Why do embedded UIStackView's, filled Proportionally, with spacing and Layout Margins causes constraint errors and how to fix?

Question

This question comes off the back of the great question and answer at:

Stackoverflow: Why does a UIStackView with a single view, fill Proportionally, and Layout Margins causes ambiguous constraint error?

I am having similar issues but with embedded stackviews inside a scrollview. I have modified the explanation code from the above question. In my working code, I have a parentVC that loads into a container view, a childVC that has the embedded scrollView and stackViews (outer stackview is vertical and then each of its subviews are a bunch of horizontal stackviews). There are a range of options from fixed width label, icons and expanding labels.

The problem is I either get UISV-spacing constraint errors or trailing errors for the expanding labels. I understand from the above question that this has to do with the sequence of how the auto layout engine is calculating the proportional widths, spacing etc. but have no idea how to fix. Any suggestions would be welcome. I'm attaching the code I'm using:

class ParentVC: UIViewController {
    
    override func viewDidLoad() {
        
        let vcContainer = UIView()
        view.addSubview(vcContainer)
        vcContainer.backgroundColor = .systemYellow
        vcContainer.translatesAutoresizingMaskIntoConstraints = false
        
        NSLayoutConstraint.activate([
            vcContainer.topAnchor.constraint(equalTo: view.topAnchor, constant: 100),
            vcContainer.bottomAnchor.constraint(equalTo: view.bottomAnchor, constant: -150),
            vcContainer.leadingAnchor.constraint(equalTo: view.leadingAnchor, constant: 50),
            vcContainer.trailingAnchor.constraint(equalTo: view.trailingAnchor, constant: -50),
        ])
        
        let vc = ChildVC()
        addChild(vc)
        vc.didMove(toParent: self)
        vcContainer.addSubview(vc.view)
        vc.view.frame = vcContainer.bounds
        vc.view.heightAnchor.constraint(equalTo: vcContainer.heightAnchor).isActive = true
    }
    
}

class ChildVC: UIViewController {

    let outerStackView: UIStackView = {
        let v = UIStackView()
        v.translatesAutoresizingMaskIntoConstraints = false
        v.axis = .vertical
        v.spacing = 8
        return v
    }()

    let scrollView: UIScrollView = {
        let sv = UIScrollView()
        sv.translatesAutoresizingMaskIntoConstraints = false
        return sv
    }()


    override func viewDidLoad() {
        super.viewDidLoad()

        for _ in 1...7 {
            let lbl = UILabel()
            lbl.font = UIFont.systemFont(ofSize: 12.0, weight: .light)
            lbl.numberOfLines = 0
            lbl.textAlignment = .center
            outerStackView.addArrangedSubview(lbl)
            let sv = UIStackView()
            sv.translatesAutoresizingMaskIntoConstraints = false
            sv.axis = .horizontal
            sv.distribution = .fillProportionally
            sv.spacing = 10
            outerStackView.addArrangedSubview(sv)
        }

        view.addSubview(scrollView)
        scrollView.addSubview(outerStackView)

        NSLayoutConstraint.activate([
            scrollView.topAnchor.constraint(equalTo: view.topAnchor),
            scrollView.widthAnchor.constraint(equalTo: view.widthAnchor),
            scrollView.centerXAnchor.constraint(equalTo: view.centerXAnchor),
            scrollView.heightAnchor.constraint(equalTo: view.heightAnchor),
            
            outerStackView.topAnchor.constraint(equalTo: scrollView.contentLayoutGuide.topAnchor),
            outerStackView.bottomAnchor.constraint(equalTo: scrollView.bottomAnchor),
            outerStackView.leadingAnchor.constraint(equalTo: scrollView.contentLayoutGuide.leadingAnchor),
            outerStackView.trailingAnchor.constraint(equalTo: scrollView.contentLayoutGuide.trailingAnchor),
            outerStackView.widthAnchor.constraint(equalTo: scrollView.widthAnchor),
        ])

