How can I get the optical bounds of an NSAttributedString?

Question

I need the optical bounds of an attributed string. I know I can call the .size() method and read its width but this obviously gives me typographic bounds with additional space to the right.

My strings would all be very short and consist only of 1-3 characters, so every string would contain exactly one glyphrun.

I found the function CTRunGetImageBounds, and after following the hints in the link from the comment I was able to extract the run and get the bounds, but obviously this does not give me the desired result.

The following swift 4 code works in an XCode9 Playground:

import Cocoa
import PlaygroundSupport

public func getGlyphWidth(glyph: CGGlyph, font: CTFont) -> CGFloat {
    var glyph = glyph
    var bBox = CGRect()
    CTFontGetBoundingRectsForGlyphs(font, .default, &glyph, &bBox, 1)
    return bBox.width
}


class MyView: NSView {

    init(inFrame: CGRect) {
        super.init(frame: inFrame)
    }

    required init?(coder decoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

    override func draw(_ rect: CGRect) {

        // setup context properties

        let context: CGContext = NSGraphicsContext.current!.cgContext

        context.setStrokeColor(CGColor.black)
        context.setTextDrawingMode(.fill)


        // prepare variables and constants

        let alphabet = ["A","B","C","D","E","F","G","H","I","J","K","L"]
        let font = CTFontCreateWithName("Helvetica" as CFString, 48, nil)
        var glyphX: CGFloat = 10

        // draw alphabet as single glyphs

        for letter in alphabet {
            var glyph = CTFontGetGlyphWithName(font, letter as CFString)
            var glyphPosition = CGPoint(x: glyphX, y: 80)
            CTFontDrawGlyphs(font, &glyph, &glyphPosition, 1, context)
            glyphX+=getGlyphWidth(glyph: glyph, font: font)
        }

        let textStringAttributes: [NSAttributedStringKey : Any] = [
                NSAttributedStringKey.font : font,
            ]
        glyphX = 10

        // draw alphabet as attributed strings

        for letter in alphabet {

            let textPosition = NSPoint(x: glyphX, y: 20)
            let text = NSAttributedString(string: letter, attributes: textStringAttributes)
            let line = CTLineCreateWithAttributedString(text)
            let runs = CTLineGetGlyphRuns(line) as! [CTRun]

            let width = (CTRunGetImageBounds(runs[0], nil, CFRange(location: 0,length: 0))).maxX
            text.draw(at: textPosition)
            glyphX += width
        }
    }
}

var frameRect = CGRect(x: 0, y: 0, width: 400, height: 150)

PlaygroundPage.current.liveView = MyView(inFrame: frameRect)

The code draws the single letters from A - L as single Glyphs in the upper row of the playground's live view. The horizontal position will be advanced after each letter by the letter's width which is retrieved via the getGlyphWidth function.

Then it uses the same letters to create attributed strings from it which will then be used to create first a CTLine, extract the (only) CTRun from it and finally measure its width. The result is seen in the second line in the live view.

The first line is the desired result: The width function returns exactly the width of every single letter, resulting in them touching each other.

I want the same result with the attributed string version, but here the ImageBounds seem to add an additional padding which I want to avoid.

How can I measure the exact width from the leftmost to the rightmost pixel of a given text?

And is there a less clumsy way to achieve this without having to cast four times (NSAtt.Str->CTLine->CTRun->CGRect->maxX) ?

Answer 1

Ok, I found the answer myself:

1. Using the .width parameter of the CTRunGetImageBounds instead of .maxX brings the right result

2. The same function also does exist for the CTLine: CTLineGetImageBounds