子类化UICollectionViewFlowLayout-在iOS 12中自动调整单元格框架计算问题

Question

我正在使用以下命令将UICollectionView中的单元格左对齐…

class LeftAlignedFlowLayout: UICollectionViewFlowLayout {
    override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
        guard let attributes = super.layoutAttributesForElements(in: rect) else { return nil }

        for attributes in attributes where attributes.representedElementCategory == .cell {
            attributes.frame = layoutAttributesForItem(at: attributes.indexPath).frame
        }
        return attributes
    }

    override func layoutAttributesForItem(at indexPath: IndexPath) -> UICollectionViewLayoutAttributes {
        let currentItemAttributes = super.layoutAttributesForItem(at: indexPath)!

        let layoutWidth = collectionView!.frame.width - sectionInset.left - sectionInset.right

        // Just left align the first item in a section
        if indexPath.item == 0 {
            currentItemAttributes.frame.origin.x = sectionInset.left
            return currentItemAttributes
        }

        let previousIndexPath = IndexPath(item: indexPath.item - 1, section: indexPath.section)
        let previousItemFrame = layoutAttributesForItem(at: previousIndexPath).frame

        let currentItemFrame = currentItemAttributes.frame
        let wholeRowFrameForCurrentItem = CGRect(x: sectionInset.left, y: currentItemFrame.minY, width: layoutWidth, height: currentItemFrame.height)

        // Left align the first item on a row
        if !previousItemFrame.intersects(wholeRowFrameForCurrentItem) {
            currentItemAttributes.frame.origin.x = sectionInset.left
            return currentItemAttributes
        }

        currentItemAttributes.frame.origin.x = previousItemFrame.maxX + minimumInteritemSpacing
        return currentItemAttributes, it's 
    }
}

iOS 11的构建提供了……

但是对于iOS 12来说……

在iOS 11中，layoutAttributesForElements(in)被调用3次-最终时间为正确的帧大小。在iOS 12中，它只被调用了两次，帧是估计的大小。

按照这个答案，https://stackoverflow.com/a/52148520/123632我尝试使用下面的UICollectionView子类（和包含视图控制器的viewDidAppear中的“蛮力”）使流布局无效，但是结果不变。

class LeftAlignedCollectionView: UICollectionView {

    private var shouldInvalidateLayout = false

    override func layoutSubviews() {
        super.layoutSubviews()
        if shouldInvalidateLayout {
            collectionViewLayout.invalidateLayout()
            shouldInvalidateLayout = false
        }
    }

    override func reloadData() {
        shouldInvalidateLayout = true
        super.reloadData()
    }
}

Answer 1

这是iOS 12中的一个已知问题（请参见https://developer.apple.com/documentation/ios_release_notes/ios_12_release_notes的UIKit部分）

解决方法是避免调用setNeedsUpdateConstraints()之前先调用updateConstraintsIfNeeded()或先调用systemLayoutSizeFitting(_:)。

Answer 2

我找到了解决方案。将contentView约束到该单元格即可对其进行修复...

override func awakeFromNib() {
    contentView.translatesAutoresizingMaskIntoConstraints = false

    NSLayoutConstraint.activate([contentView.leftAnchor.constraint(equalTo: leftAnchor),
                                 contentView.rightAnchor.constraint(equalTo: rightAnchor),
                                 contentView.topAnchor.constraint(equalTo: topAnchor),
                                 contentView.bottomAnchor.constraint(equalTo: bottomAnchor)])
}

Answer 3

我将此添加到MyCustomCollectionViewCell文件中，并且效果很好

override func systemLayoutSizeFitting(_ targetSize: CGSize, withHorizontalFittingPriority horizontalFittingPriority: UILayoutPriority, verticalFittingPriority: UILayoutPriority) -> CGSize {

    return contentView.systemLayoutSizeFitting(CGSize(width: targetSize.width, height: 1))
}