        // StackView 1
        if let lbl = outerStackView.arrangedSubviews[0] as? UILabel,
            let sv = outerStackView.arrangedSubviews[1] as? UIStackView {

            let v = ExpandingLabel()
            sv.addArrangedSubview(v)

            lbl.text = "SV1: One view \n no layoutMargins"
        }

        // StackView 2
        if let lbl = outerStackView.arrangedSubviews[2] as? UILabel,
            let sv = outerStackView.arrangedSubviews[3] as? UIStackView {


            let v = FixedLabel()
            sv.addArrangedSubview(v)


            let v2 = ExpandingLabel()
            sv.addArrangedSubview(v2)

            lbl.text = "SV2: Two views \n no layoutMargins"
        }

        // comment out this block to see the auto-layout error goes away
        // StackView 3
        if let lbl = outerStackView.arrangedSubviews[4] as? UILabel,
            let sv = outerStackView.arrangedSubviews[5] as? UIStackView {


            let v = ExpandingLabel()
            sv.addArrangedSubview(v)

            sv.isLayoutMarginsRelativeArrangement = true
            sv.layoutMargins = .init(top: 0, left: 10, bottom: 0, right: 11)

            lbl.text = "SV3: One view\nlayoutMargins left: \(sv.layoutMargins.left), right: \(sv.layoutMargins.right)"
        }

        // StackView 4
        if let lbl = outerStackView.arrangedSubviews[6] as? UILabel,
            let sv = outerStackView.arrangedSubviews[7] as? UIStackView {

            let v = FixedLabel()
            sv.addArrangedSubview(v)

            let v2 = ExpandingLabel()
            sv.addArrangedSubview(v2)

            sv.isLayoutMarginsRelativeArrangement = true
            sv.layoutMargins = .init(top: 0, left: 12, bottom: 0, right: 13)
    
            lbl.text = "SV4: Two views\nlayoutMargins\n left:  \(sv.layoutMargins.left), right: \(sv.layoutMargins.right)"
        }

        // StackView 5
        if let lbl = outerStackView.arrangedSubviews[8] as? UILabel,
            let sv = outerStackView.arrangedSubviews[9] as? UIStackView {

            let v = FixedLabel()
            sv.addArrangedSubview(v)

            let v2 = ExpandingLabel()
            sv.addArrangedSubview(v2)

            sv.isLayoutMarginsRelativeArrangement = true
            sv.layoutMargins = .init(top: 0, left: 101, bottom: 0, right: 0)

            lbl.text = "SV5: Two views \nlayoutMargins left:  \(sv.layoutMargins.left), right: \(sv.layoutMargins.right)"
        }

        // StackView 6
        if let lbl = outerStackView.arrangedSubviews[10] as? UILabel,
            let sv = outerStackView.arrangedSubviews[11] as? UIStackView {

            let v = IconView()
            sv.addArrangedSubview(v)

            let v2 = ExpandingLabel()
            v2.numberOfLines = 0
            sv.addArrangedSubview(v2)

            sv.isLayoutMarginsRelativeArrangement = true
            sv.layoutMargins = .init(top: 0, left: 14, bottom: 0, right: 15)

            lbl.text = "SV6: Two views\nlayoutMargins left:  \(sv.layoutMargins.left), right: \(sv.layoutMargins.right)"
        }
        
        
        // StackView 7
        if let lbl = outerStackView.arrangedSubviews[12] as? UILabel,
            let sv = outerStackView.arrangedSubviews[13] as? UIStackView {


            let v = ExpandingLabel()
            sv.addArrangedSubview(v)

            sv.isLayoutMarginsRelativeArrangement = true
            sv.layoutMargins = .init(top: 0, left: 16, bottom: 0, right: 17)

            let v2 = ExpandingLabel()
            sv.addArrangedSubview(v2)

            lbl.text = "SV7: One view\nlayoutMargins left:  \(sv.layoutMargins.left), right: \(sv.layoutMargins.right)"
        }
    }

}


class IconView: UIImageView {
     init() {
        super.init(frame: .zero)
    
        self.backgroundColor = UIColor.systemRed
        self.tintColor = .white

        image = UIImage(systemName: "square.and.arrow.up")?.applyingSymbolConfiguration(UIImage.SymbolConfiguration(textStyle: .body))
        contentMode = .center
        
        translatesAutoresizingMaskIntoConstraints = false
        adjustsImageSizeForAccessibilityContentSizeCategory = true
    }
    
