Better string metrics on OS X.

NSString+IBNMetrics.h

#import <Foundation/Foundation.h>

@interface NSString (IBNMetrics)

- (CGSize)ibn_sizeConstrainedToSize:(CGSize)size font:(NSFont *)font;
- (CGSize)ibn_sizeWithFont:(NSFont *)font;

@end

NSString+IBNMetrics.m

#import "NSString+IBNMetrics.h"

@implementation NSString (IBNMetrics)

- (CGSize)ibn_sizeWithFont:(NSFont *)font {
	return [self ibn_sizeConstrainedToSize:CGSizeMake(CGFLOAT_MAX, CGFLOAT_MAX) font:font];
}

- (CGSize)ibn_sizeConstrainedToSize:(CGSize)size font:(NSFont *)font {
	CTFontRef coreTextFont = CTFontCreateWithName((__bridge CFStringRef)font.fontName, font.pointSize, NULL);
	
	CGFloat leading = font.leading;
	CTParagraphStyleSetting paragraphSettings[1] = { kCTParagraphStyleSpecifierLineSpacingAdjustment, sizeof(CGFloat), &leading };
	
	CTParagraphStyleRef  paragraphStyle = CTParagraphStyleCreate(paragraphSettings, 1);
	CFRange textRange = CFRangeMake(0, self.length);
	
	CFMutableAttributedStringRef mutableString = CFAttributedStringCreateMutable(kCFAllocatorDefault, self.length);
	
	CFAttributedStringReplaceString(mutableString, CFRangeMake(0, 0), (CFStringRef)self);
	
	CFAttributedStringSetAttribute(mutableString, textRange, kCTFontAttributeName, coreTextFont);
	CFAttributedStringSetAttribute(mutableString, textRange, kCTParagraphStyleAttributeName, paragraphStyle);
	
	CTFramesetterRef framesetter = CTFramesetterCreateWithAttributedString(mutableString);
	CFRange fitRange;
	
	CGSize frameSize = CTFramesetterSuggestFrameSizeWithConstraints(framesetter, textRange, NULL, size, &fitRange);
	
	CFRelease(coreTextFont);
	CFRelease(paragraphStyle);
	CFRelease(framesetter);
	CFRelease(mutableString);
	
	return frameSize;
}

@end

This seems to chop off descenders:

@tonyarnold Thanks for that, I think I didn't actually try this out with multi-line text. I'll look into this more.