Specifying NSLineBreakMode.byTruncatingTail causes issues when displaying NSAttributedString in UILabel – Ios swift

I’m posting this answer, not because it is a solution but because I think it is worth discussion…

Edit

Personally, I would say Attributed Text and .lineBreakMode = .byTruncatingTail is rather buggy.

This does not appear to be related to Attributed Text, but rather to the presence of a newline character.

For example, if the label is set to .numberOfLines = 3 and we use this text:

myLabel.text = "Let's test this:nPlenty of text to wrap onto more than three lines. Note this is plain text, not attributed text. It may or may not exhibit the same problems."

we will see the same corrupted text wrapping occur.

Also note: this does NOT happen on iOS prior to 15.4!

/Edit

In all the following examples, the same attributed string is used for pairs of rows, but only the cyan labels have myLabel.lineBreakMode = .byTruncatingTail.

First note:

If we set myLabel.numberOfLines = 0, we get this output:

Which doesn’t make much sense… Apparently, a newline character gets factored in at the end of the attributed string — but even so, we get almost, but not enough space for TWO additional lines (I’m guessing it’s a newline and a paragraph vertical space?):

Second note:

If we DO NOT have enough text to exceed the bounds of the label (again, we’re setting myLabel.numberOfLines = 3 here), we get another oddity. This is when I rotate the device:

Even though the text is NOT truncated, we get the ... — because UIKit is trying to add 1.5 additional blank lines at the end of the string.

Third note:

As I mentioned in a comment, I know we came across this once before here on SO — unfortunately, I can’t remember if we resolved it or (probably) not.

To reproduce with Plain Text

class WrapBugVC: UIViewController {
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        let stackView = UIStackView()
        stackView.axis = .vertical
        stackView.spacing = 4
        stackView.translatesAutoresizingMaskIntoConstraints = false
        view.addSubview(stackView)
        
        let g = view.safeAreaLayoutGuide
        NSLayoutConstraint.activate([
            
            stackView.topAnchor.constraint(equalTo: g.topAnchor, constant: 20.0),
            stackView.widthAnchor.constraint(equalToConstant: 315.0),
            stackView.centerXAnchor.constraint(equalTo: g.centerXAnchor),
            
        ])

        var vInfo: UILabel
        
        let testStr1: String = "Let's test this (no newline character): Plenty of text to wrap onto more than three lines. Note this is plain text, not attributed text."
        let testStr2: String = "Let's test this:nPlenty of text to wrap onto more than three lines. Note this is plain text, not attributed text."

        vInfo = UILabel()
        vInfo.font = .italicSystemFont(ofSize: 15.0)
        vInfo.text = "NO newline, .byWordWrapping"
        stackView.addArrangedSubview(vInfo)
        
        let v1 = UILabel()
        v1.numberOfLines = 3
        v1.text = testStr1
        v1.backgroundColor = .yellow
        v1.lineBreakMode = .byWordWrapping
        stackView.addArrangedSubview(v1)
        
        vInfo = UILabel()
        vInfo.font = .italicSystemFont(ofSize: 15.0)
        vInfo.text = "NO newline, .byTruncatingTail"
        stackView.addArrangedSubview(vInfo)
        
        let v2 = UILabel()
        v2.numberOfLines = 3
        v2.text = testStr1
        v2.backgroundColor = .cyan
        v2.lineBreakMode = .byTruncatingTail
        stackView.addArrangedSubview(v2)
        
        vInfo = UILabel()
        vInfo.font = .italicSystemFont(ofSize: 15.0)
        vInfo.text = "WITH newline, .byWordWrapping"
        stackView.addArrangedSubview(vInfo)
        
        let v3 = UILabel()
        v3.numberOfLines = 3
        v3.text = testStr2
        v3.backgroundColor = .yellow
        v3.lineBreakMode = .byWordWrapping
        stackView.addArrangedSubview(v3)

        vInfo = UILabel()
        vInfo.font = .italicSystemFont(ofSize: 15.0)
        vInfo.text = "WITH newline, .byTruncatingTail"
        stackView.addArrangedSubview(vInfo)
        
        let v4 = UILabel()
        v4.numberOfLines = 3
        v4.text = testStr2
        v4.backgroundColor = .cyan
        v4.lineBreakMode = .byTruncatingTail
        stackView.addArrangedSubview(v4)

        [v1, v2, v3, v4].forEach { v in
            stackView.setCustomSpacing(16.0, after: v)
        }
        
    }
    
}

Result:

Workaround

You may want to try this as a workaround…

class TextViewLabel: UITextView {
    
    public var numberOfLines: Int = 0 {
        didSet {
            textContainer.maximumNumberOfLines = numberOfLines
        }
    }
    public var lineBreakMode: NSLineBreakMode = .byTruncatingTail {
        didSet {
            textContainer.lineBreakMode = lineBreakMode
        }
    }
    
    override init(frame: CGRect, textContainer: NSTextContainer?) {
        super.init(frame: frame, textContainer: textContainer)
        commonInit()
    }
    
    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
        commonInit()
    }
    
    private func commonInit() -> Void {
        isScrollEnabled = false
        isEditable = false
        isSelectable = false
        textContainerInset = UIEdgeInsets.zero
        textContainer.lineFragmentPadding = 0
    }
    
}

Using TextViewLabel in place of UILabel will avoid the bug:

Edit 2

As of iOS 16.2 there are still inconsistencies with word-wrapping and tail-truncation between UILabel and UITextView.

