写代码颜色值的时候会用到#ffffff 这样的格式，UIColor不能直接识别，我们要转换一下。

在SwiftTheme中看到了这个功能，在这里分享一下。

public enum UIColorInputError : Error {
    case missingHashMarkAsPrefix,unableToScanHexValue,mismatchedHexStringLength
}

extension UIColor {
    /**
     The shorthand three-digit hexadecimal representation of color.
     #RGB defines to the color #RRGGBB.
     
     - parameter hex3: Three-digit hexadecimal value.
     - parameter alpha: 0.0 - 1.0. The default is 1.0.
     */
    public convenience init(hex3: UInt16,alpha: CGFloat = 1) {
        let divisor = CGFloat(15)
        let red     = CGFloat((hex3 & 0xF00) >> 8) / divisor
        let green   = CGFloat((hex3 & 0x0F0) >> 4) / divisor
        let blue    = CGFloat( hex3 & 0x00F      ) / divisor
        self.init(red: red,green: green,blue: blue,alpha: alpha)
    }
    
    /**
     The shorthand four-digit hexadecimal representation of color with alpha.
     #RGBA defines to the color #RRGGBBAA.
     
     - parameter hex4: Four-digit hexadecimal value.
     */
    public convenience init(hex4: UInt16) {
        let divisor = CGFloat(15)
        let red     = CGFloat((hex4 & 0xF000) >> 12) / divisor
        let green   = CGFloat((hex4 & 0x0F00) >>  8) / divisor
        let blue    = CGFloat((hex4 & 0x00F0) >>  4) / divisor
        let alpha   = CGFloat( hex4 & 0x000F       ) / divisor
        self.init(red: red,alpha: alpha)
    }
    
    /**
     The six-digit hexadecimal representation of color of the form #RRGGBB.
     
     - parameter hex6: Six-digit hexadecimal value.
     */
    public convenience init(hex6: UInt32,alpha: CGFloat = 1) {
        let divisor = CGFloat(255)
        let red     = CGFloat((hex6 & 0xFF0000) >> 16) / divisor
        let green   = CGFloat((hex6 & 0x00FF00) >>  8) / divisor
        let blue    = CGFloat( hex6 & 0x0000FF       ) / divisor
        self.init(red: red,alpha: alpha)
    }
    
    /**
     The six-digit hexadecimal representation of color with alpha of the form #RRGGBBAA.
     
     - parameter hex8: Eight-digit hexadecimal value.
     */
    public convenience init(hex8: UInt32) {
        let divisor = CGFloat(255)
        let red     = CGFloat((hex8 & 0xFF000000) >> 24) / divisor
        let green   = CGFloat((hex8 & 0x00FF0000) >> 16) / divisor
        let blue    = CGFloat((hex8 & 0x0000FF00) >>  8) / divisor
        let alpha   = CGFloat( hex8 & 0x000000FF       ) / divisor
        self.init(red: red,alpha: alpha)
    }
    
    /**
     The rgba string representation of color with alpha of the form #RRGGBBAA/#RRGGBB,throws error.
     
     - parameter rgba: String value.
     */
    public convenience init(rgba_throws rgba: String) throws {
        guard rgba.hasPrefix("#") else {
            throw UIColorInputError.missingHashMarkAsPrefix
        }
        
        /*guard let hexString: String = rgba.substring(from: rgba.characters.index(rgba.startIndex,offsetBy: 1)),var   hexValue:  UInt32 = 0,Scanner(string: hexString).scanHexInt32(&hexValue) else {
                throw UIColorInputError.unableToScanHexValue
        }*/
        
        let hexString: String = String(rgba[rgba.index(rgba.startIndex,offsetBy: 1)...])
        var hexValue:  UInt32 = 0
        Scanner(string: hexString).scanHexInt32(&hexValue)
        
        guard hexString.characters.count  == 3
            || hexString.characters.count == 4
            || hexString.characters.count == 6
            || hexString.characters.count == 8 else {
                throw UIColorInputError.mismatchedHexStringLength
        }
        
        switch (hexString.characters.count) {
        case 3:
            self.init(hex3: UInt16(hexValue))
        case 4:
            self.init(hex4: UInt16(hexValue))
        case 6:
            self.init(hex6: hexValue)
        default:
            self.init(hex8: hexValue)
        }
    }
    
    /**
     The rgba string representation of color with alpha of the form #RRGGBBAA/#RRGGBB,fails to default color.
     
     - parameter rgba: String value.
     */
    public convenience init(rgba: String,defaultColor: UIColor = UIColor.clear) {
        guard let color = try? UIColor(rgba_throws: rgba) else {
            self.init(cgColor: defaultColor.cgColor)
            return
        }
        self.init(cgColor: color.cgColor)
    }
    
    /**
     Hex string of a UIColor instance.
     
     - parameter rgba: Whether the alpha should be included.
     */
    public func hexString(_ includeAlpha: Bool) -> String {
        var r: CGFloat = 0
        var g: CGFloat = 0
        var b: CGFloat = 0
        var a: CGFloat = 0
        self.getRed(&r,green: &g,blue: &b,alpha: &a)
        
        if (includeAlpha) {
            return String(format: "#%02X%02X%02X%02X",Int(r * 255),Int(g * 255),Int(b * 255),Int(a * 255))
        } else {
            return String(format: "#%02X%02X%02X",Int(b * 255))
        }
    }
    
    open override var description: String {
        return self.hexString(true)
    }
    
    open override var debugDescription: String {
        return self.hexString(true)
    }
}