swift - 当特征集合发生变化时，会出现约束冲突，就像 stackview 轴没有变化一样

Question

我有一个带有两个控件的堆栈 View 。

当 UI 不受垂直约束时: Vertical1

当 UI 受到垂直约束时:Horizontal1

我得到了如图所示的两个用户界面。当我第一次显示 UI 时，没有约束冲突。但是，当我从垂直约束变为垂直=常规时，我会遇到约束冲突。

当我注释掉 stackview 空间时(请参阅下面的代码注释)，我没有遇到约束冲突。

class ViewController: UIViewController {

    var rootStack: UIStackView!
    var aggregateStack: UIStackView!
    var field1: UITextField!
    var field2: UITextField!
    var f1f2TrailTrail: NSLayoutConstraint!

    override func viewDidLoad() {

        super.viewDidLoad()
        view.backgroundColor = .white
        createIntializeViews()
        createInitializeAddStacks()
    }

    private func createIntializeViews() {

        field1 = UITextField()
        field2 = UITextField()
        field1.text = "test 1"
        field2.text = "test 2"
    }

    private func createInitializeAddStacks() {

        rootStack = UIStackView()             

        aggregateStack = UIStackView()

        // If I comment out the following, there are no constraint conflicts
        aggregateStack.spacing = 2            

        aggregateStack.addArrangedSubview(field1)
        aggregateStack.addArrangedSubview(field2)
        rootStack.addArrangedSubview(aggregateStack)

        view.addSubview(rootStack)

        rootStack.translatesAutoresizingMaskIntoConstraints = false
        aggregateStack.translatesAutoresizingMaskIntoConstraints = false
        field1.translatesAutoresizingMaskIntoConstraints = false
        field2.translatesAutoresizingMaskIntoConstraints = false

        f1f2TrailTrail = field2.trailingAnchor.constraint(equalTo: field1.trailingAnchor)
    }


    override public func traitCollectionDidChange(_ previousTraitCollection: UITraitCollection?) {

        super.traitCollectionDidChange(previousTraitCollection)

        if traitCollection.verticalSizeClass == .regular {
            aggregateStack.axis = .vertical
            f1f2TrailTrail.isActive = true
        } else if traitCollection.verticalSizeClass == .compact {
            f1f2TrailTrail.isActive = false
            aggregateStack.axis = .horizontal
        } else {
            print("Unexpected")
        }
    }
}

约束冲突在这里 -

(
    "<NSLayoutConstraint:0x600001e7d1d0 UITextField:0x7f80b2035000.trailing == UITextField:0x7f80b201d000.trailing   (active)>",
    "<NSLayoutConstraint:0x600001e42800 'UISV-spacing' H:[UITextField:0x7f80b201d000]-(2)-[UITextField:0x7f80b2035000]   (active)>"
)

Will attempt to recover by breaking constraint 
    <NSLayoutConstraint:0x600001e42800 'UISV-spacing' H:[UITextField:0x7f80b201d000]-(2)-[UITextField:0x7f80b2035000]   (active)>

当我将输出放入 www.wtfautolayout.com 时，我得到以下结果: Easier to Read Output

上图中显示的第二个约束让我认为在评估约束之前并未发生对 stackview 垂直轴的更改。

谁能告诉我我做错了什么或者如何正确设置它(最好没有 Storyboard)？

[编辑] 文本字段后缘对齐以具有以下内容:

More of the form - portrait

More of the form - landscape

Answer 1

情侣笔记...

• “嵌套”堆栈 View 存在一个固有问题，会导致约束冲突。通过将受影响元素的优先级设置为 999(而不是默认的 1000)可以避免这种情况。
• 您的布局变得有点复杂...标签“附加”到文本字段；元素需要在纵向的两“线”上或横向的一根“线”上；具有不同高度的“多元件线”的一个元件(步进器)；等等。
• 要使“field2”和“field3”大小相等，您需要将它们的宽度限制为相等，即使它们不是同一 subview 的 subview 。只要它们是同一 View 层次结构的后代，这是完全有效的。
• Stackview 很棒——除非它们不是。我几乎建议仅使用约束。您需要添加更多约束，但它可能避免堆栈 View 的一些问题。

但是，这里有一个示例，应该可以帮助您上路。

我添加了一个名为 LabeledFieldStackView 的 UIStackView 子类...它在堆栈 View 中设置 Label-above-Field。比将其混合在所有其他布局代码中要干净一些。

class LabeledFieldStackView: UIStackView {

    var theLabel: UILabel = {
        let v = UILabel()
        v.translatesAutoresizingMaskIntoConstraints = false
        return v
    }()

    var theField: UITextField = {
        let v = UITextField()
        v.translatesAutoresizingMaskIntoConstraints = false
        v.borderStyle = .roundedRect
        return v
    }()

    convenience init(with labelText: String, fieldText: String, verticalGap: CGFloat) {

        self.init()

        axis = .vertical
        alignment = .fill
        distribution = .fill
        spacing = 2

        addArrangedSubview(theLabel)
        addArrangedSubview(theField)

        theLabel.text = labelText
        theField.text = fieldText

        self.translatesAutoresizingMaskIntoConstraints = false

    }

}

class LargentViewController: UIViewController {

    var rootStack: UIStackView!

    var fieldStackView1: LabeledFieldStackView!
    var fieldStackView2: LabeledFieldStackView!
    var fieldStackView3: LabeledFieldStackView!
    var fieldStackView4: LabeledFieldStackView!

    var stepper: UIStepper!

    var fieldAndStepperStack: UIStackView!

    var twoLineStack: UIStackView!

    var fieldAndStepperStackWidthConstraint: NSLayoutConstraint!

