UICollectionViewFlowLayout EstimatedItemSize在旋转期间打破了约束，但itemSize完全可以

Question

我想显示一堆带有图像和标签的UICollection视图单元，并根据标签的宽度调整它们的宽度。 我既不想覆盖sizeThatFits()也不想覆盖preferredLayoutAttributesFitting

据我了解，我需要做的只是确保单元格中的AutoLayout正确，然后使用属性estimatedItemSize使单元格自动调整大小。

不幸的是，我的情况是 kinda 。 UI看起来不错，但是当我旋转设备时，我遇到了很多破损的约束。但是，当我只使用属性itemSize并将所有单元格设置为固定大小（例如100x150）时，旋转时一切都很好，没有自动布局问题。

这就是我约束UICollectionViewCell的方式。我真的不知道为什么我必须将虚拟UIView用作UIStackView的“主”视图，但是Xcode在没有它的情况下对AutoLayout问题大为关注。出于测试目的，我将此视图放在单元格之外，但放在单独的UIViewController中，以测试AutoLayout是否正常-看起来确实如此。

这是我的ViewController的代码：

import UIKit

class ViewController: UIViewController {
@IBOutlet var myCollectionView: UICollectionView!

let items: [Item] = [
    Item(title: "Cell 1"),
    Item(title: "Cell 2"),
    Item(title: "Cell 3"),
    Item(title: "Cell 4"),
    Item(title: "Cell 5"),
    Item(title: "Cell 6")
]

override func viewDidLoad() {
    super.viewDidLoad()

    guard let layout = myCollectionView.collectionViewLayout as? UICollectionViewFlowLayout else {
        fatalError("Impossibruuu - layout is not FlowLayout")
    }

    myCollectionView.register(UINib(nibName: "MyCollectionViewCell", bundle: nil), forCellWithReuseIdentifier: "MyCollectionViewCell")
    layout.minimumInteritemSpacing = 10
    layout.minimumLineSpacing = 50
    // line below makes AutoLayout angry during rotations
    layout.estimatedItemSize = UICollectionViewFlowLayout.automaticSize 
    // this is completely fine
    //layout.itemSize = CGSize(width: 100, height: 150) 
    }
}

extension ViewController: UICollectionViewDataSource {
func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
    return items.count
}

func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
    guard let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "MyCollectionViewCell", for: indexPath) as? MyCollectionViewCell else { return UICollectionViewCell() }

    cell.text = items[indexPath.row].title
    return cell
    }
}

不幸的是，使用layout.estimatedItemSize时，我得到了完全奇怪的日志：

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.
2019-06-22 18:53:18.841709+0200 CollectionViewSelfSizing[55280:2953587] [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:0x600003d6e170 UIImageView:0x7fe820c1ac90.height == 100   (active)>",
    "<NSLayoutConstraint:0x600003d6ebc0 V:[UIStackView:0x7fe820c1a020]-(0)-|   (active, names: '|':UIView:0x7fe820c19e40 )>",
    "<NSLayoutConstraint:0x600003d6ec60 V:|-(0)-[UIStackView:0x7fe820c1a020]   (active, names: '|':UIView:0x7fe820c19e40 )>",
    "<NSLayoutConstraint:0x600003d6ed00 V:[UIView:0x7fe820c19e40]-(0)-|   (active, names: '|':UIView:0x7fe820c19c60 )>",
    "<NSLayoutConstraint:0x600003d6eda0 V:|-(0)-[UIView:0x7fe820c19e40]   (active, names: '|':UIView:0x7fe820c19c60 )>",
    "<NSLayoutConstraint:0x600003d6cd70 'UISV-canvas-connection' UIStackView:0x7fe820c1a020.top == UIImageView:0x7fe820c1ac90.top   (active)>",
    "<NSLayoutConstraint:0x600003d6cf50 'UISV-canvas-connection' V:[UILabel:0x7fe820c1d0c0'Cell 1']-(0)-|   (active, names: '|':UIStackView:0x7fe820c1a020 )>",
    "<NSLayoutConstraint:0x600003d6ce60 'UISV-spacing' V:[UIImageView:0x7fe820c1ac90]-(19)-[UILabel:0x7fe820c1d0c0'Cell 1']   (active)>",
    "<NSLayoutConstraint:0x600003d644b0 'UIView-Encapsulated-Layout-Height' UIView:0x7fe820c19c60.height == 50   (active)>"
)

Will attempt to recover by breaking constraint 
<NSLayoutConstraint:0x600003d6e170 UIImageView:0x7fe820c1ac90.height == 100   (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.