//创建collectionView布局
func setepUI() {
//CustomLayout是自定义的UICollectionViewFlowLayout
 layout = CustomLayout()
 layout?.itemSize = CGSize(width: SCREEN_WIDTH-80, height: SCREEN_HEIGHT-64-120)

 let rect = CGRect(x: 0, y: 64, width:SCREEN_WIDTH , height: SCREEN_HEIGHT-64)
 collectionView = UICollectionView(frame: rect, collectionViewLayout: layout!)
 collectionView?.delegate = self
 collectionView?.dataSource = self
 view.addSubview(collectionView!)
 collectionView?.register(CustomViewCell.self, forCellWithReuseIdentifier: "identifier")
 collectionView?.backgroundColor = UIColor.red

 }

// MARK: -- delegate and datasource
extension ViewController:
UICollectionViewDelegate,
UICollectionViewDataSource,
UICollectionViewDelegateFlowLayout{

 func numberOfSections(in collectionView: UICollectionView) -> Int {
 return 1
 }

 func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
 return 10
 }
 func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
//CustomViewCell是自定义的cell
 let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "identifier", for: indexPath) as! CustomViewCell
 cell.backgroundColor = UIColor.orange
 cell.lable?.text = "\(indexPath.row)/\(10)"
 return cell
 }
}

//重写prepare方法
 //布局之前的准备工作 初始化 这个方法每次layout发生改变就调用一次
override func prepare() {
  scrollDirection = UICollectionViewScrollDirection.horizontal
   minimumLineSpacing = 20.0
  sectionInset = UIEdgeInsets(top: 0, left: 40, bottom: 0, right: 40)
  super.prepare()
 }

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

 override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
  //根据当前滚动进行对每个cell进行缩放
  //首先获取 当前rect范围内的 attributes对象
  let array = super.layoutAttributesForElements(in: rect)
  private let ScaleFactor:CGFloat = 0.001//缩放因子
  //计算缩放比 首先计算出整体中心点的X值 和每个cell的中心点X的值
  //用着两个x值的差值 ，计算出绝对值
  //colleciotnView中心点的值
  let centerX = (collectionView?.contentOffset.x)! + (collectionView?.bounds.size.width)!/2
  //循环遍历每个attributes对象 对每个对象进行缩放
  for attr in array! {
   //计算每个对象cell中心点的X值
   let cell_centerX = attr.center.x

   //计算两个中心点的便宜（距离）
   //距离越大缩放比越小，距离小 缩放比越大，缩放比最大为1，即重合
   let distance = abs(cell_centerX-centerX)
   let scale:CGFloat = 1/(1+distance*ScaleFactor)
   attr.transform3D = CATransform3DMakeScale(1.0, scale, 1.0)

  }

  return array
 }

/// <#Description#>
 ///
 /// - Parameter proposedContentOffset: 当手指滑动的时候 最终的停止的偏移量
 /// - Returns: 返回最后停止后的点
 override func targetContentOffset(forProposedContentOffset proposedContentOffset: CGPoint, withScrollingVelocity velocity: CGPoint) -> CGPoint {
  let visibleX = proposedContentOffset.x
  let visibleY = proposedContentOffset.y
  let visibleW = collectionView?.bounds.size.width
  let visibleH = collectionView?.bounds.size.height
  //获取可视区域
  let targetRect = CGRect(x: visibleX, y: visibleY, width: visibleW!, height: visibleH!)

  //中心点的值
  let centerX = proposedContentOffset.x + (collectionView?.bounds.size.width)!/2

  //获取可视区域内的attributes对象
  let attrArr = super.layoutAttributesForElements(in: targetRect)!
  //如果第0个属性距离最小
  var min_attr = attrArr[0]
  for attributes in attrArr {
   if (abs(attributes.center.x-centerX) < abs(min_attr.center.x-centerX)) {
    min_attr = attributes
   }
  }
  //计算出距离中心点 最小的那个cell 和整体中心点的偏移
  let ofsetX = min_attr.center.x - centerX
  return CGPoint(x: proposedContentOffset.x+ofsetX, y: proposedContentOffset.y)
 }