    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
}


class FixedLabel : UILabel {
    
    override init(frame: CGRect) {
        super.init(frame: frame)
        text = "fxd"
        backgroundColor = .systemRed
        
        translatesAutoresizingMaskIntoConstraints = false
        setContentCompressionResistancePriority(.required, for: .horizontal)
        setContentHuggingPriority(.required, for: .horizontal)
    }
    
    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
}


class ExpandingLabel: UILabel {
  
    override init(frame: CGRect) {
        super.init(frame: frame)
        text = "expanding label"
        backgroundColor = .systemGreen
        numberOfLines = 3
        font = UIFont.preferredFont(forTextStyle: .body)
        adjustsFontForContentSizeCategory = true

        translatesAutoresizingMaskIntoConstraints = false
        setContentHuggingPriority(.defaultLow, for: .horizontal)
        setContentCompressionResistancePriority(.defaultHigh, for: .vertical)
        setContentCompressionResistancePriority(.defaultHigh, for: .horizontal)
    }
    
    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
}

The above produces the desired result as per below:

The log errors are:

2021-03-16 10:01:22.815997+1100 stackviewsWithProportionalFill[8528:4745644] Sherlock initialised 2021-03-16 10:01:23.051125+1100 stackviewsWithProportionalFill[8528:4745644] [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:0x600003317e80 'UISV-canvas-connection' UIStackView:0x7feb0270f930.leading == stackviewsWithProportionalFill.FixedLabel:0x7feb02715f80.leading   (active)>",
    "<NSLayoutConstraint:0x600003317930 'UISV-canvas-connection' H:[stackviewsWithProportionalFill.ExpandingLabel:0x7feb0270b710]-(0)-| (active, names: '|':UIStackView:0x7feb0270f930 )>",
    "<NSLayoutConstraint:0x6000033172a0 'UISV-fill-proportionally' stackviewsWithProportionalFill.ExpandingLabel:0x7feb0270b710.width == UIStackView:0x7feb0270f930.width   (active)>",
    "<NSLayoutConstraint:0x6000033178e0 'UISV-spacing' H:[stackviewsWithProportionalFill.FixedLabel:0x7feb02715f80]-(10)-[stackviewsWithProportionalFill.ExpandingLabel:0x7feb0270b710] (active)>" )

Will attempt to recover by breaking constraint  <NSLayoutConstraint:0x6000033178e0 'UISV-spacing' H:[stackviewsWithProportionalFill.FixedLabel:0x7feb02715f80]-(10)-[stackviewsWithProportionalFill.ExpandingLabel:0x7feb0270b710] (active)>

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. 2021-03-16 10:01:23.057974+1100 stackviewsWithProportionalFill[8528:4745644] [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:0x60000332cb40 UIScrollView:0x7feb0381ea00.width == UIView:0x7feb0270aeb0.width   (active)>",
    "<NSLayoutConstraint:0x60000332d270 UIStackView:0x7feb02709160.width == UIScrollView:0x7feb0381ea00.width  (active)>",
    "<NSLayoutConstraint:0x600003370fa0 'UISV-alignment' UILabel:0x7feb0270b2a0.leading == UIStackView:0x7feb02711930.leading   (active)>",
    "<NSLayoutConstraint:0x6000033713b0 'UISV-alignment' UILabel:0x7feb0270b2a0.trailing == UIStackView:0x7feb02711930.trailing (active)>",
    "<NSLayoutConstraint:0x600003310eb0 'UISV-canvas-connection' UILayoutGuide:0x6000029208c0'UIViewLayoutMarginsGuide'.leading == stackviewsWithProportionalFill.FixedLabel:0x7feb02717140.leading   (active)>",
    "<NSLayoutConstraint:0x6000033123f0 'UISV-canvas-connection' UILayoutGuide:0x6000029208c0'UIViewLayoutMarginsGuide'.trailing == stackviewsWithProportionalFill.ExpandingLabel:0x7feb02717530.trailing  (active)>",
    "<NSLayoutConstraint:0x600003370c80 'UISV-canvas-connection' UIStackView:0x7feb02709160.leading == UILabel:0x7feb0270b2a0.leading   (active)>",
    "<NSLayoutConstraint:0x600003370cd0 'UISV-canvas-connection' H:[UILabel:0x7feb0270b2a0]-(0)-|   (active, names: '|':UIStackView:0x7feb02709160 )>",
    "<NSLayoutConstraint:0x600003312530 'UISV-spacing' H:[stackviewsWithProportionalFill.FixedLabel:0x7feb02717140]-(10)-[stackviewsWithProportionalFill.ExpandingLabel:0x7feb02717530] (active)>",
    "<NSLayoutConstraint:0x600003371540 'UIView-Encapsulated-Layout-Width' UIView:0x7feb0270aeb0.width == 0   (active)>",
    "<NSLayoutConstraint:0x600003312300 'UIView-leftMargin-guide-constraint' H:|-(101)-[UILayoutGuide:0x6000029208c0'UIViewLayoutMarginsGuide'](LTR) (active, names: '|':UIStackView:0x7feb02711930 )>",
    "<NSLayoutConstraint:0x6000033123a0 'UIView-rightMargin-guide-constraint' H:[UILayoutGuide:0x6000029208c0'UIViewLayoutMarginsGuide']-(0)-|(LTR)  (active, names: '|':UIStackView:0x7feb02711930 )>" )