    // horizontal gap between elements on the same "line"
    var horizontalSpacing: CGFloat!

    // vertical gap between "lines"
    var verticalSpacing: CGFloat!

    // vertical gap between labels above text fields
    var labelToFieldSpacing: CGFloat!

    override func viewDidLoad() {

        super.viewDidLoad()

        view.backgroundColor = UIColor(white: 0.9, alpha: 1.0)

        horizontalSpacing = CGFloat(2)
        verticalSpacing = CGFloat(8)
        labelToFieldSpacing = CGFloat(2)

        createIntializeViews()
        createInitializeStacks()
        fillStacks()

    }

    private func createIntializeViews() {

        fieldStackView1 = LabeledFieldStackView(with: "label 1", fieldText: "field 1", verticalGap: labelToFieldSpacing)
        fieldStackView2 = LabeledFieldStackView(with: "label 2", fieldText: "field 2", verticalGap: labelToFieldSpacing)
        fieldStackView3 = LabeledFieldStackView(with: "label 3", fieldText: "field 3", verticalGap: labelToFieldSpacing)
        fieldStackView4 = LabeledFieldStackView(with: "label 4", fieldText: "field 4", verticalGap: labelToFieldSpacing)

        stepper = UIStepper()

    }

    private func createInitializeStacks() {

        rootStack = UIStackView()
        fieldAndStepperStack = UIStackView()
        twoLineStack = UIStackView()

        [rootStack, fieldAndStepperStack, twoLineStack].forEach {
            $0?.translatesAutoresizingMaskIntoConstraints = false
        }

        // rootStack has spacing of horizontalSpacing (inter-line vertical spacing)
        rootStack.axis = .vertical
        rootStack.alignment = .fill
        rootStack.distribution = .fill
        rootStack.spacing = verticalSpacing

        // fieldAndStepperStack has spacing of horizontalSpacing (space between field and stepper)
        // and .alignment of .bottom (so stepper aligns vertically with field)
        fieldAndStepperStack.axis = .horizontal
        fieldAndStepperStack.alignment = .bottom
        fieldAndStepperStack.distribution = .fill
        fieldAndStepperStack.spacing = horizontalSpacing

        // twoLineStack has inter-line vertical spacing of
        //   verticalSpacing in portrait orientation
        // for landscape orientation, the two "lines" will be changed to one "line"
        //  and the spacing will be changed to horizontalSpacing
        twoLineStack.axis = .vertical
        twoLineStack.alignment = .leading
        twoLineStack.distribution = .fill
        twoLineStack.spacing = verticalSpacing

    }

    private func fillStacks() {

        self.view.addSubview(rootStack)

        // constrain rootStack Top, Leading, Trailing = 20
        // no height or bottom constraint
        NSLayoutConstraint.activate([
            rootStack.topAnchor.constraint(equalTo: view.safeAreaLayoutGuide.topAnchor, constant: 20.0),
            rootStack.leadingAnchor.constraint(equalTo: view.safeAreaLayoutGuide.leadingAnchor, constant: 20.0),
            rootStack.trailingAnchor.constraint(equalTo: view.safeAreaLayoutGuide.trailingAnchor, constant: -20.0),
            ])

        rootStack.addArrangedSubview(fieldStackView1)

        fieldAndStepperStack.addArrangedSubview(fieldStackView2)
        fieldAndStepperStack.addArrangedSubview(stepper)

        twoLineStack.addArrangedSubview(fieldAndStepperStack)
        twoLineStack.addArrangedSubview(fieldStackView3)

        rootStack.addArrangedSubview(twoLineStack)

        // fieldAndStepperStack needs width constrained to its superview (the twoLineStack) when
        //  in portrait orientation
        // setting the priority to 999 prevents "nested stackView" constraint breaks
        fieldAndStepperStackWidthConstraint = fieldAndStepperStack.widthAnchor.constraint(equalTo: twoLineStack.widthAnchor, multiplier: 1.0)
        fieldAndStepperStackWidthConstraint.priority = UILayoutPriority(rawValue: 999)

        // constrain fieldView3 width to fieldView2 width to keep them the same size
        NSLayoutConstraint.activate([
            fieldStackView3.widthAnchor.constraint(equalTo: fieldStackView2.widthAnchor, multiplier: 1.0)
            ])

        rootStack.addArrangedSubview(fieldStackView4)

    }

    override public func traitCollectionDidChange(_ previousTraitCollection: UITraitCollection?) {

        super.traitCollectionDidChange(previousTraitCollection)

        if traitCollection.verticalSizeClass == .regular {
            fieldAndStepperStackWidthConstraint.isActive = true
            twoLineStack.axis = .vertical
            twoLineStack.spacing = verticalSpacing
        } else if traitCollection.verticalSizeClass == .compact {
            fieldAndStepperStackWidthConstraint.isActive = false
            twoLineStack.axis = .horizontal
            twoLineStack.spacing = horizontalSpacing
        } else {
            print("Unexpected")
        }
    }

}

结果: