This protocol defines functions for determining contrasting colors and other color related values.

UIColor+ContrastColor.swift

// Created by Nedrow, David E on 3/25/19.
// Attribution 4.0 International (CC 4.0), see bottom.

import UIKit

// MARK: - UIColor+ContrastColor.swift
/// This protocol defines functions for determining contrasting colors and other
/// color related values.
public protocol ContrastColorProtocol {
    /// Calculates the luminance of the given color.
    ///
    /// - Parameter color: The color for which the luminance is needed.
    /// - Returns: The luminance of the input color. If this could not be
    ///   calculated, nil is returned.
    static func luminance(_ color: UIColor) -> Double?

    /// Returns a color that is a contrast to the given color.
    ///
    /// - Parameter color: The color for which a contrasting color is needed.
    /// - Returns: The contrasting color.
    static func contrastColor(_ color: UIColor) -> UIColor

    /// Indicates whether a light contrast color is needed for this color instance.
    ///
    /// - Returns: True if a light contrast color should be used with this color.
    ///   `nil` is returned if the calculation failed.
    func useLightContrastingColor() -> Bool?
}

// MARK: - ContrastColorProtocol default implementation.
/// Default implementations for static ContrastColorProtocol functions.
public extension ContrastColorProtocol {
    /// Default implementation of luminance(_ color: UIColor)
    static func luminance(_ color: UIColor) -> Double? {
        // These weightings were proposed by StackOverflow user Gacek in post:
        // https://stackoverflow.com/a/1855903/1227012
        let redWeight: CGFloat = 0.299
        let greenWeight: CGFloat = 0.587 // The human eye favors the green spectrum.
        let blueWeight: CGFloat = 0.114
        let alphaWeight: CGFloat = 0.0 // Alpha is deliberately ignored.

        var redValue: CGFloat = 0.0
        var greenValue: CGFloat = 0.0
        var blueValue: CGFloat = 0.0
        var alphaValue: CGFloat = 0.0

        if !color.getRed(&redValue, green: &greenValue, blue: &blueValue, alpha: &alphaValue) {
            return nil
        }

        return Double((redWeight * redValue) + (greenWeight * greenValue) +
            (blueWeight * blueValue) + (alphaWeight * alphaValue))
    }

    /// Default implementation of contrastColor(_ color: UIColor)
    static func contrastColor(_ color: UIColor) -> UIColor {
        fatalError("contrastColor(_:) has not been implemented")
    }
}

// MARK: - ContrastColorProtocol instance methods.
extension UIColor: ContrastColorProtocol {
    public func useLightContrastingColor() -> Bool? {
        guard let thisLuminance = UIColor.luminance(self) else {
            return nil
        }
        return !(thisLuminance >= 0.5)
    }
}

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//
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