Will attempt to recover by breaking constraint  <NSLayoutConstraint:0x600003312530 'UISV-spacing' H:[stackviewsWithProportionalFill.FixedLabel:0x7feb02717140]-(10)-[stackviewsWithProportionalFill.ExpandingLabel:0x7feb02717530] (active)>

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. 2021-03-16 10:01:23.065566+1100 stackviewsWithProportionalFill[8528:4745644] [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:0x60000332cb40 UIScrollView:0x7feb0381ea00.width == UIView:0x7feb0270aeb0.width   (active)>",
    "<NSLayoutConstraint:0x60000332d270 UIStackView:0x7feb02709160.width == UIScrollView:0x7feb0381ea00.width  (active)>",
    "<NSLayoutConstraint:0x6000033710e0 'UISV-alignment' UILabel:0x7feb0270b2a0.leading == UIStackView:0x7feb02712730.leading   (active)>",
    "<NSLayoutConstraint:0x6000033714f0 'UISV-alignment' UILabel:0x7feb0270b2a0.trailing == UIStackView:0x7feb02712730.trailing (active)>",
    "<NSLayoutConstraint:0x6000033138e0 'UISV-canvas-connection' UILayoutGuide:0x600002920a80'UIViewLayoutMarginsGuide'.leading == stackviewsWithProportionalFill.ExpandingLabel:0x7feb0260e660.leading   (active)>",
    "<NSLayoutConstraint:0x600003313930 'UISV-canvas-connection' UILayoutGuide:0x600002920a80'UIViewLayoutMarginsGuide'.trailing == stackviewsWithProportionalFill.ExpandingLabel:0x7feb0260ecd0.trailing  (active)>",
    "<NSLayoutConstraint:0x600003370c80 'UISV-canvas-connection' UIStackView:0x7feb02709160.leading == UILabel:0x7feb0270b2a0.leading   (active)>",
    "<NSLayoutConstraint:0x600003370cd0 'UISV-canvas-connection' H:[UILabel:0x7feb0270b2a0]-(0)-|   (active, names: '|':UIStackView:0x7feb02709160 )>",
    "<NSLayoutConstraint:0x600003370050 'UISV-spacing' H:[stackviewsWithProportionalFill.ExpandingLabel:0x7feb0260e660]-(10)-[stackviewsWithProportionalFill.ExpandingLabel:0x7feb0260ecd0] (active)>",
    "<NSLayoutConstraint:0x600003371540 'UIView-Encapsulated-Layout-Width' UIView:0x7feb0270aeb0.width == 0   (active)>",
    "<NSLayoutConstraint:0x6000033137f0 'UIView-leftMargin-guide-constraint' H:|-(16)-[UILayoutGuide:0x600002920a80'UIViewLayoutMarginsGuide'](LTR) (active, names: '|':UIStackView:0x7feb02712730 )>",
    "<NSLayoutConstraint:0x600003313890 'UIView-rightMargin-guide-constraint' H:[UILayoutGuide:0x600002920a80'UIViewLayoutMarginsGuide']-(17)-|(LTR) (active, names: '|':UIStackView:0x7feb02712730 )>" )

