在使用自定义布局的UICollectionView中遇到collectionViewContentSize时遇到问题

Question

我的UICollectionView中有40个单元格，如果我想滚动到特定单元格（通过单击按钮），我会遇到问题。

它永远不会进入理想的细胞。

它总是关闭一两个单元格。

这是项目的链接： https://www.dropbox.com/sh/jey73k0tjubx9ld/AAA-RSpqmP18DiOuGRNDNHvaa?dl=0

class VerticalLayout: UICollectionViewFlowLayout {


 private var cache = [UICollectionViewLayoutAttributes]()
  var contentWidth: CGFloat = 0

     override func collectionViewContentSize() -> CGSize {

        return CGSize(width: contentWidth  , height: CGFloat(statics.height))
 }




  override func     targetContentOffsetForProposedContentOffset(proposedContentOffset: CGPoint, withScrollingVelocity velocity: CGPoint) -> CGPoint {
    let midWidth = collectionView!.bounds.size.width * 0.5
    let centerX = proposedContentOffset.x + midWidth
    let proposedRect = self.collectionView!.bounds

     var candidateAttributes: UICollectionViewLayoutAttributes?
       for attributes in layoutAttributesForElementsInRect(proposedRect)! {
       // set candidate if none
       if candidateAttributes == nil {
     candidateAttributes = attributes as?   UICollectionViewLayoutAttributes

        // else set if better match
       } else if let
         candidate = candidateAttributes,
         attributes = attributes as? UICollectionViewLayoutAttributes
         where fabs(attributes.center.x - centerX) < fabs(candidate.center.x - centerX) {
         candidateAttributes = attributes
    }
  }
    if let candidateAttributes = candidateAttributes {
      let lowerBound: CGFloat = 0
      let upperBound: CGFloat = cache.last!.frame.origin.x -   cache.first!.frame.origin.x - 1
      return CGPoint(
        x: min(upperBound, max(lowerBound, candidateAttributes.center.x - midWidth)),
        y: proposedContentOffset.y
  )
    } else {
      return proposedContentOffset
   }
 }


  override func prepareLayout() {
     super.prepareLayout()

      self.scrollDirection = .Horizontal

      let start = Int(collectionView!.bounds.size.width * 0.6 ) - statics.width / 2
      let length = collectionView!.numberOfItemsInSection(0)



     if cache.isEmpty && length > 0 {
        for item in 0..<length {
        let indexPath = NSIndexPath(forItem: item, inSection: 0)
        let attributes =   UICollectionViewLayoutAttributes(forCellWithIndexPath: indexPath)
        attributes.frame = CGRect(
          x: start + (statics.width + statics.padding ) * indexPath.item ,
          y: 0,
          width: statics.width ,
          height: statics.height
    )
       cache.append(attributes)
  }
     contentWidth = CGRectGetMaxX(cache.last!.frame) + CGFloat(start)
}

  }



      // 3
       override func layoutAttributesForElementsInRect(rect: CGRect) -> [AnyObject]? {
        var layoutAttributes = [UICollectionViewLayoutAttributes]()
        for attributes in cache {
        if CGRectIntersectsRect(attributes.frame, rect) {
        layoutAttributes.append(attributes)
    }
 }
    println(layoutAttributes)
    return layoutAttributes
  }



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