将十六进制的颜色字符串转为UIColor

extension UIColor {
    
    static func colorWithHexString(hex:String) -> UIColor {
        var cString:String = hex.stringByTrimmingCharactersInSet(NSCharacterSet.whitespaceAndNewlineCharacterSet()).uppercaseString
        
        if cString.hasPrefix("#") {
            let index:String.CharacterView.Index = cString.startIndex.advancedBy(1)
             cString = cString.substringFromIndex(index)
        }
        
        if cString.characters.count != 6 {
            return UIColor.grayColor()
        }
        
        //按两位分割字符内容
        let rString = cString.substringToIndex(cString.startIndex.advancedBy(2))
        let gString = cString.substringFromIndex(cString.startIndex.advancedBy(2)).substringToIndex(cString.startIndex.advancedBy(2))
        let bString = cString.substringFromIndex(cString.startIndex.advancedBy(4)).substringToIndex(cString.startIndex.advancedBy(2))
        
        var r:CUnsignedInt = 0, g:CUnsignedInt = 0, b:CUnsignedInt = 0
        NSScanner(string: rString).scanHexInt(&r)
        NSScanner(string: gString).scanHexInt(&g)
        NSScanner(string: bString).scanHexInt(&b)
        
        return UIColor(red: CGFloat(r) / 255.0 , green: CGFloat(g) / 255.0, blue: CGFloat(b) / 255.0, alpha: 1)
    }
}