Extension to UIFont that allows to create a monospaced digits font from a instance of UIFont. If the font is not a San Francisco font it simply returns self.

UIFont+MonospacedDigitsFont.swift

//
//  UIFont+MonospacedDigitsFont.swift
//
//  Created by Samuel Mellert on 29/12/15.
//

import UIKit

extension UIFont {
    
    func monospacedDigitsFont() -> UIFont {
        
        if #available(iOS 9, *) {

            guard self.fontName.hasPrefix(".SFUI") else {
                return self
            }
            
            guard self.fontName.componentsSeparatedByString("-").count == 2,
                let weightSuffix = self.fontName.componentsSeparatedByString("-").last else {
                    
                return UIFont.monospacedDigitSystemFontOfSize(self.pointSize, weight: UIFontWeightRegular)
            }
            
            let fontWeight: CGFloat
            switch weightSuffix {
            case "Ultralight":
                fontWeight = UIFontWeightUltraLight
            case "Light":
                fontWeight = UIFontWeightLight
            case "Thin":
                fontWeight = UIFontWeightThin
            case "Regular":
                fontWeight = UIFontWeightRegular
            case "Medium":
                fontWeight = UIFontWeightMedium
            case "Semibold":
                fontWeight = UIFontWeightSemibold
            case "Bold":
                fontWeight = UIFontWeightBold
            case "Heavy":
                fontWeight = UIFontWeightHeavy
            case "Black":
                fontWeight = UIFontWeightBlack
            default:
                fontWeight = UIFontWeightRegular
            }
            
            return UIFont.monospacedDigitSystemFontOfSize(self.pointSize, weight: fontWeight)
        }
        
        return self
    }
    
}