Can't subclass UIFont - swift

Question

I use custom fonts in my iOS application and have setup the fonts like so:
private enum MalloryProWeight: String {
case book = "MalloryMPCompact-Book"
case medium = "MalloryMPCompact-Medium"
case bold = "MalloryMPCompact-Bold"}
extension UIFont {
enum Caption {
private static var bookFont: UIFont {
UIFont(name: MalloryProWeight.book.rawValue, size: 1)!
}
private static var mediumFont: UIFont {
UIFont(name: MalloryProWeight.medium.rawValue, size: 1)!
}
private static var boldFont: UIFont {
UIFont(name: MalloryProWeight.bold.rawValue, size: 1)!
}
static var book: UIFont {
return bookFont.withSize(10)
}
static var medium: UIFont {
mediumFont.withSize(10)
}
static var bold: UIFont {
boldFont.withSize(10)
}
}
So that at the call site I can do the following:
UIFont.Caption.bold
This works well; I have an NSAttributed extension that takes in. UIFont and color and returns an attributed string = so it all fits nicely.
However, I now have a requirement to set the LetterSpacing and LineHeight on each of my fonts.
I don't want to go and update the NSAttributed extension to take in these values to set them - I ideally want them accessible from UIFont
So, I tried to subclass UIFont to add my own properties to it - like so:
class MrDMyCustomFontFont: UIFont {
var letterSpacing: Double?
}
And use it like so
private static var boldFont: UIFont {
MrDMyCustomFontFont(name: MalloryProWeight.bold.rawValue, size: 1)!
}
However the compiler complains and I am unsure how to resolve it:
Argument passed to call that takes no arguments
So my question is two part:
How can I add my own custom property (and set it on a per-instance base) on UIFont
Else how do I properly subclass UIFont so that I can add my own properties there?
Thanks!

Answer 1

You can't subclass UIFont because it is bridged to CTFont via UICTFont. That's why the init methods are marked "not inherited" in the header. It's not a normal kind of class.
You can easily add a new property to UIFont, but it won't work the way you want it to. It'll be exactly what you asked for: per-instance. But it won't be copied, so the instance returned from boldFont.withSize(10) won't have the same value as boldFont. If you want the code, this is how you do it:
private var letterSpacingKey: String? = nil
extension UIFont {
var letterSpacing: Double? {
get {
(objc_getAssociatedObject(self, &letterSpacingKey) as? NSNumber)?.doubleValue
}
set {
objc_setAssociatedObject(self, &letterSpacingKey, newValue.map(NSNumber.init(value:)),
.OBJC_ASSOCIATION_RETAIN)
}
}
}
And then you can set it:
let font = UIFont.boldSystemFont(ofSize: 1)
font.letterSpacing = 1
print(font.letterSpacing) // Optional(1)
But you'll lose it anytime a derived font is created:
let newFont = font.withSize(10)
print(newFont.letterSpacing) // nil
So I don't think you want that.
But most of this doesn't really make sense. What would you do with these properties? "Letter spacing" isn't a font characteristic; it's a layout/style characteristic. Lying about the font's height metric is probably the wrong tool as well; configuring that is also generally a paragraph characteristic.
What you likely want is a "Style" that tracks all the things in question (font, spacing, paragraph styles, etc) and can be applied to an AttributedString. Luckily that already exists in iOS 15+: AttributeContainer. Prior to iOS 15, you can just use a [NSAttributedString.Key: Any].
Then, instead of an (NS)AttributedString extension to merge your font in, you can just merge your Container/Dictionary directly (which is exactly how it's designed to work).
extension AttributeContainer {
enum Caption {
private static var boldAttributes: AttributeContainer {
var container = AttributeContainer()
container.font = UIFont(name: MalloryProWeight.bold.rawValue, size: 1)!
container.expansion = 1
let paragraphStyle = NSMutableParagraphStyle()
paragraphStyle.lineSpacing = 1.5
container.paragraphStyle = paragraphStyle
return container
}
static var bold: AttributeContainer {
var attributes = boldAttributes
attributes.font = boldAttributes.font.withsize(10)
return attributes
}
}
}