我想了解为什么 Collection View 仅将集合中的最后一个单元格居中对齐。
我创建了一个简单的基于 Flow 布局的 Collection View 。我正在使用 Autolayout 标志 - 我不确定这是导致此问题的原因。

每当我从 Collection View 中删除一个单元格时 - 前几个单元格似乎工作正常并向左滚动。但是，当我删除倒数第二个时，最后一个单元格突然从左对齐变为居中对齐。 有人有解释吗？看起来很奇怪。

如何使所有单元格向左滚动并保持向左对齐。

编辑:这里是 View 层次调试: http://imgur.com/a/NxidO

这是 View 行为

我做了一个简单的 github 来演示它:https://github.com/grantkemp/CollectionViewIssue

代码如下:

        var dataforCV = ["short","longer phrase", "Super long Phrase"]

override func viewDidLoad() {
        super.viewDidLoad()
        demoCollectionView.reloadData()
    let layout = UICollectionViewFlowLayout()

    // Bug Description  - > UICollectionViewFlowLayoutAutomaticSize will center Align the layout if it has only a single cell - but it will left align the content if there is more than one cell.

    // I would expect this behaviour to be consistently left aligning the content.

    //How to repeat: If you comment out UICollection​View​Flow​Layout​Automatic​Size then the collection view will show 3 cells being left aligned and it will continue to left align all the content no matter how many cells you remove.

    // But: if you leave turn UICollection​View​Flow​Layout​Automatic​Size on - then it will intially show 3 cells being left aligned, and then as you click to remove each cell they will stay left aligned until the last single cell will suddenly center align in the collection view

    //  see here for the screen recording:/image/bledY.gif
    //  see here for the view hierachy debuggins screen: http://imgur.com/a/NxidO

    layout.estimatedItemSize = UICollectionViewFlowLayoutAutomaticSize


    // <-- End Bug Description
    demoCollectionView.collectionViewLayout = layout
}

    //MARk: CollectionView

    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "cell", for: indexPath) as? collectionCell
        cell?.text.text  = dataforCV[indexPath.row]

        return cell!

    }

    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return dataforCV.count
    }
    //Remove the item from the array if selected
    func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
        dataforCV.remove(at: indexPath.row)
        demoCollectionView.deleteItems(at: [indexPath])
    }

最佳答案

我设法通过以下两个步骤解决了这个问题:

1. 向 Apple 记录了一个错误，关于我注意到使用 UICollectionViewFlowLayoutAutomaticSize
时单元格行为发生变化的奇怪行为
2. 我通过创建一个修改过的 CustomViewFlowLayout 做了一个解决方法(找不到我看到使用这种方法的原始 SO/github 问题 - 如果找到它我会添加它)然后我添加了UICollectionViewFlowLayoutAutomaticSize 设置 - 它运行良好。

示例代码:

class MyLeftCustomFlowLayout:UICollectionViewFlowLayout {
    override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {

        let attributes = super.layoutAttributesForElements(in: rect)

        var leftMargin = sectionInset.left
        var maxY: CGFloat = 2.0

        let horizontalSpacing:CGFloat = 5

        attributes?.forEach { layoutAttribute in
            if layoutAttribute.frame.origin.y >= maxY
                || layoutAttribute.frame.origin.x == sectionInset.left {
                leftMargin = sectionInset.left
            }

            if layoutAttribute.frame.origin.x == sectionInset.left {
                leftMargin = sectionInset.left
            }
            else {
                layoutAttribute.frame.origin.x = leftMargin
            }

            leftMargin += layoutAttribute.frame.width + horizontalSpacing
            maxY = max(layoutAttribute.frame.maxY, maxY)
        }

        return attributes
    }

在 ViewController 中 - 您必须将流布局添加到 Collection View 以使其工作:

let layout = MyLeftCustomFlowLayout() 
layout.estimatedItemSize = UICollectionViewFlowLayoutAutomaticSize
myCollectionViewReference.collectionViewLayout = layout

我认为 Collection View 的实现绝对可以简化，但我将进一步研究它，因为我可以看到它的强大之处。

关于ios - 停止单个 UICollectionView 单元格流向屏幕中央，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/43266924/