Will attempt to recover by breaking constraint  <NSLayoutConstraint:0x600003370050 'UISV-spacing' H:[stackviewsWithProportionalFill.ExpandingLabel:0x7feb0260e660]-(10)-[stackviewsWithProportionalFill.ExpandingLabel:0x7feb0260ecd0] (active)>

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. 2021-03-16 10:01:23.066504+1100 stackviewsWithProportionalFill[8528:4745644] [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:0x60000332cb40 UIScrollView:0x7feb0381ea00.width == UIView:0x7feb0270aeb0.width   (active)>",
    "<NSLayoutConstraint:0x60000332d270 UIStackView:0x7feb02709160.width == UIScrollView:0x7feb0381ea00.width  (active)>",
    "<NSLayoutConstraint:0x600003371040 'UISV-alignment' UILabel:0x7feb0270b2a0.leading == UIStackView:0x7feb02712130.leading   (active)>",
    "<NSLayoutConstraint:0x600003371450 'UISV-alignment' UILabel:0x7feb0270b2a0.trailing == UIStackView:0x7feb02712130.trailing (active)>",
    "<NSLayoutConstraint:0x600003312e90 'UISV-canvas-connection' UILayoutGuide:0x600002920b60'UIViewLayoutMarginsGuide'.leading == stackviewsWithProportionalFill.IconView:0x7feb027179a0.leading   (active)>",
    "<NSLayoutConstraint:0x600003312ee0 'UISV-canvas-connection' UILayoutGuide:0x600002920b60'UIViewLayoutMarginsGuide'.trailing == stackviewsWithProportionalFill.ExpandingLabel:0x7feb0260e1f0.trailing  (active)>",
    "<NSLayoutConstraint:0x600003370c80 'UISV-canvas-connection' UIStackView:0x7feb02709160.leading == UILabel:0x7feb0270b2a0.leading   (active)>",
    "<NSLayoutConstraint:0x600003370cd0 'UISV-canvas-connection' H:[UILabel:0x7feb0270b2a0]-(0)-|   (active, names: '|':UIStackView:0x7feb02709160 )>",
    "<NSLayoutConstraint:0x600003313020 'UISV-spacing' H:[stackviewsWithProportionalFill.IconView:0x7feb027179a0]-(10)-[stackviewsWithProportionalFill.ExpandingLabel:0x7feb0260e1f0] (active)>",
    "<NSLayoutConstraint:0x600003371540 'UIView-Encapsulated-Layout-Width' UIView:0x7feb0270aeb0.width == 0   (active)>",
    "<NSLayoutConstraint:0x600003312da0 'UIView-leftMargin-guide-constraint' H:|-(14)-[UILayoutGuide:0x600002920b60'UIViewLayoutMarginsGuide'](LTR) (active, names: '|':UIStackView:0x7feb02712130 )>",
    "<NSLayoutConstraint:0x600003312e40 'UIView-rightMargin-guide-constraint' H:[UILayoutGuide:0x600002920b60'UIViewLayoutMarginsGuide']-(15)-|(LTR) (active, names: '|':UIStackView:0x7feb02712130 )>" )

Will attempt to recover by breaking constraint  <NSLayoutConstraint:0x600003313020 'UISV-spacing' H:[stackviewsWithProportionalFill.IconView:0x7feb027179a0]-(10)-[stackviewsWithProportionalFill.ExpandingLabel:0x7feb0260e1f0] (active)>

