UIFont extension to derive new fonts by adding CoreText Font Features, like Proportional Number or Alternate Punctuation.

UIFont+Features.swift

import UIKit
import CoreText

extension UIFont {
    
    typealias Feature = (type: Int, selector: Int)
    struct Features {
        static var ProportionalNumbers: Feature = (kNumberSpacingType, kProportionalNumbersSelector)
        static var AlternatePunctuation: Feature = (kCharacterAlternativesType, 1) // Magic!
    }
    
    func fontByAddingFeatures(features: [Feature]) -> UIFont {
        typealias FeatureDictionary = [String: Int]
        
        let attributes = self.fontDescriptor().fontAttributes()
        let attribute: AnyObject! = attributes[UIFontDescriptorFeatureSettingsAttribute]
        
        var featureDictionaries: [FeatureDictionary] = []
        if let existingFeatureDictionaries = attribute as? [FeatureDictionary] {
            featureDictionaries = existingFeatureDictionaries
        }
        
        for feature in features {
            featureDictionaries += [
                UIFontFeatureTypeIdentifierKey: feature.type,
                UIFontFeatureSelectorIdentifierKey: feature.selector,
            ]
        }
        
        let descriptor = self.fontDescriptor().fontDescriptorByAddingAttributes([
            UIFontDescriptorFeatureSettingsAttribute: featureDictionaries,
            ])
        return UIFont(descriptor: descriptor, size: 0)
    }
    
}

Usage.swift

label.font.fontByAddingFeatures([
    UIFont.Features.ProportionalNumbers,
    UIFont.Features.AlternatePunctuation,
    ])

Fixed thanks to @krzyzanowskim.

I think featureDictionaries += should use .append now.