UIFont's extension which serves FontDescriptor attributes

UIFont+.swift

//
//  UIFont+.swift
//  Autofriend
//
//  Created by Pilipenko Dima on 10/9/17.
//  Copyright © 2017 dimpiax. All rights reserved.
//

import Foundation
import UIKit

private typealias FeatureType = [UIFontDescriptor.FeatureKey: Int]

extension UIFont {
    func addFeatures(_ value: FeatureOptions) -> UIFont {
        var features = [FeatureType]()
        
        let applyExistFeature = { (option: FeatureOptions) in
            guard value.contains(option) else { return }
            
            guard let feature = option.getFeature() else {
                print("Option for \(option.rawValue) are not served")
                return
            }
            
            features.append(feature)
        }
        
        applyExistFeature(.lowerSmallCaps)
        applyExistFeature(.upperSmallCaps)
        applyExistFeature(.stylisticAltSixOn)
        
        let descriptor = fontDescriptor.addingAttributes([.featureSettings: features])
        return UIFont(descriptor: descriptor, size: descriptor.pointSize)
    }
}

struct FeatureOptions: OptionSet {
    let rawValue: Int
    
    static let lowerSmallCaps = FeatureOptions(rawValue: 1 << 0)
    static let upperSmallCaps = FeatureOptions(rawValue: 1 << 1)
    static let stylisticAltSixOn = FeatureOptions(rawValue: 1 << 2)
    
    static let all: FeatureOptions = [.lowerSmallCaps, .upperSmallCaps, .stylisticAltSixOn]
    
    fileprivate func getFeature() -> FeatureType? {
        switch rawValue {
            case FeatureOptions.lowerSmallCaps.rawValue:
                return buildFeature(identifier: kLowerCaseType, type: kLowerCaseSmallCapsSelector)
            
            case FeatureOptions.upperSmallCaps.rawValue:
                return buildFeature(identifier: kUpperCaseType, type: kUpperCaseSmallCapsSelector)
            
            case FeatureOptions.stylisticAltSixOn.rawValue:
                return buildFeature(identifier: kStylisticAlternativesType, type: kStylisticAltSixOnSelector)
            
            // executes, only if some options not declared here
            default: return nil
        }
    }
    
    private func buildFeature(identifier: Int, type: Int) -> FeatureType {
        return [.featureIdentifier: identifier, .typeIdentifier: type]
    }
}

// Usage:
let font = UIFont.systemFont(ofSize: 13, weight: .light).addFeatures([.lowerSmallCaps])