Font Extension for Monospaced Digits (Swift 4.0)

Font+MonospacedDigits.swift

import UIKit

extension UIFont {
    /// Returns a font with monospaced digits, preserving other font attributes.
    var withMonospacedDigits: UIFont {
        let monospacedDescriptor = fontDescriptor.withMonospacedDigits
        return UIFont(descriptor: monospacedDescriptor, size: 0) // Size 0 preserves the original font size.
    }
}

private extension UIFontDescriptor {
    /// Returns a font descriptor with monospaced digits feature enabled.
    var withMonospacedDigits: UIFontDescriptor {
        let featureSettings = [
            [UIFontDescriptor.FeatureKey.type: kNumberSpacingType,
             UIFontDescriptor.FeatureKey.selector: kMonospacedNumbersSelector]
        ]
        return addingAttributes([.featureSettings: featureSettings])
    }
}