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

Question

目标是为包含固定纵横比UIImageView（加上下面的一些其他视图，包括可变高度UILabels 和UIStackViews）的全宽单元格配置UICollectionViewCompositionalLayout，并让单元格适当地自动调整大小。我的天真尝试不起作用，我不知道为什么。我在这里介绍的简单案例在运行时给了我一堆约束错误。

我抄袭了this answer 的代码，但看起来很简单：

    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
    }

约束如下所示：

一旦我滚动，我就会得到这些约束错误：

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, names: '|':UIView:0x13881c4d0 )>",
    "<NSLayoutConstraint:0x6000020a8500 V:[UIImageView:0x138818600]-(0)-|   (active, names: '|':UIView:0x13881c4d0 )>",
    "<NSLayoutConstraint:0x6000020a8550 H:|-(0)-[UIImageView:0x138818600]   (active, names: '|':UIView:0x13881c4d0 )>",
    "<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, names: '|':CompositionalLayoutDynamicHeight.CollectionViewCell:0x13881c280 )>",
    "<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.

似乎 AutoLayout 正在创建与纵横比约束冲突的 UIView-Encapsulated-Layout-Height 约束，但为什么不只是调整大小以适应它呢？

Answer 1

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

通常，尤其是在使用可变大小的单元格内容时，自动布局会在通过约束时引发警告。

消除该警告的最简单方法可能是为您的纵横比约束提供一个小于必需的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）。

Answer 2

我在具有动态项目宽度的水平滑块上遇到了同样的问题。我在我的案例中找到了解决方案，这对我来说似乎是一个错误。

有两个函数可以创建NSCollectionLayoutGroup。使用另一个为我修复了它。

这打破了我的限制：

let group = NSCollectionLayoutGroup.horizontal(layoutSize: size, subitem: item, count: 1)

这不是：

let group = NSCollectionLayoutGroup.horizontal(layoutSize: size, subitems: [item])