Is there a way to allow certain classes to access private properties of certain other classes?

Question

I have 2 classes that I want to be able to access each others properties, but I don't want those properties accessed from anywhere else. Is there a way to do this? Is the only way to accomplish this through subclassing? Is there no way to establish a "special" relationship between two classes?

Answer 1

If I understand your question, you effectively want class A and class B, which are unrelated by inheritance, to be aware of more innards than are publicly advertised?

Say A has a property called innardsForB that only instances of B should access. You can use class extensions to declare the non-public interface to A.

---

A.h

@interface A:NSObject
... regular class goop here ...
@end

---

A-Private.h

@interface A()
@property(nonatomic, strong) Innards *innardsForB;
@end

---

A.m

#import "A.h"
#import "A-Private.h"

@implementation A
// because "A-Private.h" is #import'd, `innardsForB` will be automatically @synthesized
...
@end

---

B.m

#import "B.h"
#import "A-Private.h"

@implementation B
...
- (void)someMethod
{
     A *a = [self someASomewhere];
     a.innardsForB = [[Innards alloc] initForMeaning:@(42)];
}