Apply font style (bold / italic) & features (monospaced digit).

UIFont+Traits.swift

import CoreText
import class UIKit.UIFont
import class UIKit.UIFontDescriptor

extension UIFont {
	public func asBold() -> UIFont? {
		return asTraits(.traitBold)
	}
	
	public func asItaric() -> UIFont? {
		return asTraits(.traitItalic)
	}
	
	private func asTraits(_ traits: UIFontDescriptor.SymbolicTraits) -> UIFont? {
		guard let fontDesc = self.fontDescriptor.withSymbolicTraits(traits) else { return nil }
		return UIFont(descriptor: fontDesc, size: 0)
	}
	
	public func asMonospacedDigit() -> UIFont {
		return customized(by: [
			.typeIdentifier: kNumberSpacingType,
			.featureIdentifier: kMonospacedNumbersSelector,
		])
	}
	
	public func customized(by features: [UIFontDescriptor.FeatureKey: Any]) -> UIFont {
		let fontDesc = self.fontDescriptor.addingAttributes([.featureSettings: features])
		return UIFont(descriptor: fontDesc, size: 0)
	}
}