永远不会调用UICollectionViewLayout和方法layoutAttributesForItem

Question

我正在尝试实施UICollectionViewFlowLayout或UICollectionViewLayout - 任何方式都不会调用layoutAttributesForItem。

我可以从其他人那里看到他们从layoutAttributesForItem致电self.layoutAttributesForItem。

赞this flowout示例：

我创建了一个你可以看到的Playground项目。总的来说，我只是想扩大中心视野。

Answer 1

  

尝试通过Sub-Classing UICollectionViewFlowLayout

来使用它

let flowLayout = LeftAgignedFlowLayout()
flowLayout.minimumLineSpacing = 18
flowLayout.minimumInteritemSpacing = 8
flowLayout.scrollDirection = .vertical
self.collectionViewLayout = flowLayout



class LeftAgignedFlowLayout : UICollectionViewFlowLayout {        

    override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
        let arr = super.layoutAttributesForElements(in: rect)!
        return arr.map {
            atts in 

            var atts = atts
            if atts.representedElementCategory == .cell {
                let ip = atts.indexPath
                atts = self.layoutAttributesForItem(at:ip)!
            }
            return atts
        }
    }

    override func layoutAttributesForItem(at indexPath: IndexPath) -> UICollectionViewLayoutAttributes? {
        var atts = super.layoutAttributesForItem(at:indexPath)!
        if indexPath.item == 0 {
            return atts 

        }
        if atts.frame.origin.x - 1 <= self.sectionInset.left {
            return atts 

        }
        let ipPv = IndexPath(item:indexPath.row-1, section:indexPath.section)
        let fPv = self.layoutAttributesForItem(at:ipPv)!.frame
        let rightPv = fPv.origin.x + fPv.size.width + self.minimumInteritemSpacing
        atts = atts.copy() as! UICollectionViewLayoutAttributes
        atts.frame.origin.x = rightPv
        return atts
    }

}

Answer 2

所以我遇到了同样的问题，我开始探索苹果的 this 项目，我认为这是一个错误，在 viewWillAppear 中滚动到集合视图中的第一项解决了这个问题。

override func viewWillAppear(_ animated: Bool) {
    super.viewWillAppear(animated)
    if collectionView.numberOfItems(inSection: 0) > 0 {
        collectionView.scrollToItem(at: IndexPath(item: 0, section: 0), at: .top, animated: false)
    }
}