Instantiate a UIColor from a hex string with 6 or 8 chars

UIColor+Hex.swift

extension UIColor {
    // Adapted from https://gist.github.com/yannickl/16f0ed38f0698d9a8ae7, modified to accept alpha
    // values. Works with strings with or without "#", 6 char strings like "a735b5", 8 char strings
    // with alpha value like "836be64"
    convenience init(hexString: String) {
        let hexString = hexString.trimmingCharacters(in: .whitespacesAndNewlines)
        let scanner = Scanner(string: hexString)

        if (hexString.hasPrefix("#")) {
            scanner.scanLocation = 1
        }

        let initialPosition = scanner.scanLocation
        var color: UInt32 = 0
        scanner.scanHexInt32(&color)
        let numCharsScanned = scanner.scanLocation - initialPosition

        let alphaOffset = numCharsScanned == 8 ? 8 : 0
        let mask = 0x000000FF
        let r = Int(color >> (16 + alphaOffset)) & mask
        let g = Int(color >> (8 + alphaOffset)) & mask
        let b = Int(color >> (alphaOffset)) & mask

        let red = CGFloat(r) / 255.0
        let green = CGFloat(g) / 255.0
        let blue = CGFloat(b) / 255.0

        let alpha: CGFloat
        if alphaOffset > 0 {
            let a = Int(color) & mask
            alpha = CGFloat(a) / 255.0
        } else {
            alpha = 1
        }

        self.init(red: red, green: green, blue: blue, alpha: alpha)
    }
}