UITextView doesn't display all of the text on small screens when using exclusions

I have a UITextView:

var textView: UITextView = {
    let textView = UITextView()
    textView.textColor = .orange
    textView.isScrollEnabled = false
    textView.isEditable = false
    return textView
}()

I have it constrained to another view, which is in turn part of a UIStackView

    mainStack.axis = .vertical
    mainStack.addArrangedSubview(titleView)
    mainStack.addArrangedSubview(bodyView)
    
    titleView.addSubview(textView)
    textView.translatesAutoresizingMaskIntoConstraints = false
    
    let padding: CGFloat = 5
    
    NSLayoutConstraint.activate([
        textView.leadingAnchor.constraint(equalTo: titleView.leadingAnchor, constant: padding),
        textView.trailingAnchor.constraint(equalTo: titleView.trailingAnchor, constant: -padding),
        textView.topAnchor.constraint(equalTo:  titleView.topAnchor, constant: padding),
        textView.bottomAnchor.constraint(equalTo: titleView.bottomAnchor, constant: -padding)
    ])

In addition I have a button constrained partially in the same area

let mainActionButton = UIButton()
titleView.addSubview(mainActionButton)
        
mainActionButton.translatesAutoresizingMaskIntoConstraints = false
        
        NSLayoutConstraint.activate([
            mainActionButton.trailingAnchor.constraint(equalTo: titleView.trailingAnchor),
            mainActionButton.topAnchor.constraint(equalTo:  titleView.topAnchor),
            mainActionButton.heightAnchor.constraint(equalTo: mainActionButton.widthAnchor)
        ])

At runtime, after viewDidLoad is called, I use the following to make sure the text flows around the button

    textView.textContainer.exclusionPaths = [UIBezierPath(rect: mainActionButton.frame)]
    self.layoutSubviews()

Generally, this works nicely, as you can see from the screenshot:

working

However, when I changed the padding to anything less than 4 in the constraints for the uitextview, on small screens only, it does not display the last line, as you can see below:

not working

Again, this doesn't happen if:

There is no exclusion path (button)
It is on a large screen
The padding is more than 3 px

Does anyone have any idea why this might be happening?

Solution

Based on your "minimum reproducible example" ...

I've found in the past that when setting .exclusionPaths in a UITextView, the layout needs to be "prompted" to update. It also seems much more reliable to set the .text after setting the exclusion paths.

Also - while not related to what's going on here - I find it very helpful during development to give UI elements contrasting background colors to make it easy to see the framing at run-time.

So, here is your example, almost as-is, with these changes:

constrained PrimaryView to the safe-area
gave bodyView a height, and added a label as a subview to identify it
gave all the views background colors

your "helper" function - unmodified

// Helper function
extension UIView {
    enum ConstraintType: CaseIterable { case leading, trailing, top, bottom }
    func constrain(_ child: UIView, padding: CGFloat = 0, except: [ConstraintType] = []) {
        
        self.addSubview(child)
        child.translatesAutoresizingMaskIntoConstraints = false
        
        for type in ConstraintType.allCases where !except.contains(type) {
            switch type {
            case .leading:
                child.leadingAnchor.constraint(equalTo: self.leadingAnchor, constant: padding).isActive = true
            case .trailing:
                child.trailingAnchor.constraint(equalTo: self.trailingAnchor, constant: -padding).isActive = true
            case .top:
                child.topAnchor.constraint(equalTo: self.topAnchor, constant: padding).isActive = true
            case .bottom:
                child.bottomAnchor.constraint(equalTo: self.bottomAnchor, constant: -padding).isActive = true
            }
        }
    }
}

view controller class - modified to respect safe-area

class ExclusionTestViewController: UIViewController {
    
    let primaryView = MyPrimaryView()

    override func viewDidLoad() {
        super.viewDidLoad()
        
        //self.view.constrain(primaryView)
        
        primaryView.translatesAutoresizingMaskIntoConstraints = false
        view.addSubview(primaryView)
        
        let g = view.safeAreaLayoutGuide
        NSLayoutConstraint.activate([
            primaryView.topAnchor.constraint(equalTo: g.topAnchor, constant: 0.0),
            primaryView.leadingAnchor.constraint(equalTo: g.leadingAnchor, constant: 0.0),
            primaryView.trailingAnchor.constraint(equalTo: g.trailingAnchor, constant: 0.0),
            primaryView.bottomAnchor.constraint(equalTo: g.bottomAnchor, constant: 0.0),
        ])
        
