UICollectionView scrollToItem() 滚动到上一个单元格无法正常工作

Question

在过去的几周里，我一直在为此烦恼，但我没有想法..

短背景

我用自定义的UICollectionViewFlowLayout 构建了自己的UICollectionView 框架，用于垂直卡片分页和创建卡片堆叠效果。

这是它的 gif 图像：

问题

我正在尝试实现一项功能，该功能允许用户滚动到特定索引处的特定卡片（例如使用scrollToItem 函数）。

现在问题是，由于某种原因，尝试滚动回上一张卡片时无法正常工作。

下面是发生的事情的记录，我已经附加了一个 tapGestureRecognizer 到它，当我点击聚焦（当前居中）的卡片时，它应该滚动到那个索引 - 1（所以上一个卡片）。
由于某种原因，只有当底部卡片的 frame.origin.y 高于焦点（当前居中卡片）的 frame.origin.maxY 时，它才会这样做。

请注意，我在卡片上敲击两次以使其工作，因为底部卡片的 frame.origin.y 值需要低于顶部卡片的 frame.origin.maxY 值，因为某种原因它可以工作。

到目前为止我已经尝试过的基本答案

collectionView.scrollToItem(at: convertIndexToIndexPath(for: index), at: .top, animated: animated)

和

if let frame = collectionView.layoutAttributesForItem(at: convertIndexToIndexPath(for: index))?.frame {
    collectionView.scrollRectToVisible(frame, animated: true)
}

需要了解的一些重要事项

我已经弄清楚是哪一行导致了这个问题，在UICollectionViewFlowLayout（称为VerticalCardSwiperFlowLayout）的子类中，有一个名为updateCellAttributes 的函数，它修改了框架的一些属性以便能够达到这个效果。问题出现在以下行：

let finalY = max(cvMinY, cardMinY)

下面是有问题的函数，包括在代码 cmets 顶部如何工作的非常详细的解释：

/**
 Updates the attributes.
 Here manipulate the zIndex of the cards here, calculate the positions and do the animations.
 
 Below we'll briefly explain how the effect of scrolling a card to the background instead of the top is achieved.
 Keep in mind that (x,y) coords in views start from the top left (x: 0,y: 0) and increase as you go down/to the right,
 so as you go down, the y-value increases, and as you go right, the x value increases.
 
 The two most important variables we use to achieve this effect are cvMinY and cardMinY.
 * cvMinY (A): The top position of the collectionView + inset. On the drawings below it's marked as "A".
 This position never changes (the value of the variable does, but the position is always at the top where "A" is marked).
 * cardMinY (B): The top position of each card. On the drawings below it's marked as "B". As the user scrolls a card,
 this position changes with the card position (as it's the top of the card).
 When the card is moving down, this will go up, when the card is moving up, this will go down.
 
 We then take the max(cvMinY, cardMinY) to get the highest value of those two and set that as the origin.y of the card.
 By doing this, we ensure that the origin.y of a card never goes below cvMinY, thus preventing cards from scrolling upwards.
 
 ```
 +---------+   +---------+
 |         |   |         |
 | +-A=B-+ |   |  +-A-+  | ---> The top line here is the previous card
 | |     | |   | +--B--+ |      that's visible when the user starts scrolling.
 | |     | |   | |     | |
 | |     | |   | |     | |  |  As the card moves down,
 | |     | |   | |     | |  v  cardMinY ("B") goes up.
 | +-----+ |   | |     | |
 |         |   | +-----+ |
 | +--B--+ |   | +--B--+ |
 | |     | |   | |     | |
 +-+-----+-+   +-+-----+-+
 ```
 
 - parameter attributes: The attributes we're updating.
 */
fileprivate func updateCellAttributes(_ attributes: UICollectionViewLayoutAttributes) {
    
    guard let collectionView = collectionView else { return }
    
    var cvMinY = collectionView.bounds.minY + collectionView.contentInset.top
    let cardMinY = attributes.frame.minY
    var origin = attributes.frame.origin
    let cardHeight = attributes.frame.height
            
    if cvMinY > cardMinY + cardHeight + minimumLineSpacing + collectionView.contentInset.top {
        cvMinY = 0
    }
    
    let finalY = max(cvMinY, cardMinY)
    
    let deltaY = (finalY - cardMinY) / cardHeight
    transformAttributes(attributes: attributes, deltaY: deltaY)
    
    // Set the attributes frame position to the values we calculated
    origin.x = collectionView.frame.width/2 - attributes.frame.width/2 - collectionView.contentInset.left
    origin.y = finalY
    attributes.frame = CGRect(origin: origin, size: attributes.frame.size)
    attributes.zIndex = attributes.indexPath.row
}

// Creates and applies a CGAffineTransform to the attributes to recreate the effect of the card going to the background.
fileprivate func transformAttributes(attributes: UICollectionViewLayoutAttributes, deltaY: CGFloat) {
    
    let translationScale = CGFloat((attributes.zIndex + 1) * 10)
    
    if let itemTransform = firstItemTransform {
        let scale = 1 - deltaY * itemTransform
        
        var t = CGAffineTransform.identity
        
        t = t.scaledBy(x: scale, y: 1)
        if isPreviousCardVisible {
            t = t.translatedBy(x: 0, y: (deltaY * translationScale))
        }
        attributes.transform = t
    }
}

下面是调用该特定代码的函数：

internal override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
    
    let items = NSArray(array: super.layoutAttributesForElements(in: rect)!, copyItems: true)
    for object in items {
        if let attributes = object as? UICollectionViewLayoutAttributes {
            self.updateCellAttributes(attributes)
        }
    }
    return items as? [UICollectionViewLayoutAttributes]
}

如果你想亲自看看，可以在这里下载完整的项目： https://github.com/JoniVR/VerticalCardSwiper/archive/master.zip

（如果你通过 github 结账，请务必下载开发分支）

可以在here 找到特定的 Github 问题以及一些讨论和可能的额外信息。

感谢您抽出宝贵时间，任何有关此问题的帮助或信息将不胜感激！另外，如果您有任何其他问题或缺少任何重要信息，请告诉我

编辑 1： 注意：奇怪的效果只有在滚动到currentIndex - 1时才会发生，currentIndex + 1没有给出任何问题，我认为这在一定程度上解释了为什么它发生在let finalY = max(cvMinY, cardMinY)上，但我还没有找到合适的解决方案.

Answer 1

通过使用setContentOffset 并手动计算偏移量，我设法让它以另一种方式工作。

guard index >= 0 && index < verticalCardSwiperView.numberOfItems(inSection: 0) else { return }

let y = CGFloat(index) * (flowLayout.cellHeight + flowLayout.minimumLineSpacing) - topInset
let point = CGPoint(x: verticalCardSwiperView.contentOffset.x, y: y)
verticalCardSwiperView.setContentOffset(point, animated: animated)

verticalCardSwiperView 基本上只是UICollectionView 的子类，我这样做是因为我想缩小用户可以访问的范围，因为这是一个特定的库，并且允许完全的 collectionView 访问会使事情变得混乱。

如果您确实知道究竟是什么导致了这个问题，或者我可以如何更好地解决它，我仍然很乐意知道:)