Here is some code to see them side-by-side:

// a replacement for UILabel
class TextViewLabel: UITextView {
    
    public var numberOfLines: Int = 0 {
        didSet {
            textContainer.maximumNumberOfLines = numberOfLines
        }
    }
    public var lineBreakMode: NSLineBreakMode = .byTruncatingTail {
        didSet {
            textContainer.lineBreakMode = lineBreakMode
        }
    }
    
    override init(frame: CGRect, textContainer: NSTextContainer?) {
        super.init(frame: frame, textContainer: textContainer)
        commonInit()
    }
    
    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
        commonInit()
    }
    
    private func commonInit() -> Void {
        isScrollEnabled = false
        isEditable = false
        isSelectable = false
        textContainerInset = UIEdgeInsets.zero
        textContainer.lineFragmentPadding = 0
    }
    
}

class ViewController: UIViewController {
    
    let labelStackView = UIStackView()
    let textViewStackView = UIStackView()
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        let scrollView = UIScrollView()
        
        let hStack = UIStackView()
        hStack.axis = .horizontal
        hStack.spacing = 8
        hStack.distribution = .fillEqually
        hStack.alignment = .top
        
        [labelStackView, textViewStackView].forEach { v in
            v.axis = .vertical
            v.spacing = 4
            hStack.addArrangedSubview(v)
        }
        
        hStack.translatesAutoresizingMaskIntoConstraints = false
        scrollView.addSubview(hStack)

        scrollView.translatesAutoresizingMaskIntoConstraints = false
        view.addSubview(scrollView)
        
        let g = view.safeAreaLayoutGuide
        let cg = scrollView.contentLayoutGuide
        let fg = scrollView.frameLayoutGuide

        NSLayoutConstraint.activate([
            
            scrollView.topAnchor.constraint(equalTo: g.topAnchor, constant: 20.0),
            scrollView.leadingAnchor.constraint(equalTo: g.leadingAnchor, constant: 20.0),
            scrollView.trailingAnchor.constraint(equalTo: g.trailingAnchor, constant: -20.0),
            scrollView.bottomAnchor.constraint(equalTo: g.bottomAnchor, constant: -20.0),

            hStack.topAnchor.constraint(equalTo: cg.topAnchor, constant: 8.0),
            hStack.leadingAnchor.constraint(equalTo: cg.leadingAnchor, constant: 8.0),
            hStack.trailingAnchor.constraint(equalTo: cg.trailingAnchor, constant: -8.0),
            hStack.bottomAnchor.constraint(equalTo: cg.bottomAnchor, constant: -8.0),

            hStack.widthAnchor.constraint(equalTo: fg.widthAnchor, constant: -16.0),
            
        ])
        
        let infoFont: UIFont = .italicSystemFont(ofSize: 10.0)
        let testFont: UIFont = .systemFont(ofSize: 12.0)
        
        var testStr: String = "This is some example text to test truncation consistency. At the end of this line, is a pair of newline chars:nnA HEADLINEnnThe headline was also followed by a pair of newline chars."

        for i in 3...10 {
            
            let infoLabel = UILabel()
            infoLabel.font = infoFont
            infoLabel.text = "UILabel - numLines: (i)"
            labelStackView.addArrangedSubview(infoLabel)
            
            let v = UILabel()
            v.font = testFont
            v.numberOfLines = i
            v.text = testStr
            v.backgroundColor = .yellow
            v.lineBreakMode = .byTruncatingTail
            labelStackView.addArrangedSubview(v)
            
            labelStackView.setCustomSpacing(16.0, after: v)
            
        }
        
        for i in 3...10 {
            
            let infoLabel = UILabel()
            infoLabel.font = infoFont
            infoLabel.text = "TextViewLabel - numLines: (i)"
            textViewStackView.addArrangedSubview(infoLabel)
            
            let v = TextViewLabel()
            v.font = testFont
            v.numberOfLines = i
            v.text = testStr
            v.backgroundColor = .cyan
            v.lineBreakMode = .byTruncatingTail
            textViewStackView.addArrangedSubview(v)
            
            textViewStackView.setCustomSpacing(16.0, after: v)
            
        }
        
        scrollView.backgroundColor = UIColor(white: 0.95, alpha: 1.0)
        
    }
    
}

Differences also exist from one iOS version to another – for example: