为什么具有单个视图的UIStackView按比例填充，并且布局边距导致模棱两可的约束错误？

Question

以下代码是尝试将UIStackView添加到视图控制器，并以很小的余量固定在所有边缘上，并为其添加标签。

我希望StackView在以后添加更多视图时准备使用.fillProportionally作为其分发模式。

似乎对于单个布置的子视图，每当使用分配模式为.fillProportionally和布局边距时，我都会得到一个模棱两可的约束错误（如下所示）。此错误的原因是什么？

override func viewDidLoad() {
    super.viewDidLoad()

    let label = UILabel(frame: .zero)
    label.text = "ABC"

    let stack = UIStackView(arrangedSubviews: [label])
    stack.translatesAutoresizingMaskIntoConstraints = false
    stack.distribution = .fillProportionally
    stack.isLayoutMarginsRelativeArrangement = true
    stack.layoutMargins = .init(top: 10, left: 10, bottom: 10, right: 10)
    view.addSubview(stack)

    NSLayoutConstraint.activate([
        stack.centerXAnchor.constraint(equalTo: view.centerXAnchor),
        stack.centerYAnchor.constraint(equalTo: view.centerYAnchor),
        stack.widthAnchor.constraint(equalTo: view.widthAnchor),
        stack.heightAnchor.constraint(equalTo: view.heightAnchor),
    ])
}

}

模糊约束错误（WTFAutoLayout）：

(
    "<NSLayoutConstraint:0x600001a432f0 'UISV-canvas-connection' UILayoutGuide:0x6000000f3100'UIViewLayoutMarginsGuide'.leading == UILabel:0x7ff470913280'ABC'.leading   (active)>",
    "<NSLayoutConstraint:0x600001a423f0 'UISV-canvas-connection' UILayoutGuide:0x6000000f3100'UIViewLayoutMarginsGuide'.trailing == UILabel:0x7ff470913280'ABC'.trailing   (active)>",
    "<NSLayoutConstraint:0x600001a425d0 'UISV-fill-proportionally' UILabel:0x7ff470913280'ABC'.width == UIStackView:0x7ff46d510030.width   (active)>",
    "<NSLayoutConstraint:0x600001a77f70 'UIView-leftMargin-guide-constraint' H:|-(10)-[UILayoutGuide:0x6000000f3100'UIViewLayoutMarginsGuide'](LTR)   (active, names: '|':UIStackView:0x7ff46d510030 )>",
    "<NSLayoutConstraint:0x600001a42940 'UIView-rightMargin-guide-constraint' H:[UILayoutGuide:0x6000000f3100'UIViewLayoutMarginsGuide']-(10)-|(LTR)   (active, names: '|':UIStackView:0x7ff46d510030 )>"
)

Will attempt to recover by breaking constraint 
<NSLayoutConstraint:0x600001a423f0 'UISV-canvas-connection' UILayoutGuide:0x6000000f3100'UIViewLayoutMarginsGuide'.trailing == UILabel:0x7ff470913280'ABC'.trailing   (active)>

Answer 1

首先，将标签的translatesAutoresizingMaskIntoConstraints设置为false。

    label.translatesAutoresizingMaskIntoConstraints = false

由于您使用了4个全部的layoutMargins，因此宽度或高度不能等于视图的宽度和高度。

您需要容纳宽度和高度相差20（双面）。

因此约束应该是，

    NSLayoutConstraint.activate([
        stack.centerXAnchor.constraint(equalTo: view.centerXAnchor),
        stack.centerYAnchor.constraint(equalTo: view.centerYAnchor),
        stack.widthAnchor.constraint(equalTo: view.widthAnchor, multiplier: 1, constant: -20),
        stack.heightAnchor.constraint(equalTo: view.heightAnchor, multiplier: 1, constant: -20),
        ])

也请删除以下几行

    stack.isLayoutMarginsRelativeArrangement = true
    stack.layoutMargins = .init(top: 10, left: 10, bottom: 10, right: 10)