SFPro Font Features

UIFont.Features.swift

import UIKit

extension UIFont {
    
    /// Watch [WWDC Session](https://developer.apple.com/videos/play/wwdc2015/804/).
    /// [Font Feature Registry](https://developer.apple.com/fonts/TrueType-Reference-Manual/RM09/AppendixF.html)
    /// - Parameters:
    ///   - size: The size (in points) to which the font is scaled. This value must be greater than 0.0.
    ///   - weight: The weight of the font, specified as a font weight constant.
    ///   - features: Font features supported by SFPro
    ///   - italic: Italic variant of the font.
    public static func systemFont(
        ofSize size: CGFloat,
        weight: Weight = .regular,
        features: [Feature] = [],
        italic: Bool = false
    ) -> UIFont {
        let key = Key(size: size, weight: weight, features: features, italic: italic)
        if let font = cache[key] { return font }
        let descriptor = UIFont.systemFont(ofSize: size, weight: weight).fontDescriptor
        let font = UIFont(
            descriptor: descriptor
                .addingAttributes([.featureSettings: features.map { $0.setting }])
                .withSymbolicTraits(descriptor
                    .symbolicTraits
                    .union(italic ? .traitExpanded : .init(rawValue: 0))
                ) ?? descriptor,
            size: size
        )
        cache[key] = font
        return font
    }
    
    public struct Feature: Hashable {
        private let type: Int
        private let selector: Int
        fileprivate var setting: [UIFontDescriptor.FeatureKey : Int] { [.featureIdentifier: type, .typeIdentifier: selector] }
        
        public static let straightSidedSixAndNine = Feature(type: kStylisticAlternativesType, selector: kStylisticAltOneOnSelector)
        public static let openFour = Feature(type: kStylisticAlternativesType, selector: kStylisticAltTwoOnSelector)
        public static let verticallyAlignedColon = Feature(type: kStylisticAlternativesType, selector: kStylisticAltThreeOnSelector)
        public static let highLegibility = Feature(type: kStylisticAlternativesType, selector: kStylisticAltSixOnSelector)
        public static let oneStoreyA = Feature(type: kStylisticAlternativesType, selector: kStylisticAltSixOnSelector)
        public static let monospacedNumbers = Feature(type: kNumberSpacingType, selector: kMonospacedNumbersSelector)
        public static let diagonalFractions = Feature(type: kFractionsType, selector: kDiagonalFractionsSelector)
        public static let loverCaseSmallCaps = Feature(type: kLowerCaseType, selector: kLowerCaseSmallCapsSelector)
        public static let upperCaseSmallCaps = Feature(type: kUpperCaseType, selector: kUpperCaseSmallCapsSelector)
    }
    
    // For performance reasons fonts are cached
    private static var cache = [Key: UIFont]()
    
    private struct Key: Hashable {
        let size: CGFloat
        let weight: Weight
        let features: [Feature]
        let italic: Bool
    }

}

oneStoreyA seems to be kStylisticAltSevenOnSelector