/// 参考 WeexSDK `WXConvert.m` 中的 `+UIColor:` 方法 （且在其基础上添加了对 #rrggbbaa 格式的支持）
/// @param colorString 颜色字符串, 支持以下格式:
///         #fff
///         #rrggbb
///         #rrggbbaa
///         rgb(r,g,b)
///         rgba(r,g,b,a)
+ (UIColor *)colorWithColorString:(NSString *)colorString {
    // 1. check cache
    static NSCache *colorCache;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        colorCache = [[NSCache alloc] init];
        colorCache.countLimit = 64;
    });

    if ([colorString isKindOfClass:[NSNull class]] || !colorString) {
        return nil;
    }

    UIColor *color = [colorCache objectForKey:colorString];
    if (color) {
        return color;
    }

    // Default color is white
    double red = 255, green = 255, blue = 255, alpha = 1.0;

    if([colorString isKindOfClass:[NSString class]]){

        /* `WXConvert.m` 中的 knownColors 相关功能用不上, 已删除*/

        NSString *rgba = colorString;

        if ([rgba hasPrefix:@"#"]) {
            // #fff
            if ([rgba length] == 4) {
              unichar f =   [rgba characterAtIndex:1];
              unichar s =   [rgba characterAtIndex:2];
              unichar t =   [rgba characterAtIndex:3];
              rgba = [NSString stringWithFormat:@"#%C%C%C%C%C%C", f, f, s, s, t, t];
            }

            uint32_t colorValue = 0;
            sscanf(rgba.UTF8String, "#%x", &colorValue);

            if ([rgba length] == 7) {
                // #rrggbb
                red     = ((colorValue & 0xFF0000) >> 16) / 255.0;
                green   = ((colorValue & 0x00FF00) >> 8) / 255.0;
                blue    = (colorValue & 0x0000FF) / 255.0;
            } else if ([rgba length] == 9) {
                // #rrggbbaa
                red     = ((colorValue & 0xFF000000) >> 24) / 255.0;
                green     = ((colorValue & 0x00FF0000) >> 16) / 255.0;
                blue   = ((colorValue & 0x0000FF00) >> 8) / 255.0;
                alpha    = (colorValue & 0x000000FF) / 255.0;
            }

        } else if ([rgba hasPrefix:@"rgb("]) {
            // rgb(r,g,b)
            int r,g,b;
            sscanf(rgba.UTF8String, "rgb(%d,%d,%d)", &r, &g, &b);
            red = r / 255.0;
            green = g / 255.0;
            blue = b / 255.0;
        } else if ([rgba hasPrefix:@"rgba("]) {
            // rgba(r,g,b,a)
            int r,g,b;
            sscanf(rgba.UTF8String, "rgba(%d,%d,%d,%lf)", &r, &g, &b, &alpha);
            red = r / 255.0;
            green = g / 255.0;
            blue = b / 255.0;
        }
    }

    color = [UIColor colorWithRed:red green:green blue:blue alpha:alpha];
    // 6. cache color
    if (color && colorString) {
        [colorCache setObject:color forKey:colorString];
    }

    return color;
}