当排列视图的大小发生变化时，如何确保 UIStackView 通知它的所有后代？

Question

我有一个 UIStackView 有 2 个排列的子 UIView。第二个 UIView（一个标题视图）将 contentHugging 设置为high，当它的内容发生变化时，第一个 UIView（一个内容视图）会正确拉伸它的大小。一切看起来都很好。

问题在于，当 UIStackView 布局其视图以响应第二个视图更改大小时，不会通知第一个视图的子视图和后代。

我以为viewDidLayoutSubviews应该是向下传播的，所以我加了

override func viewDidLayoutSubviews() {
    let parentView = self.superview!

    // This is always .zero
    print(parentView.frame)

    super.viewDidLayoutSubviews()

    // This is ALSO always .zero
    print(parentView.frame)

}

尽可能多的子视图，但它似乎从来没有报告任何视图变化超过一次，所以有这个理论。

既然我们无法观察UIView 框架值，那么确保后代视图始终知道内容视图的框架大小并且可以相对于它调整自身大小的正确方法是什么？

Answer 1

这是一个完整的示例，显示layoutSubviews() 在堆栈视图的所有arrangedSubviews 上被调用...和它们的所有子视图。

下面是开始的样子：

• 堆栈视图有一个“虚线轮廓”
• “底部”（绿色）视图是我们的“标题”视图 - UIView 中的一个多行标签
• “顶部”（黄色）视图是我们的“内容”视图 - 两个标签和一个“圆形”视图

每次我们点击按钮，标题视图中的文本都会改变：

“圆形视图”仍然是圆形的事实告诉我们它的layoutSubviews()（我们更新cornerRadius）正在被调用。如果不是，它看起来像这样：

这是此示例的代码（无 @IBOutlet 或 @IBAction 连接）：

class StackExampleViewController: UIViewController {

    let testButton: UIButton = {
        let v = UIButton()
        v.setTitle("Tap Me", for: [])
        v.setTitleColor(.lightGray, for: .highlighted)
        v.backgroundColor = .blue
        return v
    }()

    let contentView: ContentView = {
        let v = ContentView()
        return v
    }()

    let titleView: TitleView = {
        let v = TitleView()
        return v
    }()

    let stackView: UIStackView = {
        let v = UIStackView()
        v.axis = .vertical
        v.alignment = .fill
        v.distribution = .fill
        v.spacing = 8
        return v
    }()

    let dashedView: DashedBorderView = {
        let v = DashedBorderView()
        return v
    }()

    let sampleData: [String] = [
        "This is the Title View",
        "A label can contain an arbitrary amount of text, but UILabel may shrink, wrap, or truncate the text, depending on the size of the bounding rectangle and properties you set.",
        "You can control the font, text color, alignment, highlighting, and shadowing of the text in the label.",
        "What's a UIButton?",
        "You can set the title, image, and other appearance properties of a button. In addition, you can specify a different appearance for each button state."
    ]

    var idx: Int = 0

    override func viewDidLoad() {
        super.viewDidLoad()

        // standard auto-layout
        [testButton, contentView, titleView, stackView, dashedView].forEach {
            $0.translatesAutoresizingMaskIntoConstraints = false
        }

        // add contentView and titleView to stackView
        stackView.addArrangedSubview(contentView)
        stackView.addArrangedSubview(titleView)

        // add button, dashedView and stackView
        //  (dashedView will be used to show the frame of stackView)
        view.addSubview(testButton)
        view.addSubview(dashedView)
        view.addSubview(stackView)

        // respect safe-area
        let g = view.safeAreaLayoutGuide

        NSLayoutConstraint.activate([

            // testButton 40-pts from top, centeredX
            testButton.topAnchor.constraint(equalTo: g.topAnchor, constant: 40.0),
            testButton.centerXAnchor.constraint(equalTo: g.centerXAnchor),

            // stackView 40-pts from testButton
            //  40-pts on each side
            stackView.topAnchor.constraint(equalTo: testButton.bottomAnchor, constant: 40.0),
            stackView.leadingAnchor.constraint(equalTo: g.leadingAnchor, constant: 40.0),
            stackView.trailingAnchor.constraint(equalTo: g.trailingAnchor, constant: -40.0),

            // stackView height = 300
            stackView.heightAnchor.constraint(equalToConstant: 300.0),

            // constrain dashedView centered to stackView
            //  width and height 2-pts greater (so we can "outline" the stackView frame)
            dashedView.widthAnchor.constraint(equalTo: stackView.widthAnchor, constant: 2),
            dashedView.heightAnchor.constraint(equalTo: stackView.heightAnchor, constant: 2),
            dashedView.centerXAnchor.constraint(equalTo: stackView.centerXAnchor),
            dashedView.centerYAnchor.constraint(equalTo: stackView.centerYAnchor),

        ])

        // add touchUp target
        testButton.addTarget(self, action: #selector(self.didTap(_:)), for: .touchUpInside)

        // this will track the text for updating titleView's label
        idx = sampleData.count
        updateText()
    }

    func updateText() -> Void {
        // change the text in titleView's titleLabel
        //  let auto-layout handle ALL of the resizing
        titleView.titleLabel.text = sampleData[idx % sampleData.count]
        idx += 1
    }

    @objc func didTap(_ sender: Any) {
        updateText()
    }

}

class TitleView: UIView {

    // TitleView has a multi-line UILabel
    //  with 20-pts "padding" on each side
    //  and 12-pts "padding" on top and bottom

    var titleLabel: UILabel = {
        let v = UILabel()
        v.text = "Title Label"
        v.numberOfLines = 0
        return v
    }()

    override init(frame: CGRect) {
        super.init(frame: frame)
        commonInit()
    }
    required init?(coder: NSCoder) {
        super.init(coder: coder)
        commonInit()
    }

    func commonInit() -> Void {

        backgroundColor = .green

        [titleLabel].forEach {
            $0.translatesAutoresizingMaskIntoConstraints = false
            $0.backgroundColor = UIColor(white: 0.9, alpha: 1.0)
            $0.textAlignment = .center
            addSubview($0)
        }

