UICollectionViewCompositionLayout 具有自动调整大小的单元格，包含固定比例 UIImageView

如何解决UICollectionViewCompositionLayout 具有自动调整大小的单元格，包含固定比例 UIImageView

目标是为包含固定纵横比 UICollectionViewCompositionalLayout 的全角单元格配置 UIImageView（以及下面的一些其他视图，包括可变高度 UILabels 和 {{1 }}s) 并让单元格正确自动调整大小。我天真的尝试不起作用，我不知道为什么。我在这里展示的简单案例在运行时给了我一堆约束错误。

我抄袭了 this answer 中的代码，但它看起来很简单：

UIStackView

约束看起来像这样：

一旦我滚动，我就会收到这些约束错误：

    override func viewDidLoad() {
        super.viewDidLoad()

        let size = NSCollectionLayoutSize(
            widthDimension: NSCollectionLayoutDimension.fractionalWidth(1),heightDimension: NSCollectionLayoutDimension.estimated(100)
        )
        let item = NSCollectionLayoutItem(layoutSize: size)
        let group = NSCollectionLayoutGroup.horizontal(layoutSize: size,subitem: item,count: 1)

        let section = NSCollectionLayoutSection(group: group)
        section.interGroupSpacing = 10
        
        let layout = UICollectionViewCompositionalLayout(section: section)

        collectionView.collectionViewLayout = layout
    }

似乎 AutoLayout 正在创建与纵横比约束冲突的 2021-01-21 20:29:18.311491+0100 CompositionalLayoutDynamicHeight[96399:2597528] [LayoutConstraints] Unable to simultaneously satisfy constraints. Probably at least one of the constraints in the following list is one you don't want. Try this: (1) look at each constraint and try to figure out which you don't expect; (2) find the code that added the unwanted constraint or constraints and fix it. ( "<NSLayoutConstraint:0x6000020a82d0 UIImageView:0x138818600.width == 1.33333*UIImageView:0x138818600.height (active)>","<NSLayoutConstraint:0x6000020a8460 V:|-(0)-[UIImageView:0x138818600] (active,names: '|':UIView:0x13881c4d0 )>","<NSLayoutConstraint:0x6000020a84b0 H:[UIImageView:0x138818600]-(0)-| (active,"<NSLayoutConstraint:0x6000020a8500 V:[UIImageView:0x138818600]-(0)-| (active,"<NSLayoutConstraint:0x6000020a8550 H:|-(0)-[UIImageView:0x138818600] (active,"<NSLayoutConstraint:0x6000020a85a0 'UIIBSystemGenerated' CompositionalLayoutDynamicHeight.CollectionViewCell:0x13881c280.leading == UIView:0x13881c4d0.leading (active)>","<NSLayoutConstraint:0x6000020a85f0 'UIIBSystemGenerated' H:[UIView:0x13881c4d0]-(0)-| (active,names: '|':CompositionalLayoutDynamicHeight.CollectionViewCell:0x13881c280 )>","<NSLayoutConstraint:0x6000020a8640 'UIIBSystemGenerated' CompositionalLayoutDynamicHeight.CollectionViewCell:0x13881c280.top == UIView:0x13881c4d0.top (active)>","<NSLayoutConstraint:0x6000020a8690 'UIIBSystemGenerated' V:[UIView:0x13881c4d0]-(0)-| (active,"<NSLayoutConstraint:0x6000020a60d0 'UIView-Encapsulated-Layout-Height' CompositionalLayoutDynamicHeight.CollectionViewCell:0x13881c280.height == 292.667 (active)>","<NSLayoutConstraint:0x6000020a6120 'UIView-Encapsulated-Layout-Width' CompositionalLayoutDynamicHeight.CollectionViewCell:0x13881c280.width == 390 (active)>" ) Will attempt to recover by breaking constraint <NSLayoutConstraint:0x6000020a82d0 UIImageView:0x138818600.width == 1.33333*UIImageView:0x138818600.height (active)> Make a symbolic breakpoint at UIViewAlertForUnsatisfiableConstraints to catch this in the debugger. The methods in the UIConstraintBasedLayoutDebugging category on UIView listed in <UIKitCore/UIView.h> may also be helpful. 约束——但为什么不只是调整大小以适应它？

解决方法

在为 Collection 视图（和 Table 视图）布置 UI 元素时，自动布局会进行多次“传递”。

经常，尤其是在使用可变大小的单元格内容时，自动布局会在遍历约束时发出警告。

可能消除该警告的最简单方法是为您的纵横比约束设置小于必需的 Priority。

如果您将其设置为 Default High (750)，则表示允许自动布局“破坏”它而不抛出警告（但仍会应用）。