我可以使用类别来添加类方法吗？

我想添加一些类方法到UIColor。我实现了它们，一切编译正常，但在运行时我得到以下错误：

I want to add some class methods to UIColor. I've implemented them and everything compiles fine, but at runtime I get the following error:

由于未捕获异常终止应用程序'NSInvalidArgumentException' ，原因：'+ [UIColor colorWithHex：]：无法识别的选择器发送到类0x8d1d68'

Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '+[UIColor colorWithHex:]: unrecognized selector sent to class 0x8d1d68'

这是头文件：

@interface UIColor (Hex) 
+ (UIColor*) colorWithHex: (NSUInteger) hex;
@end

这是实现：

#import "UIColor+Hex.h"


@implementation UIColor (Hex)

+ (UIColor*) colorWithHex: (NSUInteger) hex {
    CGFloat red, green, blue, alpha;

    red = ((CGFloat)((hex >> 16) & 0xFF)) / ((CGFloat)0xFF);
    green = ((CGFloat)((hex >> 8) & 0xFF)) / ((CGFloat)0xFF);
    blue = ((CGFloat)((hex >> 0) & 0xFF)) / ((CGFloat)0xFF);
    alpha = hex > 0xFFFFFF ? ((CGFloat)((hex >> 24) & 0xFF)) / ((CGFloat)0xFF) : 1;

    return [UIColor colorWithRed: red green:green blue:blue alpha:alpha];
}
@end

我找到了一些关于添加-all_load链接器标志，但这样做给出相同的结果。这是在iPhone上，如果不清楚。

I've found something about adding -all_load to the linker flags, but doing that gives the same result. This is on the iPhone, if it wasn't clear.