Question

If I have this class defined, how do I access the someObject property in subclasses without compiler errors?

@interface MyBaseClass
  // someObject property not declared here because I want it to be scoped 
  // protected. Only this class instance and subclass instances should be
  // able to see the someObject property.
@end

// This is a private interface extension...properties declared here
// won't be visible to subclasses. However, I don't see any way to 
// declare protected properties...
@interface MyBaseClass (private)
   @property (nonatomic, readwrite, retain) NSObject *someObject;
@end

@interface MySubclass : MyBaseClass 
@end

@implementation MySubclass

- (id) init {
    // Try to do something with the super classes' someObject property. 
    // Always throws compile errors.

    // Semantic Issue: Property 'someObject' not found 
    // object of type 'MySubclass *'
    self.someObject = nil; 

}
@end

I’m obviously not understanding how inheritance works in objective-c. Could someone enlighten me?

Answer 1

The solution you’re after is to declare the MyBaseClass private property in a class extension:

@interface MyBaseClass ()
@property (nonatomic, readwrite, retain) NSObject *someObject;
@end

You are then free to make that declaration both in MyBaseClass and in MySubclass. This lets MySubclass know about these properties so that its code can talk about them.

If the repetition bothers you, put the class extension in a .h file of its own and import it into both .m files.

I will give an example from my own code. Here is MyDownloaderPrivateProperties.h:

@interface MyDownloader ()
@property (nonatomic, strong, readwrite) NSURLConnection* connection;
@property (nonatomic, copy, readwrite) NSURLRequest* request;
@property (nonatomic, strong, readwrite) NSMutableData* mutableReceivedData;
@end

There is no corresponding .m file and that’s all that’s in this file; it is, as it were, purely declarative. Now here’s the start of MyDownloader.m:

#import "MyDownloader.h"
#import "MyDownloaderPrivateProperties.h"
@implementation MyDownloader
@synthesize connection=_connection;
@synthesize request=_request;
@synthesize mutableReceivedData=_mutableReceivedData;
// ...

And here’s the start of its subclass MyImageDownloader.m:

#import "MyImageDownloader.h"
#import "MyDownloaderPrivateProperties.h"

Problem solved. Privacy is preserved, as these are the only classes that import MyDownloaderPrivateProperties.h so they are the only classes that know about these properties as far as the compiler is concerned (and that’s all that privacy is in Objective-C). The subclass can access the private properties whose accessors are synthesized by the superclass. I believe that’s what you wanted to accomplish in the first place.