ios - 具有自定义约束的 UIStackView - 更改轴时出现 "Unable to simultaneously satisfy constraints"

Question

我偶然发现了 UIStackView 的问题.
它与 UIStackView 与其排列的 subview 之间存在一些自定义约束的情况有关。更改堆栈 View 的轴和自定义约束时，错误消息 [LayoutConstraints] Unable to simultaneously satisfy constraints.出现在控制台中。
这是一个代码示例，您可以将其粘贴到新项目中以观察问题:

import UIKit

public class ViewController: UIViewController {
    private var stateToggle = false

    private let viewA: UIView = {
        let view = UIView()
        view.translatesAutoresizingMaskIntoConstraints = false
        view.accessibilityIdentifier = "viewA"
        view.backgroundColor = .red
        return view
    }()

    private let viewB: UIView = {
        let view = UIView()
        view.translatesAutoresizingMaskIntoConstraints = false
        view.accessibilityIdentifier = "viewB"
        view.backgroundColor = .green
        return view
    }()

    private lazy var stackView: UIStackView = {
        let stackView = UIStackView(arrangedSubviews: [viewA, viewB])
        stackView.translatesAutoresizingMaskIntoConstraints = false
        stackView.distribution = .fill
        stackView.alignment = .fill
        stackView.accessibilityIdentifier = "stackView"
        return stackView
    }()

    private lazy var viewAOneFourthOfStackViewHightConstraint = viewA.heightAnchor.constraint(equalTo: stackView.heightAnchor, multiplier: 0.25)
    private lazy var viewAOneFourthOfStackViewWidthConstraint = viewA.widthAnchor.constraint(equalTo: stackView.widthAnchor, multiplier: 0.25)

    public override func viewDidLoad() {
        super.viewDidLoad()
        view.accessibilityIdentifier = "rootView"

        view.addSubview(stackView)

        NSLayoutConstraint.activate([
            stackView.topAnchor.constraint(equalTo: view.safeAreaLayoutGuide.topAnchor),
            stackView.leadingAnchor.constraint(equalTo: view.safeAreaLayoutGuide.leadingAnchor),
            stackView.trailingAnchor.constraint(equalTo: view.safeAreaLayoutGuide.trailingAnchor),
            stackView.bottomAnchor.constraint(equalTo: view.safeAreaLayoutGuide.bottomAnchor),
        ])

        let button = UIButton(type: .system)
        button.frame = .init(x: 50, y: 50, width: 100, height: 44)
        button.setTitle("toggle layout", for: .normal)
        button.tintColor = .white
        button.backgroundColor = .black
        button.addTarget(self, action: #selector(buttonTap), for: .touchUpInside)
        view.addSubview(button)

        updateConstraintsBasedOnState()
    }

    @objc func buttonTap() {
        stateToggle.toggle()
        updateConstraintsBasedOnState()
    }

    private func updateConstraintsBasedOnState() {
        switch (stateToggle) {
        case true:
            stackView.axis = .horizontal
            viewAOneFourthOfStackViewHightConstraint.isActive = false
            viewAOneFourthOfStackViewWidthConstraint.isActive = true
        case false:
            stackView.axis = .vertical
            viewAOneFourthOfStackViewWidthConstraint.isActive = false
            viewAOneFourthOfStackViewHightConstraint.isActive = true
        }
    }
}

在这个例子中，我们正在制作一个带有两个 subview 的堆栈 View 。我们希望其中一个占据 25% 的区域 - 所以我们有实现这一目标的限制(每个方向一个)，但我们需要能够相应地切换它们。当切换发生时(在这种情况下从垂直堆栈到水平堆栈)，会出现错误消息:

