Ios swift定位中心UICollectionViewCell

我正在尝试实现“快速生效”，但无法使其工作，是否有什么地方我做错了

子类化

UICollectionViewFlowLayout

并重写

targetContentOffsetForProposedContentOffset

函数：

override func targetContentOffsetForProposedContentOffset(proposedContentOffset: CGPoint, withScrollingVelocity velocity: CGPoint) -> CGPoint {

    var offsetAdjustment:CGFloat = CGFloat(MAXFLOAT)
    let verticalCenter:CGFloat = proposedContentOffset.y + (CGRectGetHeight(self.collectionView!.bounds) / 2.0)
    let proposedRect:CGRect = CGRectMake(proposedContentOffset.x,0.0,self.collectionView!.bounds.size.width,self.collectionView!.bounds.size.height)

    let array:NSArray = self.layoutAttributesForElementsInRect(proposedRect)!

    for layoutAttributes : AnyObject in array {

        if let _layoutAttributes = layoutAttributes as? UICollectionViewLayoutAttributes {

            if _layoutAttributes.representedElementCategory != UICollectionElementCategory.Cell {
                continue
            }

            let itemVerticalCenter:CGFloat = layoutAttributes.center.y

            let _verticalCenter = fabsf(Float(itemVerticalCenter) - Float(verticalCenter))
            let _offsetAdjustment = fabsf(Float(offsetAdjustment))

            if (_verticalCenter < _offsetAdjustment) {
                offsetAdjustment = itemVerticalCenter - verticalCenter
            }
        }
    }
    return CGPointMake(proposedContentOffset.x + offsetAdjustment, proposedContentOffset.y)
}

覆盖func targetContentOffsetForProposedContentOffset（proposedContentOffset:CGPoint，使用CrollingVelocity:CGPoint）->CGPoint{
var补偿调整：CGFloat=CGFloat（MAXFLOAT）
让verticalCenter:CGFloat=proposedContentOffset.y+（CGRectGetHeight（self.collectionView！.bounds）/2.0）
让proposedRect:CGRect=CGRectMake（proposedContentOffset.x，0.0，self.collectionView！.bounds.size.width，self.collectionView！.bounds.size.height）
让数组：NSArray=self.layouttributesforementsinrect（proposedRect）！
对于LayoutAttribute：数组中的任意对象{
如果让_layouttributes=layouttributes作为UICollectionViewLayouttributes{
如果_layouttributes.representedElementCategory！=UICollectionElementCategory.Cell{
持续
}
让itemVerticalCenter:CGFloat=layouttributes.center.y
设_verticalCenter=fabsf（Float（itemVerticalCenter）-Float（verticalCenter））
设_offsetAdjustment=fabsf（Float（offsetAdjustment））
如果（_垂直中心<_偏移调整）{
偏移调整=项目垂直中心-垂直中心
}
}
}
返回CGPointMake（proposedContentOffset.x+offsetAdjustment，proposedContentOffset.y）
}

---

最后我终于找到了答案

override func targetContentOffsetForProposedContentOffset(proposedContentOffset: CGPoint, withScrollingVelocity velocity: CGPoint) -> CGPoint {

    let rectBounds:CGRect = self.collectionView!.bounds
    let halfHeight:CGFloat = rectBounds.size.height * CGFloat(0.45)
    let proposedContentOffsetCenterY:CGFloat = proposedContentOffset.y + halfHeight

    let attributesArray:NSArray = self.layoutAttributesForElementsInRect(rectBounds)!

    var candidateAttributes:UICollectionViewLayoutAttributes?

    for layoutAttributes : AnyObject in attributesArray {

        if let _layoutAttributes = layoutAttributes as? UICollectionViewLayoutAttributes {

            if _layoutAttributes.representedElementCategory != UICollectionElementCategory.Cell {
                continue
            }

            if candidateAttributes == nil {
                candidateAttributes = _layoutAttributes
                self.delegate?.cellCenteredAtIndexPathWithCollectionView(self.collectionView!, layout: self, indexPath: candidateAttributes!.indexPath)
                continue
            }

            if fabsf(Float(_layoutAttributes.center.y) - Float(proposedContentOffsetCenterY)) < fabsf(Float(candidateAttributes!.center.y) - Float(proposedContentOffsetCenterY)) {
                candidateAttributes = _layoutAttributes
                self.delegate?.cellCenteredAtIndexPathWithCollectionView(self.collectionView!, layout: self, indexPath: candidateAttributes!.indexPath)
            }
        }
    }

    if attributesArray.count == 0 {
        return CGPointMake(proposedContentOffset.x,proposedContentOffset.y - halfHeight * 2)
    }

    return CGPointMake(proposedContentOffset.x,candidateAttributes!.center.y - halfHeight)
}

代表本人：

protocol MyFlowLayoutDelegate
{
    func cellCenteredAtIndexPathWithCollectionView(collectionView: UICollectionView, layout: UICollectionViewLayout, indexPath: NSIndexPath) -> Void
}

protocol MyFlowLayoutDelegate
{
    func cellCenteredAtIndexPathWithCollectionView(collectionView: UICollectionView, layout: UICollectionViewLayout, indexPath: NSIndexPath) -> Void
}