简短的背景

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

这是它的样子：

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（例如使用该 scrollToItem功能）。

现在的问题是，由于某种原因，当试图滚动回上一张卡片时，它不能正常工作。

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（所以上一张卡片）。
出于某种原因，它仅frame.origin.y在底部卡片的高于frame.origin.maxY焦点（当前居中的卡片）的上方时才会这样做。

请注意，我在卡上敲击两次以使其起作用，因为出于某种原因，frame.origin.y底部卡的值需要低于顶部卡的值才能起作用。frame.origin.maxY

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)
}

一些重要的事情要知道

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，有一个名为的函数updateCellAttributes，它修改了框架的一些属性以实现这种效果。问题出现在以下行：

let finalY = max(cvMinY, cardMinY)

这是有问题的函数，包括在代码注释顶部对它如何工作的非常扩展的解释：

/**
 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
    }
}

这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，你可以在这里下载完整的项目： https ://github.com/JoniVR/VerticalCardSwiper/archive/master.zip

（如果您通过 github 结帐，请务必下载开发分支）

可以在此处找到带有一些讨论和可能额外信息的特定 Github 问题。

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：奇怪的效果只在滚动到时发生currentIndex - 1，currentIndex + 1不会产生任何问题，我认为这在一定程度上解释了它发生在上的原因let finalY = max(cvMinY, cardMinY)，但我还没有找到合适的解决方案。

标签： iosswiftcocoa-touchuicollectionviewuikit

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

问题描述

简短的背景

问题

到目前为止我已经尝试过的基本答案

一些重要的事情要知道

解决方案

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