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. 2021-03-16 10:01:23.067723+1100 stackviewsWithProportionalFill[8528:4745644] [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:0x60000332cb40 UIScrollView:0x7feb0381ea00.width == UIView:0x7feb0270aeb0.width   (active)>",
    "<NSLayoutConstraint:0x60000332d270 UIStackView:0x7feb02709160.width == UIScrollView:0x7feb0381ea00.width  (active)>",
    "<NSLayoutConstraint:0x600003370f00 'UISV-alignment' UILabel:0x7feb0270b2a0.leading == UIStackView:0x7feb02710b30.leading   (active)>",
    "<NSLayoutConstraint:0x600003371310 'UISV-alignment' UILabel:0x7feb0270b2a0.trailing == UIStackView:0x7feb02710b30.trailing (active)>",
    "<NSLayoutConstraint:0x6000033118b0 'UISV-canvas-connection' UILayoutGuide:0x600002920380'UIViewLayoutMarginsGuide'.leading == stackviewsWithProportionalFill.FixedLabel:0x7feb02716a60.leading   (active)>",
    "<NSLayoutConstraint:0x600003311900 'UISV-canvas-connection' UILayoutGuide:0x600002920380'UIViewLayoutMarginsGuide'.trailing == stackviewsWithProportionalFill.ExpandingLabel:0x7feb02716cd0.trailing  (active)>",
    "<NSLayoutConstraint:0x600003370c80 'UISV-canvas-connection' UIStackView:0x7feb02709160.leading == UILabel:0x7feb0270b2a0.leading   (active)>",
    "<NSLayoutConstraint:0x600003370cd0 'UISV-canvas-connection' H:[UILabel:0x7feb0270b2a0]-(0)-|   (active, names: '|':UIStackView:0x7feb02709160 )>",
    "<NSLayoutConstraint:0x600003311a40 'UISV-spacing' H:[stackviewsWithProportionalFill.FixedLabel:0x7feb02716a60]-(10)-[stackviewsWithProportionalFill.ExpandingLabel:0x7feb02716cd0] (active)>",
    "<NSLayoutConstraint:0x600003371540 'UIView-Encapsulated-Layout-Width' UIView:0x7feb0270aeb0.width == 0   (active)>",
    "<NSLayoutConstraint:0x6000033117c0 'UIView-leftMargin-guide-constraint' H:|-(12)-[UILayoutGuide:0x600002920380'UIViewLayoutMarginsGuide'](LTR) (active, names: '|':UIStackView:0x7feb02710b30 )>",
    "<NSLayoutConstraint:0x600003311860 'UIView-rightMargin-guide-constraint' H:[UILayoutGuide:0x600002920380'UIViewLayoutMarginsGuide']-(13)-|(LTR) (active, names: '|':UIStackView:0x7feb02710b30 )>" )

Will attempt to recover by breaking constraint  <NSLayoutConstraint:0x600003311a40 'UISV-spacing' H:[stackviewsWithProportionalFill.FixedLabel:0x7feb02716a60]-(10)-[stackviewsWithProportionalFill.ExpandingLabel:0x7feb02716cd0] (active)>

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. 2021-03-16 10:01:23.068649+1100 stackviewsWithProportionalFill[8528:4745644] [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:0x60000332cb40 UIScrollView:0x7feb0381ea00.width == UIView:0x7feb0270aeb0.width   (active)>",
    "<NSLayoutConstraint:0x60000332d270 UIStackView:0x7feb02709160.width == UIScrollView:0x7feb0381ea00.width  (active)>",
    "<NSLayoutConstraint:0x600003370e60 'UISV-alignment' UILabel:0x7feb0270b2a0.leading == UIStackView:0x7feb02710130.leading   (active)>",
    "<NSLayoutConstraint:0x600003371270 'UISV-alignment' UILabel:0x7feb0270b2a0.trailing == UIStackView:0x7feb02710130.trailing (active)>",
    "<NSLayoutConstraint:0x600003316df0 'UISV-canvas-connection' UILayoutGuide:0x600002939880'UIViewLayoutMarginsGuide'.leading == stackviewsWithProportionalFill.ExpandingLabel:0x7feb027167f0.leading   (active)>",
    "<NSLayoutConstraint:0x600003316da0 'UISV-canvas-connection' UILayoutGuide:0x600002939880'UIViewLayoutMarginsGuide'.trailing == stackviewsWithProportionalFill.ExpandingLabel:0x7feb027167f0.trailing  (active)>",
    "<NSLayoutConstraint:0x600003370c80 'UISV-canvas-connection' UIStackView:0x7feb02709160.leading == UILabel:0x7feb0270b2a0.leading   (active)>",
    "<NSLayoutConstraint:0x600003370cd0 'UISV-canvas-connection' H:[UILabel:0x7feb0270b2a0]-(0)-|   (active, names: '|':UIStackView:0x7feb02709160 )>",
    "<NSLayoutConstraint:0x600003371540 'UIView-Encapsulated-Layout-Width' UIView:0x7feb0270aeb0.width == 0   (active)>",
    "<NSLayoutConstraint:0x600003316ee0 'UIView-leftMargin-guide-constraint' H:|-(10)-[UILayoutGuide:0x600002939880'UIViewLayoutMarginsGuide'](LTR) (active, names: '|':UIStackView:0x7feb02710130 )>",
    "<NSLayoutConstraint:0x600003316e40 'UIView-rightMargin-guide-constraint' H:[UILayoutGuide:0x600002939880'UIViewLayoutMarginsGuide']-(11)-|(LTR) (active, names: '|':UIStackView:0x7feb02710130 )>" )

