在运行时将Objective-C方法添加到类中

我试图在旧系统上支持10.9中新添加的NSColor方法.10.9之前我在一个类别中有这些允许我统一Mac和iOS之间的代码.现在10.9有这些方法,无论我在哪里使用它,我都会得到奇怪的绘图.我想动态地将这些方法添加到较旧的运行时,并且我已经找到了几个使用class_addMethod来完成它的参考.问题是,即使addMethod返回成功,也不会调用这些方法.

NSColor *
fColorWithWhite(id self, SEL _cmd, float white, float alpha) {
    return [NSColor colorWithDeviceWhite: white
                                   alpha: alpha];
}

NSColor *
fColorWithRedGreenBlue(id self, SEL _cmd, float red, float green, float blue, float alpha) {
    return [NSColor colorWithDeviceRed: red
                                 green: green
                                  blue: blue
                                 alpha: alpha];
}

+ (void)addLegacySupport {
    Class class = NSClassFromString(@"NSColor");

    BOOL success = class_addMethod(class, @selector(colorWithWhite:alpha:), (IMP)fColorWithWhite, "@@:ff");
    NSLog(@"colorWithWhite:alpha: - %i", success);

    success = class_addMethod(class, @selector(colorWithRed:green:blue:alpha:), (IMP)fColorWithRedGreenBlue, "@@:ffff");
    NSLog(@"colorWithRed:green:blue:alpha: - %i", success);
}

任何指针都将非常感激.