Conditional categories in Mountain Lion

Question

Mountain Lion introduced new APIs, some of which we had implemented as categories in our project.

For examples, we have a category NSColor+CGColorAdditions

Answer 1

NSColor *_NSColor_colorWithCGColor_(Class self, SEL cmd, CGColorRef cgColor)
{
    // make an NSColor outta `cgColor` and return it
    return nsColor;
}

// inside some initialization code

if ([[NSColor class] respondsToSelector:@selector(colorWithCGColor:)]) {
    // on ML, don't do anything
} else {
    // older system, add your own category
    class_addMethod(objc_getMetaClass("NSColor"), @selector(colorWithCGColor:), (IMP)_NSColor_colorWithCGColor_, "@@:@");
}

Answer 2

I thought it might also be helpful to post what I did based on H2CO3's answer.

NSColor+CGColorAdditions.m became:

static CGColorRef _NSColor_CGColor_(Class self, SEL cmd) {
    return nil; // TODO: Do something
}

static NSColor* _NSColor_colorWithCGColor_(Class self, SEL cmd, CGColorRef aColor) {
    return nil; // TODO: Do something    
}

__attribute__((constructor))
static void initialize_NSColor_CGColorAdditions() {
    if (![[NSColor class] respondsToSelector:@selector(colorWithCGColor:)]) {
        class_addMethod(objc_getMetaClass("NSColor"), @selector(colorWithCGColor:), (IMP)_NSColor_colorWithCGColor_, "@@:@");
    }
    if (![[NSColor class] instancesRespondToSelector:@selector(CGColor)]) {
        class_addMethod(objc_getClass("NSColor"), @selector(CGColor), (IMP)_NSColor_CGColor_, "@@:");
    }
}