Reference properteries declared in a protocol and implemented in the anonymous category? - objective-c

Question

I have the following protocol:
#protocol MyProtocol
#property (nonatomic, retain) NSObject *myProtocolProperty;
-(void) myProtocolMethod;
#end
and I have the following class:
#interface MyClass : NSObject {
}
#end
I have a class extension declared, I have to redeclare my protocol properties here or else I can't implement them with the rest of my class.
#interface()<MyProtocol>
#property (nonatomic, retain) NSObject *myExtensionProperty;
/*
* This redeclaration is required or my #synthesize myProtocolProperty fails
*/
#property (nonatomic, retain) NSObject *myProtocolProperty;
- (void) myExtensionMethod;
#end
#implementation MyClass
#synthesize myProtocolProperty = _myProtocolProperty;
#synthesize myExtensionProperty = _myExtensionProperty;
- (void) myProtocolMethod {
}
- (void) myExtensionMethod {
}
- (void) useMyConsumer {
[[[MyConsumer new] autorelease] consumeMyClassWithMyProtocol:self];
}
#end
MyConsumer will only be called from MyClass, so I don't want any other classes to see that MyClass implements methods on MyProtocol because they aren't public API. Similarly, I don't want MyConsumer to see the class extension within MyClass.
#interface MyConsumer : NSObject {
}
#end
#implementation MyConsumer
- (void) consumeMyClassWithMyProtocol: (MyClass<MyProtocol> *) myClassWithMyProtocol {
myClassWithMyProtocol.myProtocolProperty; // works, yay!
[myClassWithMyProtocol myProtocolMethod]; // works, yay!
myClassWithMyProtocol.myExtensionProperty; // compiler error, yay!
[myClassWithMyProtocol myExtensionMethod]; // compiler warning, yay!
}
#end
Is there any way I can avoid redeclaring the properties in MyProtocol within my class extension in order to implement MyProtocol privately?

Answer 1

What you've been referring to as an "anonymous category" is actually known as a class extension, and is used to declare private functionality in an implementation file. That last part is important, because it means that other classes will not be able to see the declarations that you put into a class extension (and they won't be able to see that your class implements the methods of MyProtocol). This is also probably what is responsible for #synthesize failing without redeclaring the properties.
Instead, declare your conformance to the protocol in the interface of your class, and add whatever methods you want to be public:
#interface MyClass : NSObject <MyProtocol> {
}
// public methods and properties go here
#end
If you add the protocol declaration to your interface, then it also removes the need for your consumer to specify it explicitly. Your consumer method can have the following signature instead:
- (void) consumeMyClassWithMyProtocol: (MyClass *) myClassWithMyProtocol;
EDIT: It sounds like you're looking for a way to selectively expose private functionality. First, I would try to consider a different architecture for what you're trying to accomplish, because what's about to follow is a rather unpleasant solution, and it's generally better OOP if everything is public or private.
With that said, Apple typically solves this problem by having a separate header file for the class in question, which declares the methods that should be visible. So you would have your class interface, in which you expose everything that should be completely public:
// MyClass.h
#interface MyClass : NSObject {
}
#end
And a separate header, in which you declare a category for pseudo-private stuff:
// MyClass+Private.h
#import "MyClass.h"
#interface MyClass (Private) <MyProtocol>
- (void)mySortaPrivateMethod;
#end
MyClass.m would implement everything from those two files, and could still have a class extension:
// MyClass.m
#import "MyClass.h"
#import "MyClass+Private.h"
#interface MyClass ()
- (void)myClassExtensionMethod;
#end
#implementation MyClass
// everything can go here
#end
Then your consumer would include MyClass+Private.h so that it can see the declarations there, and everyone else would simply use MyClass.h.