Calculate frame of String, that fits given width

StringGetSizeThatFits.swift

// Excerpt from https://github.com/krzyzanowskim/CoreTextWorkshop
// Licence BSD-2 clause
// Marcin Krzyzanowski [email protected]

func getSizeThatFits(_ attributedString: NSAttributedString, maxWidth: CGFloat) -> CGSize {
    let framesetter = CTFramesetterCreateWithAttributedString(attributedString)
    let rectPath = CGRect(origin: .zero, size: CGSize(width: maxWidth, height: 50000))

    let ctFrame = CTFramesetterCreateFrame(framesetter, CFRange(), CGPath(rect: rectPath, transform: nil), nil)

    guard let ctLines = CTFrameGetLines(ctFrame) as? [CTLine], !ctLines.isEmpty else {
	return .zero
    }

    var ctLinesOrigins = Array<CGPoint>(repeating: .zero, count: ctLines.count)
    // Get origins in CoreGraphics coodrinates
    CTFrameGetLineOrigins(ctFrame, CFRange(), &ctLinesOrigins)

    // Transform last origin to iOS coordinates
    let transform: CGAffineTransform
    #if os(macOS)
    transform = CGAffineTransform.identity
    #else
    transform = CGAffineTransform(scaleX: 1, y: -1).concatenating(CGAffineTransform.init(translationX: 0, y: rectPath.height))
    #endif

    guard let lastCTLineOrigin = ctLinesOrigins.last?.applying(transform), let lastCTLine = ctLines.last else {
	return .zero
    }

    // Get last line metrics and get full height (relative to from origin)
    var ascent: CGFloat = 0
    var descent: CGFloat = 0
    var leading: CGFloat = 0
    CTLineGetTypographicBounds(lastCTLine, &ascent, &descent, &leading)
    let lineSpacing = (floor(ascent + descent + leading) * 0.2) + 0.5 // 20% by default, actual value depends on Paragraph
    let lineHeight = floor(ascent + descent + leading) + 0.5

    // Calculate maximum height of the frame
    let maxHeight = lastCTLineOrigin.y + descent + leading + (lineSpacing / 2)
    return CGSize(width: maxWidth, height: maxHeight)
}

It should be floor(value) + 0.5 for pixel aligning.

P.S. Personally I've been using NSAttributedString's boundingRect(with: options:) method for quite some time and it's been okay for the most part.

it's broken tho, it always has been. It works mostly, and when it fails - it depends on the font, characters, and given frames - depends on the context in general. I did use it by myself until it break the layout randomly, which pushed me to debug it further. You will find a bunch of questions about it eg. on StackOverflow.

Missed bracket on 36 line.