Enables creating a UIFont that's a variant of another font in a different family

NSMutableAttributedString+FontWithFamily.h

@import Foundation;

@interface NSMutableAttributedString (FontWithFamily)
// Modifies the attributed text to change the font family while keeping
// font size and traits.
- (void)setFontFamily:(NSString *)fontFamily inRange:(NSRange)range;
- (void)setFontFamily:(NSString *)fontFamily;
@end

NSMutableAttributedString+FontWithFamily.m

#import "NSMutableAttributedString+FontWithFamily.h"
#import "UIFont+FontWithFamily.h"

@implementation NSMutableAttributedString (FontWithFamily)

- (void)setFontFamily:(NSString *)fontFamily inRange:(NSRange)range {
    [self enumerateAttribute:NSFontAttributeName inRange:range options:0 usingBlock:^(id value, NSRange blockRange, BOOL *stop) {
        *stop = NO;
        UIFont *oldFont = value;
        UIFont *newFont = [oldFont fontWithFamily:fontFamily];
        [self addAttribute:NSFontAttributeName value:newFont range:blockRange];
    }];
}

- (void)setFontFamily:(NSString *)fontFamily {
    [self setFontFamily:fontFamily inRange:NSMakeRange(0, self.length)];
}

@end

UIFont+FontWithFamily.h

@import UIKit;

@interface UIFont (FontWithFamily)
// Gets a font from a different family while preserving point size
// and traits such as Bold and Italic.
- (UIFont *)fontWithFamily:(NSString *)fontFamily;
@end

UIFont+FontWithFamily.m

#import "UIFont+FontWithFamily.h"

@implementation UIFont (FontWithFamily)

- (UIFont *)fontWithFamily:(NSString *)fontFamily {
    UIFontDescriptor *desc = [self fontDescriptor];

    //
    // Change the family
    //
    UIFontDescriptor *newDesc = [desc fontDescriptorWithFamily:fontFamily];

    //
    // Preserve the traits (bold, italic)
    //
    // In symbolic traits, the lower 16 bits represent the typeface, and the upper 16 bits describe appearance of the font.
    // We're interested in preserving typeface (TraitBold, TraitItalic, etc.)
    // and NOT interested in preserving appearance (ClassSansSerif etc.)
    //
    // For example, when changing from Times to Helvetica, you cannot remove the ClassModernSerifs
    // trait from the Times' description, nor add the ClassSansSerif trait to the Helvetica description.
    // What you CAN do, is apply masked traits AFTER changing the font family.
    //
    newDesc = [newDesc fontDescriptorWithSymbolicTraits:(desc.symbolicTraits & 0xFFFF)];

    //
    // Preserve font point size.
    //
    return [UIFont fontWithDescriptor:newDesc size:self.pointSize];
}

@end