property inheritance: Auto property synthesis will not synthesize property

Question

Abstract:

This question is about inheritance of properties in combination with different read/write accesses from inside and outside of the classes inheriting the property from one another.

Details:

I have one class A and another class B, which inherits from A. There is the property someProperty declared in A. I want the property to be readonly from outside these classes and read/write from inside.

With only one class, this is dead-simple: You declare the property in the .h as readonly and you declare it again as readwrite in the .m inside of a category. Done.

But with the two classes, one deriving from the other, I get the below compiler warning in B:

Auto property synthesis will not synthesize property 'someProperty' because it is 'readwrite' but it will be synthesized 'readonly' via another property

Here is the code:

A.h:

#import <Foundation/Foundation.h>

@interface A : NSObject

// This property shall be readonly from outside, but read/write from subclasses
@property (readonly) SInt32 someProperty;

@end

A.m:

#import "A.h"

@implementation A
@end

B.h:

#import <Foundation/Foundation.h>
#import "A.h"

@interface B : A

@end

B.m:

#import "B.h"    

@interface B ()

// compiler warning in the following property declaration:
// /Users/.../B.m:12:41: Auto property synthesis will not synthesize property
// 'someProperty' because it is 'readwrite' but it will be synthesized
// 'readonly' via another property
@property (readwrite) SInt32 someProperty;

@end

@implementation B
@end

Why does this warning appear and how should I structure my code to avoid it?

Solution

You need to declare the property as read-write on the owning class (A), and then redeclare on the subclass (B) to make the compiler aware that you want to use it there. So, A hosts the accessor method and B uses it. Generally you don't want B to create another accessor method so you can use @dynamic to tell the compiler that the superclass (technically, just another class) will provide the implementation.

Note that you can also declare a category (not extension) on A, in B.m which declares the accessor method explicitly (not using a property, just a method) as that is what you're actually interested in (you don't actually want any of the other things that a property specifies and you don't really want the maintenance overhead of ensuring that the property attributes match in the super and subclass)...