Can categories be inherited?

Question

Say I have a class Parent and the child is Child1 and Child2.

I declare Parent+ParentCategory and then I declare a method:

-(void) Redraw;

Later I want the Redraw function of Child1 to be different than the Redraw function of Child2.

Can I declare Child1+Child1Category and then override the

-(void) Redraw

Can -(void)Redraw of Child1 call the function in the parent category?

Can that be done?

What would be good approach?

I know classes can inherit each other. I know protocol can extend each other. I wonder why category doesn't?

Answer 1

I do not agree with the given answers:

A.: It is not true, that category's methods win against class' method. This is an implementation detail, but the behavior is not defined.

If the name of a method declared in a category is the same as a method in the original class, or a method in another category on the same class (or even a superclass), the behavior is undefined as to which method implementation is used at runtime.

B. But this does not matter. You neither define a method in a category that is defined in the class nor define a method in two clashing categories. The cit. simply does not apply to your case. It applies to something like this:

@interface A : NSObject
- (void)doSomething;
@end
@implementation A
- (void)doSomething {}
@end

@interface A (Addition)
- (void)doSomething;
@end
@implementation A (Addition)
- (void)doSomething {}
@end

If I understand you correct, you do not want to do this. You want something like that:

Base level:

@interface A : NSObject
@end
@implementation A
@end

@interface A (Addition)
- (void)doSomething;
@end
@implementation A (Addition)
- (void)doSomething {}
@end

This does not break the rule, because there is no method in the category, which is in the class.

Subclass level:

@interface B : A
@end
@implementation B
@end

@interface B (Addition)
- (void)doSomething;
@end
@implementation B (Addition)
- (void)doSomething {}
@end

1. There is no clash on the level of B for the same reason as there is no clash on the level of A.

2. Somebody might say, that there is a clash, because B inherits -doSomething from A. But this is no clash, because in this case a priority rule applies: Subclasses overwrite methods.

You can do that.

Edit:

You can check the problem simply by adding the method to the class. Would that compile? In the clashing case, it wouldn't. In your case, it would.