inheritance - Expose a private Objective-C method or property to subclasses

Question

According to some official talk, a class in Objective-C should only expose public methods and properties in its header:

@interface MyClass : NSObject

@property (nonatomic, strong) MyPublicObject *publicObject;

- (void)publicMethod;

@end

and private methods/properties should be kept in class extension in .m file:

@interface MyClass()

@property (nonatomic, strong) MyPrivateObject *privateObject;

- (void) privateMethod;

@end

and I don't think there is a protected type for things that are private but accessible from subclasses. I wonder, is there anyway to achieve this, apart from declaring private properties/methods publicly?

Answer 1

One way to solve this is to re-declare the property in your subclass's class extension, and then add an @dynamic statement so that the compiler won't create an overriding implementation of that property. So something like:

@interface SuperClass ()

@property (nonatomic, strong) id someProperty;

@end

....


@interface SubClass ()

@property (nonatomic, strong) id someProperty;

@end

@implementation SubClass

@dynamic someProperty;

@end

This obviously isn't ideal because it duplicates a privately visible declaration. But it is quite convenient and helpful in some situations so I'd say evaluate on a case-by-case basis the dangers involved in this duplication vs. exposing the property in the public interface.

An alternative - that is used by Apple in UIGestureRecognizer - is to declare the property in a separate category header file explicitly named as "private" or "protected" e.g. "SomeClass+Protected.h". That way, other programmers will know they ought not import the file. But, if you don't control the code you're inheriting from, that's not an option.