Helpers to use UIKit font features in Swift

UIFont+Features.swift

import UIKit

// Usage:
//
// ```
// UIFont.systemFont(ofSize: 14).usingFeatures([.tabularFigures])
// ```
//

extension UIFont {
    enum Feature {
        case smallCaps
        case tabularFigures

        fileprivate var features: [[UIFontDescriptor.FeatureKey: Int]] {
            switch self {
            case .smallCaps:
                let upperCaseFeature = [
                    UIFontDescriptor.FeatureKey.featureIdentifier: kUpperCaseType,
                    .typeIdentifier: kUpperCaseSmallCapsSelector,
                ]
                let lowerCaseFeature = [
                    UIFontDescriptor.FeatureKey.featureIdentifier: kLowerCaseType,
                    .typeIdentifier: kLowerCaseSmallCapsSelector,
                ]
                return [upperCaseFeature, lowerCaseFeature]
            case .tabularFigures:
                let tabularFigures = [
                    UIFontDescriptor.FeatureKey.featureIdentifier: kNumberSpacingType,
                    .typeIdentifier: kMonospacedNumbersSelector,
                ]
                return [tabularFigures]
            }
        }
    }

    func usingFeatures(_ features: Set<Feature>) -> UIFont {
        let descriptor = fontDescriptor.addingAttributes([.featureSettings: features.flatMap { $0.features } ])
        return UIFont(descriptor: descriptor, size: pointSize)
    }
}