Will attempt to recover by breaking constraint  <NSLayoutConstraint:0x600003316da0 'UISV-canvas-connection' UILayoutGuide:0x600002939880'UIViewLayoutMarginsGuide'.trailing == stackviewsWithProportionalFill.ExpandingLabel:0x7feb027167f0.trailing  (active)>

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. 2021-03-16 10:01:24.091972+1100 stackviewsWithProportionalFill[8528:4745811] [] nw_protocol_get_quic_image_block_invoke dlopen libquic failed

Answer 1

You have inadvertently presented one of the (many) reasons I tell people to *forget you ever heard of the .fillProportionally Distribution property of a UIStackView...

For your horizontal stack views, you have:

sv.distribution = .fillProportionally

So you just told auto-layout: "Stretch the widths of the arranged subviews proportionally to each other!"

You then add a label as an arranged subview, and you set this property on the label:

setContentHuggingPriority(.required, for: .horizontal)

So you just told auto-layout: "Do NOT stretch the width of this label!"

Kinda tough for auto-layout to both stretch and not stretch the view at the same time :)

Unless you know exactly why you want .fillProportionally (and in this case, that is not what you want), don't use it. For your layout, you want the default .fill.

So, if we forget for a moment that you are loading ChildVC as a, well, child VC (grin) and set ChildVC as the "root" view controller, and leave sv.distribution = .fillProportionally, you'll get layout errors.

If you change that to sv.distribution = .fill, you won't get layout errors.

But, back to using that VC as a child ... even with .fill you'll still get layout errors. So...

Change your "load the child" code in ParentVC to this:

class ParentVC: UIViewController {
    
    override func viewDidLoad() {
        
        let vcContainer = UIView()
        view.addSubview(vcContainer)
        vcContainer.backgroundColor = .systemYellow
        vcContainer.translatesAutoresizingMaskIntoConstraints = false
        
        NSLayoutConstraint.activate([
            vcContainer.topAnchor.constraint(equalTo: view.topAnchor, constant: 100),
            vcContainer.bottomAnchor.constraint(equalTo: view.bottomAnchor, constant: -150),
            vcContainer.leadingAnchor.constraint(equalTo: view.leadingAnchor, constant: 50),
            vcContainer.trailingAnchor.constraint(equalTo: view.trailingAnchor, constant: -50),
        ])
        
        let vc = ChildVC()
        addChild(vc)
        vcContainer.addSubview(vc.view)
        
        vc.view.translatesAutoresizingMaskIntoConstraints = false

        NSLayoutConstraint.activate([
            vc.view.topAnchor.constraint(equalTo: vcContainer.topAnchor),
            vc.view.leadingAnchor.constraint(equalTo: vcContainer.leadingAnchor),
            vc.view.trailingAnchor.constraint(equalTo: vcContainer.trailingAnchor),
            vc.view.bottomAnchor.constraint(equalTo: vcContainer.bottomAnchor),
        ])
        
        vc.didMove(toParent: self)
    }
    
}

That approach, combined with sv.distribution = .fill, gets rid of the errors.