Blog post / Dynamic Type - Defines scalable fonts behaviour and extends itself when conditionally conforms to raw representable of raw type string (what we use in our custom fonts)

ScalableFont.swift

protocol ScalableFont {
    func scaled(forTextStyle textStyle: UIFont.TextStyle?, maxSize: CGFloat?) -> UIFont
}

extension UIFont: ScalableFont {
    func scaled(forTextStyle textStyle: UIFont.TextStyle? = nil, maxSize: CGFloat? = nil) -> UIFont {
        let fontMetrics = textStyle != nil ? UIFontMetrics(forTextStyle: textStyle!) : UIFontMetrics.default
        if let maxSize = maxSize {
            return fontMetrics.scaledFont(for: self, maximumPointSize: maxSize)
        }
        return fontMetrics.scaledFont(for: self)
    }
}