UIColor和十六进制颜色值得互相转换（包括透明度）

在开发中美工进行标注都是十六进制的颜色值，很少直接写具体的颜色值，有时出现的具体的颜色值也是已经规定好的十六进制的颜色值；十六进制一般以#开头，也有0x开头的，后面的才是真正的颜色值，一般都是6位数字或者字母或者两者的混合，也有8位的，这种是带有透明度的，8位的前两位就是透明度。

透明度参照表：

0%=FF（不透明）    5%=F2    10%=E5    15%=D8    20%=CC    25%=BF    30%=B2    35%=A5    40%=99    45%=8c    50%=7F   
55%=72    60%=66    65%=59    70%=4c    75%=3F    80%=33    85%=21    90%=19    95%=0c    100%=00（全透明）

要想把美工标注的颜色值显示出来必须转成UIColor，我们一般把转换方法封装起来：

//如果hexString是8位则参数alpha失效,使用hexString包含的alpha
+ (UIColor *)colorWithHexString:(NSString *)hexString alpha:(CGFloat)alpha {
    //去除空格
    NSString *cString = [[hexString stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceAndNewlineCharacterSet]] uppercaseString];
    //把开头截取
    if ([cString hasPrefix:@"0X"]) cString = [cString substringFromIndex:2];
    if ([cString hasPrefix:@"#"]) cString = [cString substringFromIndex:1];
    //6位或8位(带透明度)
    if ([cString length] < 6) {
        return nil;
    }
    //取出透明度、红、绿、蓝
    unsigned int a, r, g, b;
    NSRange range;
    range.location = 0;
    range.length = 2;
    if (cString.length == 8) {
        //a
        NSString *aString = [cString substringWithRange:range];
        //r
        range.location = 2;
        NSString *rString = [cString substringWithRange:range];
        //g
        range.location = 4;
        NSString *gString = [cString substringWithRange:range];
        //b
        range.location = 6;
        NSString *bString = [cString substringWithRange:range];
        
        [[NSScanner scannerWithString:aString] scanHexInt:&a];
        [[NSScanner scannerWithString:rString] scanHexInt:&r];
        [[NSScanner scannerWithString:gString] scanHexInt:&g];
        [[NSScanner scannerWithString:bString] scanHexInt:&b];
        
        return [UIColor colorWithRed:(r / 255.0f) green:(g / 255.0f) blue:(b / 255.0f) alpha:(a / 255.0f)];
    } else {
        //r
        NSString *rString = [cString substringWithRange:range];
        //g
        range.location = 2;
        NSString *gString = [cString substringWithRange:range];
        //b
        range.location = 4;
        NSString *bString = [cString substringWithRange:range];
        
        [[NSScanner scannerWithString:rString] scanHexInt:&r];
        [[NSScanner scannerWithString:gString] scanHexInt:&g];
        [[NSScanner scannerWithString:bString] scanHexInt:&b];
        
        return [UIColor colorWithRed:(r / 255.0f) green:(g / 255.0f) blue:(b / 255.0f) alpha:alpha];
    }
}

下面的方法是把UIColor转换成十六进制颜色值：

- (NSString *)hexString {
    //颜色值个数，rgb和alpha
    NSInteger cpts = CGColorGetNumberOfComponents(self.CGColor);
    const CGFloat *components = CGColorGetComponents(self.CGColor);
    CGFloat r = components[0];//红色
    CGFloat g = components[1];//绿色
    CGFloat b = components[2];//蓝色
    if (cpts == 4) {
        CGFloat a = components[3];//透明度
        return [NSString stringWithFormat:@"#%02lX%02lX%02lX%02lX", lroundf(a * 255), lroundf(r * 255), lroundf(g * 255), lroundf(b * 255)];
    } else {
        return [NSString stringWithFormat:@"#%02lX%02lX%02lX", lroundf(r * 255), lroundf(g * 255), lroundf(b * 255)];
    }
}