, y: scale)
   //层次关系，设置此属性使item依次上下排列
   attributes.zIndex = Int(1 - abs(delta))
   
   //value<=0表示向左移动，最前面的item停止一段距离
   if value <= 0{
    //实际中心点与开始变化时的item的中心点小于等于设定的边界值
    if delta >= 0 && delta <= itemHeight
    {
     //item的中心点固定不变
     attributes.center.x = centerX
     //根据推进值，改变item的透明度，此处的delta>10是想让item有一个达到目标区域时有一个停顿效果而不是直接进入改变透明度的阶段
     attributes.alpha = (delta > 10) ? (1 - delta/(itemHeight/4)) : 1
     //设置缩放比例，停顿阶段不进行缩放
     attributes.transform = CGAffineTransform.init(scaleX: 1, y: 1)
    } else {
     //移动大于边界值，就是从停顿阶段到透明度改变，此处是下一个item上来,当前item透明度变为0
     attributes.alpha = 0
    }
   }
  }
  return array
 }

override func targetContentOffset(forProposedContentOffset proposedContentOffset: CGPoint, withScrollingVelocity velocity: CGPoint) -> CGPoint {
  
  let targetRect = CGRect(x: proposedContentOffset.x, y: 0.0, width: self.collectionView!.bounds.size.width, height: self.collectionView!.bounds.size.height)
  
  //目标区域中包含的cell
  let attrArray = super.layoutAttributesForElements(in: targetRect) as! [UICollectionViewLayoutAttributes]
  //collectionView落在屏幕重点的x坐标
  let horizontalCenterX = proposedContentOffset.x + (self.collectionView?.frame.height ?? 0)/2
  var offsetAdjustment = CGFloat(MAXFLOAT)
  for layoutAttributes in attrArray {
   
   let itemHorizontalCenterX = layoutAttributes.center.x
   //找出离中心店最近的
   if (abs(itemHorizontalCenterX-horizontalCenterX) < abs(offsetAdjustment)) {
    offsetAdjustment = itemHorizontalCenterX - horizontalCenterX
   }
  }
  
  //返回collectionView最终停留的位置
  return CGPoint(x: proposedContentOffset.x + offsetAdjustment, y: proposedContentOffset.y)
  
 }
//当collectionView的显示范围发生改变的时候，是否重新布局
 override func shouldInvalidateLayout(forBoundsChange newBounds: CGRect) -> Bool {
  return true
 }