    }
    
    override func viewDidLayoutSubviews() {
        super.viewDidLayoutSubviews()
        primaryView.layoutTitle()
    }
}

PrimaryView class - original, except added colors and bodyView properties

class MyPrimaryView: UIView {
    
    var mainScroll = UIScrollView()
    
    var mainStack: UIStackView = {
        let stackView = UIStackView()
        stackView.axis = .vertical
        return stackView
    } ()
    
    var titleTextView: UITextView = {
        let textView = UITextView()
        textView.font = UIFont.systemFont(ofSize: 36)
        textView.isScrollEnabled = false
        textView.isEditable = false
        return textView
    }()
    var mainActionButtonArea = UIView()
    var mainActionButton: UIButton = {
        let button = UIButton()
        button.setImage(UIImage(systemName: "square.and.pencil.circle.fill", withConfiguration: UIImage.SymbolConfiguration(pointSize: 36)), for: .normal)
        return button
    }()
    
    var bodyView = UIStackView()
    
    init() {
        super.init(frame: .zero)
        
        setUpMainStack()

        // set background colors to make it easy to see framing
        self.backgroundColor = .cyan
        mainScroll.backgroundColor = .systemBlue
        mainStack.backgroundColor = .systemYellow
        titleTextView.backgroundColor = .green
        mainActionButtonArea.backgroundColor = .red.withAlphaComponent(0.75)
        mainActionButton.backgroundColor = .white
        bodyView.backgroundColor = UIColor(white: 0.95, alpha: 1.0)
        
        // let's give the bodyView a height constraint of 180
        bodyView.heightAnchor.constraint(equalToConstant: 180.0).isActive = true
        // and add a label to identify it
        let v = UILabel()
        v.font = .italicSystemFont(ofSize: 24.0)
        v.text = "Body View"
        bodyView.constrain(v, padding: 12.0, except: [.trailing, .bottom])
        
    }
    
    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
    
    func setUpMainStack() {
        
        // Constrain scrollview to view
        self.constrain(mainScroll, padding: 30)
        
        // Constrain UIStackView to scroll view
        mainScroll.constrain(mainStack)
        mainScroll.widthAnchor.constraint(equalTo: mainStack.widthAnchor).isActive = true
        
        // Add child views
        mainStack.addArrangedSubview(titleTextView)
        mainStack.addArrangedSubview(bodyView)
        
        titleTextView.text = "1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19"
        
        // Add action button area + action button
        mainScroll.constrain(mainActionButtonArea, except: [.leading, .bottom])
        mainActionButtonArea.constrain(mainActionButton, padding: 10)
        mainActionButton.heightAnchor.constraint(equalTo: mainActionButton.widthAnchor).isActive = true
        
    }
            
    func layoutTitle() {
        // This adds the exception path; needs to be called after subviews are already laid out
        let buttonPath = UIBezierPath(rect: mainActionButtonArea.frame)
        titleTextView.textContainer.exclusionPaths = [buttonPath]
    }
    
}

and it looks like this:

As you pointed out, the "17 18 19" line is missing.

So, let's force the text view to update its layout when we set the exclusion path. The only change will be a few lines added here:

func layoutTitle() {
    // This adds the exception path; needs to be called after subviews are already laid out
    let buttonPath = UIBezierPath(rect: mainActionButtonArea.frame)
    titleTextView.textContainer.exclusionPaths = [buttonPath]
    
    // "re-set" the text view's text and force a layout pass
    let str: String = titleTextView.text ?? ""
    titleTextView.text = ""
    titleTextView.text = str
    titleTextView.setNeedsLayout()
    titleTextView.layoutIfNeeded()
}

Now when we run it, we get:

and there's no need to create a temporary label with sizing calculations that may, or may not, be reliable and flexible.

As a side note, to get this to "play well" with dynamic size changes - such as device rotation - it's still got a bit of work to do.