无法满足嵌套堆栈视图的约束

Question

我在尝试调试控制台中的 Unable to satisfy constraints 警告时逐渐变得疯狂，该警告在我从 iPhone 纵向转换为 iPhone 横向时出现。我正在处理带有嵌套堆栈视图的仪表板类型视图。如果宽度大小 class 是规则的，则顶部堆栈将轴更改为 .horizontal。

查看层次结构：

查看
- 滚动视图
  - 基本视图
    - 仪表板堆栈视图
      - 顶部堆栈视图
        
        进度视图
        
        统计视图
      - 中间视图
      - 底视图

代码

所有组件在viewDidLoad

translatesAutoresizingMaskIntoConstraints = false

当检测到大小 class 变化时，约束被操纵

override func traitCollectionDidChange(_ previousTraitCollection: UITraitCollection?) {

    setupDashboardView()
}

func setupDashboardView() {
    let sizeClass = self.traitCollection

    if sizeClass.isIpad {
        topStack.axis = .horizontal
        topStackHeight.constant = (self.dashboardStack.bounds.height*0.3)
        progressViewWidth.constant = (self.dashboardStack.bounds.width*0.25)
        progressViewHeight.isActive = false
    } else if sizeClass.isIphonePortrait {
        topStack.axis = .vertical
        topStackHeight.constant = self.view.frame.width*1.5
        progressViewWidth.isActive = false
        progressViewHeight.isActive = true
        progressViewHeight.constant = self.view.frame.width
    } else if sizeClass.isIphoneLandscape {
        progressViewHeight.isActive = false
        topStack.axis = .horizontal
        topStackHeight.constant = self.view.frame.height*0.5
        progressViewWidth.isActive = true
        progressViewWidth.constant = self.view.frame.width*0.25
        }
        dashboardStack.spacing = 5
        topStack.spacing = 5

        self.updateViewConstraints()
        self.viewDidLayoutSubviews()
    }


extension UITraitCollection {
    var isIpad: Bool {
        return horizontalSizeClass == .regular && verticalSizeClass == .regular
    }
    var isIphoneLandscape: Bool {
        return verticalSizeClass == .compact
    }
    var isIphonePortrait: Bool {
        return horizontalSizeClass == .compact && verticalSizeClass == .regular
    }
    var isIphone: Bool {
        return isIphoneLandscape || isIphonePortrait
    }
}

//For constraint debugging
extension NSLayoutConstraint {
    override public var description: String {
        let id = identifier ?? ""
        return "id: \(id), constant: \(constant)"
    }
}

调试器输出

当我将 iPhone 从纵向旋转为横向时收到此消息。

[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. 
(
    "id: , constant: 0.0",
    "id: , constant: 0.0",
    "id: , constant: 0.0",
    "id: , constant: 0.0",
    "id: progressWidth, constant: 224.0",
    "id: s, constant: 5.0",
    "id: scrollTrail, constant: 5.0",
    "id: UIScrollView-frameLayoutGuide-width, constant: 0.0",
    "id: UISV-canvas-connection, constant: 0.0",
    "id: UISV-canvas-connection, constant: 0.0",
    "id: UISV-canvas-connection, constant: 0.0",
    "id: UIView-Encapsulated-Layout-Width, constant: 896.0",
    "id: UIViewSafeAreaLayoutGuide-left, constant: 44.0",
    "id: UIViewSafeAreaLayoutGuide-right, constant: 44.0"
)

Will attempt to recover by breaking constraint 
id: progressWidth, constant: 224.0

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.
2020-02-26 11:27:51.128311+0000 Haem Data PG Std[9632:2228108] [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. 
(
    "id: progressHeight, constant: 414.0",
    "id: topStackHeight, constant: 185.0",
    "id: UISV-canvas-connection, constant: 0.0",
    "id: UISV-canvas-connection, constant: 0.0"
)

Will attempt to recover by breaking constraint 
id: progressHeight, constant: 414.0

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.

Answer 1

依赖于其他布局操作的约束可能会变得棘手。自动布局会产生多个 "passes"，因为它试图满足所有约束。

这对于比例约束和可能受堆栈视图影响的约束（它们正在执行自己的布局操作）尤为明显。

为了避免警告，您通常需要调整约束优先级。

并且，您在激活/停用约束时需要注意顺序。

我按照你展示的方式布置了对象，对于你的情况，你应该能够通过以下方式解决问题：

将 progressViewHeight、progressViewWidth 和 topStackHeight 约束的优先级分别更改为 999。
约束变化顺序应该是
- 在 views/labels/etc 上设置 .isActive = false 应该先进行
- 下一步应该更改 views/labels/etc 上的 .constant 值
- 然后在 views/labels/etc
- 然后更改 .constant 堆栈视图中的值
- 最后更改堆栈视图的 .axis

并非所有这些都是硬性规定，但根据经验，这是一个很好的方法。

尝试更改上面列出的约束的优先级，并按如下方式更改 setupDashboardView() 功能：

func setupDashboardView() {
    let sizeClass = self.traitCollection

    if sizeClass.isIpad {
        progressViewHeight.isActive = false
        progressViewWidth.constant = (self.dashboardStack.bounds.width*0.25)
        topStackHeight.constant = (self.dashboardStack.bounds.height*0.3)
        topStack.axis = .horizontal
    } else if sizeClass.isIphonePortrait {
        progressViewWidth.isActive = false
        progressViewHeight.isActive = true
        progressViewHeight.constant = self.view.frame.width
        topStackHeight.constant = self.view.frame.width*1.5
        topStack.axis = .vertical
    } else if sizeClass.isIphoneLandscape {
        progressViewHeight.isActive = false
        progressViewWidth.constant = self.view.frame.width*0.25
        progressViewWidth.isActive = true
        topStackHeight.constant = self.view.frame.height*0.5
        topStack.axis = .horizontal
    }
    dashboardStack.spacing = 5
    topStack.spacing = 5

    // not needed
    //self.updateViewConstraints()

    // NEVER call this yourself
    //self.viewDidLayoutSubviews()
}

无法满足嵌套堆栈视图的约束

Unable to satisfy constraints with nested stack views

nslayoutconstraint

swift

uistackview

查看层次结构：

代码

调试器输出