NSAttributedString: Setting FontAttributes doesn't change font - ios

Question

I am trying to change the font on an NSAttributedString.
I have a method which iterates over each character, and creates a new NSFontAttribute dictionary for that character. In the end, however, the string remains un-changed. To demonstrate, here is how I setup the string:
UIFontDescriptor *fontDescriptor = [UIFontDescriptor fontDescriptorWithName:#"Avenir-Book" size:14.0f];
NSDictionary *fontAttributes = [fontDescriptor fontAttributes];
[fontAttributes setValue:[NSString stringWithFormat:#"%u",[fontDescriptor symbolicTraits]] forKey:UIFontSymbolicTrait];
[mutableAttributedString setAttributes:fontAttributes range:(NSRange){0,length}];
This produces the following NSFontAttribute dictionary for the entire string:
Font Attributes: {
NSCTFontSymbolicTrait = 2147483648;
NSFontNameAttribute = "Avenir-Book";
NSFontSizeAttribute = 14;
}
I go through and modify each character's FontAttribute to add bolding or italics, as follows:
for (int i = (int)range.location; i < (range.location + range.length); i++){
/* Extract Font Attributes */
NSDictionary *extractedAttributes = [[mutableAttributedString attributesAtIndex:i effectiveRange:NULL]mutableCopy];
/* Determine New Trait */
uint newTrait = ((uint)[[extractedAttributes valueForKey:UIFontSymbolicTrait]longLongValue] | [self symbolicTraitForMarkdownType:markdown]); // (markDown is a mask)
/* Set New Trait */
[extractedAttributes setValue:[NSString stringWithFormat:#"%u",newTrait] forKey:UIFontSymbolicTrait];
/* Create New Font Descriptor */
UIFontDescriptor *newDescriptor = [UIFontDescriptor fontDescriptorWithFontAttributes:extractedAttributes];
newDescriptor = [newDescriptor fontDescriptorWithSymbolicTraits:newTrait];
/* Apply Font Descriptor */
[mutableAttributedString setAttributes:[newDescriptor fontAttributes] range:(NSRange){i,1}];
}
This produces many different FontAttributes:
Index 1: {
NSCTFontSymbolicTrait = 2147483650;
NSFontNameAttribute = "Avenir-Black";
NSFontSizeAttribute = 14;
}
Index 2: {
NSCTFontSymbolicTrait = 2147483651;
NSFontNameAttribute = "Avenir-BlackOblique";
NSFontSizeAttribute = 14;
}
However, the NSAttributedString itself remains completely un-changed. It is still the default Font. How can I get it to reflect the changes I am making to its attributes?

Answer 1

Here is the solution:
1: The documentation for UIFontDescriptor defines the key: UIFontDescriptorNameAttribute as an NSString instance, which it is.
2: The documentation for NSAttributedString defines the key: NSFontAttributeName as a UIFont instance.
So obtaining the fontAttributes dictionary from a UIFontDescriptor initialized with a the method: (UIFontDescriptor *)fontDescriptorWithName:(NSString *)fontName size:(CGFloat)size will only set the keys UIFontDescriptorNameAttribute and UIFontDescriptorSizeAttribute. If you wish to actually modify the font of an NSAttributedString however, you need to apply attributes with a UIFont instance saved to the key NSFontAttributeName.
This is where the confusion comes from. However, it should be noted that you can actually obtain UIFontDescriptorNameAttribute from the fontAttributes dictionary of a UIFontDescriptor instance with the key NSFontAttributeName. This may also be confusing.