CTFramesetterSuggestFrameSizeWithConstraints sometimes returns incorrect size?

Working on this problem and following a lot of different answers from several posters, I had implemented a solution for the all mighty problem of correct text size, for me CTFramesetterSuggestFrameSizeWithConstraints is not working properly so we need to use CTFramesetterCreateFrame and then measure the size for that frame, (this is a UIFont extension) this is swift 100%

References

CTFramesetterSuggestFrameSizeWithConstraints sometimes returns incorrect size?

How to get the real height of text drawn on a CTFrame

Using CFArrayGetValueAtIndex in Swift with UnsafePointer (AUPreset)

func sizeOfString (string: String, constrainedToWidth width: Double) -> CGSize {
        let attributes = [NSAttributedString.Key.font:self]
        let attString = NSAttributedString(string: string,attributes: attributes)
        let framesetter = CTFramesetterCreateWithAttributedString(attString)

        let frame = CTFramesetterCreateFrame(framesetter,CFRange(location: 0,length: 0),CGPath.init(rect: CGRect(x: 0, y: 0, width: width, height: 10000), transform: nil),nil)

        return UIFont.measure(frame: frame)
    }

Then we measure our CTFrame

static func measure(frame:CTFrame) ->CGSize {

        let lines = CTFrameGetLines(frame)
        let numOflines = CFArrayGetCount(lines)
        var maxWidth : Double = 0

        for index in 0..<numOflines {
            let line : CTLine = unsafeBitCast(CFArrayGetValueAtIndex(lines, index), to: CTLine.self)
            var ascent : CGFloat = 0
            var descent : CGFloat = 0
            var leading : CGFloat = 0
            let width = CTLineGetTypographicBounds(line, &ascent, &descent, &leading)

            if(width > maxWidth) {
                maxWidth = width
            }
        }

        var ascent : CGFloat = 0
        var descent : CGFloat = 0
        var leading : CGFloat = 0


        CTLineGetTypographicBounds(unsafeBitCast(CFArrayGetValueAtIndex(lines, 0), to: CTLine.self), &ascent, &descent, &leading)
        let firstLineHeight = ascent + descent + leading

        CTLineGetTypographicBounds(unsafeBitCast(CFArrayGetValueAtIndex(lines, numOflines - 1), to: CTLine.self), &ascent, &descent, &leading)
        let lastLineHeight = ascent + descent + leading

        var firstLineOrigin : CGPoint = CGPoint(x: 0, y: 0)
        CTFrameGetLineOrigins(frame, CFRangeMake(0, 1), &firstLineOrigin);

        var lastLineOrigin : CGPoint = CGPoint(x: 0, y: 0)
        CTFrameGetLineOrigins(frame, CFRangeMake(numOflines - 1, 1), &lastLineOrigin);

        let textHeight = abs(firstLineOrigin.y - lastLineOrigin.y) + firstLineHeight + lastLineHeight

        return CGSize(width: maxWidth, height: Double(textHeight))
    }

Thanks to Chris DeSalvo for the excellent answer! Finally ending 16 hours of debugging. I had a little trouble figuring out the Swift 3 syntax. So sharing the Swift 3 version of setting the paragraph style.

let leading = uiFont.lineHeight - uiFont.ascender + uiFont.descender
let paragraphStyle = NSMutableParagraphStyle()
paragraphStyle.lineSpacing = leading      
mutableAttributedString.addAttribute(NSParagraphStyleAttributeName, value: paragraphStyle, range: textRange)

CTFramesetterSuggestFrameSizeWithConstraints() is broken. I filed a bug on this a while back. Your alternative is to use CTFramesetterCreateFrame() with a path that is sufficiently high. Then you can measure the rect of the CTFrame that you get back. Note that you cannot use CGFLOAT_MAX for the height, as CoreText uses a flipped coordinate system from the iPhone and will locate its text at the "top" of the box. This means that if you use CGFLOAT_MAX, you won't have enough precision to actually tell the height of the box. I recommend using something like 10,000 as your height, as that's 10x taller than the screen itself and yet gives enough precision for the resulting rectangle. If you need to lay out even taller text, you can do this multiple times for each section of text (you can ask CTFrameRef for the range in the original string that it was able to lay out).

CTFramesetterSuggestFrameSizeWithConstraints works correctly. The reason that you get a height that is too short is because of the leading in the default paragraph style attached to attributed strings. If you don't attach a paragraph style to the string then CoreText returns the height needed to render the text, but with no space between the lines. This took me forever to figure out. Nothing in the documentation spells it out. I just happened to notice that my heights were short by an amount equal to (number of lines x expected leading). To get the height result you expect you can use code like the following:

NSString  *text = @"This\nis\nsome\nmulti-line\nsample\ntext."
UIFont    *uiFont = [UIFont fontWithName:@"Helvetica" size:17.0];
CTFontRef ctFont = CTFontCreateWithName((CFStringRef) uiFont.fontName, uiFont.pointSize, NULL);

//  When you create an attributed string the default paragraph style has a leading 
//  of 0.0. Create a paragraph style that will set the line adjustment equal to
//  the leading value of the font.
CGFloat leading = uiFont.lineHeight - uiFont.ascender + uiFont.descender;
CTParagraphStyleSetting paragraphSettings[1] = { kCTParagraphStyleSpecifierLineSpacingAdjustment, sizeof (CGFloat), &leading };

CTParagraphStyleRef  paragraphStyle = CTParagraphStyleCreate(paragraphSettings, 1);
CFRange textRange = CFRangeMake(0, text.length);

//  Create an empty mutable string big enough to hold our test
CFMutableAttributedStringRef string = CFAttributedStringCreateMutable(kCFAllocatorDefault, text.length);

//  Inject our text into it
CFAttributedStringReplaceString(string, CFRangeMake(0, 0), (CFStringRef) text);

//  Apply our font and line spacing attributes over the span
CFAttributedStringSetAttribute(string, textRange, kCTFontAttributeName, ctFont);
CFAttributedStringSetAttribute(string, textRange, kCTParagraphStyleAttributeName, paragraphStyle);

CTFramesetterRef framesetter = CTFramesetterCreateWithAttributedString(string);
CFRange fitRange;

CGSize frameSize = CTFramesetterSuggestFrameSizeWithConstraints(framesetter, textRange, NULL, bounds, &fitRange);

CFRelease(framesetter);
CFRelease(string);