Adds a convenience init to UIColor to allow it to be created from a hex string as in HTML or CSS colour specifications

UIColorByHexString.swift

//
//  UIColorByHexString.swift
//
//  Created by Karl Wilcox on 10/05/2015.
//  Copyright (c) 2015 Karl Wilcox. All rights reserved.
//

import Foundation
import UIKit

/**
 * Adds a convenience init to UIColor to allow it to be created from a 3, 6 or 8 digit hexadecimal string,
 * with an optional '#' or '0x' prefix, as per HTML and CSS colour specifications
 * A 3 digit hex string has each digit doubled (ie. #123 becomes #112233) and becomes a 6 digit string
 * A 6 digit hex string is converted to red, green and blue components and is opaque (alpha = 1)
 * An 8 digit hex string is converted to red, green, blue and alpha components
 * There are no errors reported, the init always succeeds, if no conversion is possible opaque black is produced
 *
 * Example usage: let aliceBlue = UIColor(hexString: "#f0f8ff")
 *
 */

extension UIColor {
    convenience init(hexString:String) {
        
        var redFloat:Float = 0.0
        var greenFloat:Float = 0.0
        var blueFloat:Float = 0.0
        var alphaFloat:Float = 255.0
        
        var digits = Array(hexString)
        var numDigits = digits.count
        
        if numDigits >= 3 {
            if digits[0] ==  "#" {
                digits.removeAtIndex(0)
                numDigits -= 1
            } else if digits[0] == "0" && digits[1] == "x" {
                digits.removeAtIndex(0)
                digits.removeAtIndex(0)
                numDigits -= 2
            }
        }
        
        if numDigits == 3 { // double up hex digits
            digits = [digits[0], digits[0], digits[1], digits[1], digits[2], digits[2]]
            numDigits = 6
        }
        
        // Only go ahead if length is 6 or 8 (extra digits are ignored)
        if numDigits >= 6 {
            NSScanner(string: "0x\(digits[0])\(digits[1])").scanHexFloat(&redFloat)
            NSScanner(string: "0x\(digits[2])\(digits[3])").scanHexFloat(&greenFloat)
            NSScanner(string: "0x\(digits[4])\(digits[5])").scanHexFloat(&blueFloat)
            if numDigits >= 8 {
                NSScanner(string: "0x\(digits[6])\(digits[7])").scanHexFloat(&alphaFloat)
            }
        }
        self.init(red: CGFloat(redFloat / 255), green: CGFloat(greenFloat / 255), blue: CGFloat(blueFloat / 255), alpha: CGFloat(alphaFloat / 255))
    }
}