Contrast foreground text color

UIColor+ContrastTextColor.swift

import UIKit


extension UIColor {
    
    static func contrastTextColor(with background: UIColor) -> UIColor {
        let darkText  = UIColor.dark
        let lightText = UIColor.white
        return background.contrastRatio(with: darkText) > background.contrastRatio(with: lightText) ? darkText : lightText
    }
    
}
    
fileprivate extension UIColor {
    
    //  MARK: Контраст по WCAG 2.0
    func contrastRatio(with color: UIColor) -> CGFloat {
        return UIColor.contrastRatio(between: self, and: color)
    }
    
    private static func contrastRatio(between color1: UIColor, and color2: UIColor) -> CGFloat {
        //  Методика расчета: https://www.w3.org/TR/WCAG20-TECHS/G18.html#G18-tests
        //  Рекомендуется придерживаться контраста > 4.5
        
        let luminance1 = color1.luminance()
        let luminance2 = color2.luminance()
        
        let luminanceDarker  = min(luminance1, luminance2)
        let luminanceLighter = max(luminance1, luminance2)
        
        return (luminanceLighter + 0.05) / (luminanceDarker + 0.05)
    }

    private func luminance() -> CGFloat {
        let ciColor = CIColor(color: self)
        let adjust = { (component: CGFloat) -> CGFloat in
            (component < 0.03928) ? (component / 12.92) : pow((component + 0.055) / 1.055, 2.4)
        }
        return 0.2126 * adjust(ciColor.red) + 0.7152 * adjust(ciColor.green) + 0.0722 * adjust(ciColor.blue)
    }
}