How can I declare a private property within a named category?

Question

I know about the possibility of declaring private properties on a class by putting them inside an unnamed category on that class declared in the implementation (.m) file of that class. That's not what I want to do.

I'm dealing with a named category on a class that adds some functionality to that class. For this functionality, it would help me very much to have a private property to use in my category - so the usual way of achieving this (described above) doesn't seem to work for me. Or does it? Please enlighten me!

Answer 1

Inside your category's implementation file, declare another category and call it something like MyCategoryName_Private, and declare your private properties there. Provide implementations of the -propertyName and -setPropertyName: methods using associated objects.

For example, your implementation file might look like this:

#import "SomeClass+MyCategory.h"
#import <objc/runtime.h>

@interface SomeClass (MyCategory_Private)

@property (nonatomic, strong) id somePrivateProperty;

@end

@implementation SomeClass (MyCategory_Private)

static void *AssociationKey;

- (id)somePrivateProperty 
{
    return objc_getAssociatedObject(self, AssociationKey);
}

- (void)setSomePrivateProperty:(id)arg
{
    objc_setAssociatedObject(self, AssociationKey, arg, OBJC_ASSOCIATION_RETAIN);
}

@end

@implementation SomeClass (MyCategory)

// implement your publicly declared category methods

@end