CTFramesetterSuggestFrameSizeWithConstraints sometimes returns incorrect size?

Question

In the code below, CTFramesetterSuggestFrameSizeWithConstraints sometimes returns a CGSize with a height that is not big enough to contain all the text that is being passed into it. I did look at this answer. But in my case the width of the text box needs to be constant. Is there any other/better way to figure out the correct height for my attributed string? Thanks!

CTFramesetterRef framesetter = CTFramesetterCreateWithAttributedString(attributedString);
CGSize tmpSize = CTFramesetterSuggestFrameSizeWithConstraints(framesetter, CFRangeMake(0,0), NULL, CGSizeMake(self.view.bounds.size.width, CGFLOAT_MAX), NULL); 
CGSize textBoxSize = CGSizeMake((int)tmpSize.width + 1, (int)tmpSize.height + 1);

Answer 1

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).

Answer 2

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);

Answer 3

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)

Answer 4

I finally find out.. When uses CTFramesetterSuggestFrameSizeWithConstraints returning CGSize to draw text, the size is considered not big enough(sometimes) for the whole text, so last line is not drew.. I just add 1 to size.height for return, it appears to be right now.

Something like this:

suggestedSize = CGSizeMake(suggestedSize.width, suggestedSize.height + 1);