[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:0x60000089ca00 stackView.leading == UILayoutGuide:0x6000012b48c0'UIViewSafeAreaLayoutGuide'.leading   (active, names: stackView:0x7fe269d07b50 )>",
    "<NSLayoutConstraint:0x60000089c9b0 stackView.trailing == UILayoutGuide:0x6000012b48c0'UIViewSafeAreaLayoutGuide'.trailing   (active, names: stackView:0x7fe269d07b50 )>",
    "<NSLayoutConstraint:0x60000089c730 viewA.width == 0.25*stackView.width   (active, names: viewA:0x7fe269e14fd0, stackView:0x7fe269d07b50 )>",
    "<NSLayoutConstraint:0x6000008ba990 'UISV-canvas-connection' stackView.leading == viewA.leading   (active, names: stackView:0x7fe269d07b50, viewA:0x7fe269e14fd0 )>",
    "<NSLayoutConstraint:0x6000008ba9e0 'UISV-canvas-connection' H:[viewA]-(0)-|   (active, names: stackView:0x7fe269d07b50, viewA:0x7fe269e14fd0, '|':stackView:0x7fe269d07b50 )>",
    "<NSLayoutConstraint:0x6000008bab20 'UIView-Encapsulated-Layout-Width' rootView.width == 375   (active, names: rootView:0x7fe269c111a0 )>",
    "<NSLayoutConstraint:0x60000089ce60 'UIViewSafeAreaLayoutGuide-left' H:|-(0)-[UILayoutGuide:0x6000012b48c0'UIViewSafeAreaLayoutGuide'](LTR)   (active, names: rootView:0x7fe269c111a0, '|':rootView:0x7fe269c111a0 )>",
    "<NSLayoutConstraint:0x60000089cdc0 'UIViewSafeAreaLayoutGuide-right' H:[UILayoutGuide:0x6000012b48c0'UIViewSafeAreaLayoutGuide']-(0)-|(LTR)   (active, names: rootView:0x7fe269c111a0, '|':rootView:0x7fe269c111a0 )>"
)

Will attempt to recover by breaking constraint 
<NSLayoutConstraint:0x6000008ba990 'UISV-canvas-connection' stackView.leading == viewA.leading   (active, names: stackView:0x7fe269d07b50, viewA:0x7fe269e14fd0 )>

注意约束: "<NSLayoutConstraint:0x6000008ba9e0 'UISV-canvas-connection' H:[viewA]-(0)-| (active, names: stackView:0x7fe269d07b50, viewA:0x7fe269e14fd0, '|':stackView:0x7fe269d07b50 )>"上面写着“viewA 的尾随应该与堆栈 View 的尾随相同”。这不是水平轴布局的有效约束。但这不是我添加的约束，它是堆栈 View ( UISV-canvas-connection )的内部内容。
在我看来，因为我们正在激活/停用自定义约束，并且堆栈 View 在切换轴时在内部执行相同的操作 - 暂时存在冲突和错误。
潜在的解决方案/解决方法:

解决方法 1:使自定义约束具有 priority = 999 .这不是一个好的解决方法 - 不仅因为它很hacky，而且因为在某些情况下它会导致其他布局问题(例如，当 viewA 有一些内部布局要求发生冲突时，例如需要拥抱优先级的 subview )。

解决方法 2:在轴更改之前删除排列的 View ，并在轴更改后重新添加它们。这可行，但它也很hacky，对于一些复杂的情况在实践中可能很困难。

解决方法 3:不要使用 UIStackView完全 - 使用常规 UIView 实现作为容器并根据需要创建所需的约束。

如果这应该被视为一个错误(并报告给 Apple)以及是否有其他(更好的？)解决方案，有什么想法吗？
例如，有没有办法告诉 AutoLayout - “我即将更改一些约束以及堆栈 View 的轴 - 等到我完成然后继续评估布局”。

Answer 1

正如我所看到的，您和 iOS 都有您需要停用和激活的约束。为了正确执行并避免冲突，您和 iOS 都需要先停用旧约束，然后才能开始激活新约束。
这是一个有效的解决方案。告诉 stackView至layoutIfNeeded()在您停用旧约束之后。然后它将按顺序获取其约束，您可以激活您的新约束。

private func updateConstraintsBasedOnState() {
    switch (stateToggle) {
    case true:
        stackView.axis = .horizontal
        viewAOneFourthOfStackViewHightConstraint.isActive = false
        stackView.layoutIfNeeded()
        viewAOneFourthOfStackViewWidthConstraint.isActive = true
    case false:
        stackView.axis = .vertical
        viewAOneFourthOfStackViewWidthConstraint.isActive = false
        stackView.layoutIfNeeded()
        viewAOneFourthOfStackViewHightConstraint.isActive = true
    }
}