How to override a function that is part of a category's original class?

Question

for example let's say i have a class car with the following .h:

Car.h

@interface Car : Automobile

@property Engine * V6;

-(void)igniteEngine:(int) key;

-(void)StopEngine;

@end

and now I have a category Car(Test) with the .m file

Car(Test).m

@implementation Car(Test){
    Oil * testingOil;
}

...

@end

and i want to override a function in the following way

-(void)igniteEngine:(int) key //inside test
{
    [self applyTestingOil];
    [(reference to original class) igniteEngine:key];
}

how would I go by doing this? if there is a different way of approaching this problem that would also be helpful

Answer 1

Overriding a method in a category is not an option, as the behavior at runtime will be undefined: you have no way to know which of the base class or category implementation will be executed.

See Avoid Category Method Name Clashes.

If possible, the simplest way to do this is to subclass the base class, and override the method there.

Is subclassing is not an option, you can use method swizzling. This will exchange the method implementation with your own at runtime. But as Mattt put it: "Swizzling is widely considered a voodoo technique", so I'd avoid it if I could.

If you think swizzling could solve your problem, and you're ready to "take the risk", take a look at JRSwizzle, which will handle most of the boilerplate for you.

Considering your ivar Oil * testingOil;, subclassing is again the only simple option. As rmaddy said in the comments, categories does not allow you to add ivars to the class. You'll need to use Associated Objects for this. And well... it's not pretty.