swift 定位中心 UICollectionViewCell

Question

我正在尝试实现“快速生效”但无法正常工作，是不是我做错了什么？

继承 UICollectionViewFlowLayout 并覆盖 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)
}

Sample project here and snap effect, desired output

Answer 1

我终于找到了

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)
}

另外不要忘记设置委托：

class MyCostomFlow: UICollectionViewFlowLayout, UICollectionViewDelegateFlowLayout {

    var delegate:MyFlowLayoutDelegate?
}

和代表本身：

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

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