        NSLayoutConstraint.activate([

            titleLabel.topAnchor.constraint(equalTo: topAnchor, constant: 12.0),
            titleLabel.leadingAnchor.constraint(equalTo: leadingAnchor, constant: 20.0),
            titleLabel.trailingAnchor.constraint(equalTo: trailingAnchor, constant: -20.0),
            titleLabel.bottomAnchor.constraint(equalTo: bottomAnchor, constant: -12.0),

        ])

        // we want this label's text to control its height
        titleLabel.setContentHuggingPriority(.required, for: .vertical)

    }

    override func layoutSubviews() {
        super.layoutSubviews()
        print(NSStringFromClass(type(of: self)), #function, bounds)
    }

}

class ContentView: UIView {

    // ContentView has two labels and a "round" view

    var labelA: UILabel = {
        let v = UILabel()
        v.text = "Label A"
        return v
    }()

    var labelB: UILabel = {
        let v = UILabel()
        v.text = "The Content View is Yellow"
        return v
    }()

    var roundView: RoundView = {
        let v = RoundView()
        v.backgroundColor = .orange
        return v
    }()

    override init(frame: CGRect) {
        super.init(frame: frame)
        commonInit()
    }
    required init?(coder: NSCoder) {
        super.init(coder: coder)
        commonInit()
    }

    func commonInit() -> Void {

        backgroundColor = .yellow

        [labelA, labelB].forEach {
            $0.translatesAutoresizingMaskIntoConstraints = false
            $0.backgroundColor = .cyan
            $0.textAlignment = .center
            addSubview($0)
        }

        roundView.translatesAutoresizingMaskIntoConstraints = false
        addSubview(roundView)

        NSLayoutConstraint.activate([

            // constrain labelA 20-pts from top / leading
            labelA.topAnchor.constraint(equalTo: topAnchor, constant: 20.0),
            labelA.leadingAnchor.constraint(equalTo: leadingAnchor, constant: 20.0),

            // constrain roundView 20-pts from trailing
            roundView.trailingAnchor.constraint(equalTo: trailingAnchor, constant: -20.0),

            // constrain labelA trailing 20-pts from roundView leading
            labelA.trailingAnchor.constraint(equalTo: roundView.leadingAnchor, constant: -20.0),

            // constrain roundView height equal to lableA height
            roundView.heightAnchor.constraint(equalTo: labelA.heightAnchor),
            // keep roundView square (1:1 ratio)
            roundView.widthAnchor.constraint(equalTo: roundView.heightAnchor),

            // center roundView vertically to labelA
            roundView.centerYAnchor.constraint(equalTo: labelA.centerYAnchor),

            // labelB top is 8-pts below labelA
            labelB.topAnchor.constraint(equalTo: labelA.bottomAnchor, constant: 8.0),

            // constrain labelB 20-pts from leading / trailing / bottom
            labelB.leadingAnchor.constraint(equalTo: leadingAnchor, constant: 20.0),
            labelB.trailingAnchor.constraint(equalTo: trailingAnchor, constant: -20.0),
            labelB.bottomAnchor.constraint(equalTo: bottomAnchor, constant: -20.0),

            // keep labelA and labelB heights equal
            labelA.heightAnchor.constraint(equalTo: labelB.heightAnchor),

        ])

    }

    override func layoutSubviews() {
        super.layoutSubviews()
        print(NSStringFromClass(type(of: self)), #function, bounds)
    }

}

class RoundView: UIView {

    override func layoutSubviews() {
        super.layoutSubviews()
        print(NSStringFromClass(type(of: self)), #function, bounds)
        // update cornerRadius here to keep it round
        layer.cornerRadius = bounds.size.width * 0.5
    }

}

class DashedBorderView: UIView {

    // simple view with dashed border

    var shapeLayer: CAShapeLayer!

    override class var layerClass: AnyClass {
        return CAShapeLayer.self
    }

    override init(frame: CGRect) {
        super.init(frame: frame)
        commonInit()
    }

    required init?(coder: NSCoder) {
        super.init(coder: coder)
        commonInit()
    }

    func commonInit() -> Void {

        shapeLayer = self.layer as? CAShapeLayer
        shapeLayer.fillColor = UIColor.clear.cgColor
        shapeLayer.strokeColor = UIColor(red: 1.0, green: 0.25, blue: 0.25, alpha: 1.0).cgColor
        shapeLayer.lineWidth = 1.0
        shapeLayer.lineDashPattern = [8,8]

    }

    override func layoutSubviews() {
        super.layoutSubviews()
        print(NSStringFromClass(type(of: self)), #function, bounds)
        shapeLayer.path = UIBezierPath(rect: bounds).cgPath
    }
}

注意：当视图收到对layoutSubviews() 的调用时，它们也会打印（）它们的类名和边界，因此您会在调试控制台中看到类似这样的内容：

SampleApp.TitleView layoutSubviews() (0.0, 0.0, 295.0, 146.0)
SampleApp.ContentView layoutSubviews() (0.0, 0.0, 295.0, 146.0)
SampleApp.RoundView layoutSubviews() (0.0, 0.0, 49.0, 49.0)
SampleApp.TitleView layoutSubviews() (0.0, 0.0, 295.0, 105.5)
SampleApp.ContentView layoutSubviews() (0.0, 0.0, 295.0, 186.5)
SampleApp.RoundView layoutSubviews() (0.0, 0.0, 69.0, 69.5)