Question

背景

因此，我正在开发一个自定义框架，并且为UICollectionViewFlowLayout实现了一个自定义UICollectionView。该实现使您可以滚动卡堆栈，同时还可以向左/向右滑动卡（单元）（Tinder + Shazam Discover组合）。

我正在修改UICollectionViewLayoutAttributes以创建滚动卡堆栈效果。

问题

在堆栈末尾，当我滑动一张卡（单元）时，新卡不是从堆栈后面出现，而是从顶部出现。堆栈末端，我不知道为什么。

我的想法-我尝试过的事情

我的猜测是我需要修改initialLayoutAttributesForAppearingItem中的某些内容，我已经尝试过了，但是似乎没有任何作用。

我当前正在调用其中的updateCellAttributes函数来更新属性，但是我也尝试过手动修改其中的属性。我真的没有在这里看到这个问题，除非有另一种方法可以修改这种情况下卡片的位置。

也许是因为从技术上讲这些单元还不在“矩形”中（请参阅layoutAttributesForElements(in rect: CGRect)），所以它们没有被更新吗？

我有什么想念的吗？有谁更熟悉如何修改布局以实现所需的行为？

示例和代码

以下是它的实际效果：

以下是我要解决的错误的gif图像：

如您所见，刷掉最后一张卡时，新卡从顶部出现，而应该从前一张卡的后面出现。

下面您可以找到自定义UICollectionViewFlowLayout代码。最重要的功能是updateCellAttributes，内嵌注释已很好地记录在案（请参见下面的代码）。该函数从以下位置调用：
initialLayoutAttributesForAppearingItem
finalLayoutAttributesForDisappearingItem
layoutAttributesForItem
layoutAttributesForElements
修改布局信息并创建堆栈效果。

import UIKit

/// Custom `UICollectionViewFlowLayout` that provides the flowlayout information like paging and `CardCell` movements.
internal class VerticalCardSwiperFlowLayout: UICollectionViewFlowLayout {

    /// This property sets the amount of scaling for the first item.
    internal var firstItemTransform: CGFloat?
    /// This property enables paging per card. Default is true.
    internal var isPagingEnabled: Bool = true
    /// Stores the height of a CardCell.
    internal var cellHeight: CGFloat!
    /// Allows you to make the previous card visible or not visible (stack effect). Default is `true`.
    internal var isPreviousCardVisible: Bool = true

    internal override func prepare() {
        super.prepare()

        assert(collectionView?.numberOfSections == 1, "Number of sections should always be 1.")
        assert(collectionView?.isPagingEnabled == false, "Paging on the collectionview itself should never be enabled. To enable cell paging, use the isPagingEnabled property of the VerticalCardSwiperFlowLayout instead.")
    }

    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]
    }

    internal override func layoutAttributesForItem(at indexPath: IndexPath) -> UICollectionViewLayoutAttributes? {

        if self.collectionView?.numberOfItems(inSection: 0) == 0 { return nil }

        if let attr = super.layoutAttributesForItem(at: indexPath)?.copy() as? UICollectionViewLayoutAttributes {
            self.updateCellAttributes(attr)
            return attr
        }
        return nil
    }

    internal override func finalLayoutAttributesForDisappearingItem(at itemIndexPath: IndexPath) -> UICollectionViewLayoutAttributes? {
        // attributes for swiping card away
        return self.layoutAttributesForItem(at: itemIndexPath)
    }

    internal override func initialLayoutAttributesForAppearingItem(at itemIndexPath: IndexPath) -> UICollectionViewLayoutAttributes? {
        // attributes for adding card
        return self.layoutAttributesForItem(at: itemIndexPath)
    }

    // We invalidate the layout when a "bounds change" happens, for example when we scale the top cell. This forces a layout update on the flowlayout.
    internal override func shouldInvalidateLayout(forBoundsChange newBounds: CGRect) -> Bool {
        return true
    }

    // Cell paging
    internal override func targetContentOffset(forProposedContentOffset proposedContentOffset: CGPoint, withScrollingVelocity velocity: CGPoint) -> CGPoint {

        // If the property `isPagingEnabled` is set to false, we don't enable paging and thus return the current contentoffset.
        guard let collectionView = self.collectionView, isPagingEnabled else {
            let latestOffset = super.targetContentOffset(forProposedContentOffset: proposedContentOffset, withScrollingVelocity: velocity)
            return latestOffset
        }

        // Page height used for estimating and calculating paging.
        let pageHeight = cellHeight + self.minimumLineSpacing

        // Make an estimation of the current page position.
        let approximatePage = collectionView.contentOffset.y/pageHeight

        // Determine the current page based on velocity.
        let currentPage = (velocity.y < 0.0) ? floor(approximatePage) : ceil(approximatePage)

        // Create custom flickVelocity.
        let flickVelocity = velocity.y * 0.4

        // Check how many pages the user flicked, if <= 1 then flickedPages should return 0.
        let flickedPages = (abs(round(flickVelocity)) <= 1) ? 0 : round(flickVelocity)

        // Calculate newVerticalOffset.
        let newVerticalOffset = ((currentPage + flickedPages) * pageHeight) - collectionView.contentInset.top

        return CGPoint(x: proposedContentOffset.x, y: newVerticalOffset)
    }

    /**
     Updates the attributes.
     Here manipulate the zIndex of the cells 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.
     */
    private 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.
    private func transformAttributes(attributes: UICollectionViewLayoutAttributes, deltaY: CGFloat) {

        if let itemTransform = firstItemTransform {

            let scale = 1 - deltaY * itemTransform
            let translationScale = CGFloat((attributes.zIndex + 1) * 10)
            var t = CGAffineTransform.identity

            t = t.scaledBy(x: scale, y: 1)
            if isPreviousCardVisible {
                t = t.translatedBy(x: 0, y: (deltaY * translationScale))
            }
            attributes.transform = t
        }
    }
}

Full project zip（立即下载）

Github repo

Github issue

如果您还有其他问题，我们很乐意为您解答。感谢您的时间和精力，我们将不胜感激！

Answer 1

似乎删除最后一个单元格后，我们得到了两个动画同时发生。内容提示（由于内容大小更改）随动画而变化，新的最后一个单元格移到其新位置。但是新的可见细胞已经处于其位置。遗憾的是，但我看不出解决此问题的快捷方法。

Answer 2

首先，您应该了解super.layoutAttributesForElements(in: rect)仅返回标准FlowLayout中可见的单元格。因此，当您在底部跳UICollectionView时，可以看到顶部卡下方的卡消失的原因。这就是为什么您应该自己管理属性的原因。我的意思是将所有属性复制到prepare()中，甚至创建它们。 @ team-orange描述了另一个问题。他是正确的，UIKit的动画类将其作为简单的动画处理，并且在您的逻辑中，您将基于当前contentOffset计算单元格的位置，当前contentOffset在动画块中已经更改。我不确定您在这里真正能做什么，也许您可以通过设置所有单元格的更新属性来实现它，但是即使使用isHidden = true，它也会降低性能。

<VerticalCardSwiper.VerticalCardSwiperView: 0x7f9a63810600; baseClass = UICollectionView; contentOffset: {-20, 13636}; contentSize: {374, 14320}; adjustedContentInset: {40, 20, 124, 20}>
<VerticalCardSwiper.VerticalCardSwiperView: 0x7f9a63810600; baseClass = UICollectionView; contentOffset: {-20, 12918}; contentSize: {374, 14320}; adjustedContentInset: {40, 20, 124, 20}>

如何更改UICollectionViewCell的初始布局位置？

背景

问题

我的想法-我尝试过的事情

示例和代码

2 个答案: