Can a category simultaneously implement a protocol?

Question

If a category I'm creating for a class adds methods that also fulfill the contract set out by a protocol, I'd like to flag that category class as implementing the protocol, and thereby indicate to the Obj-C pre-processor that the class effectively implements the protocol as well.

Example delegate (for clarity, thanks Ole!):

@protocol SomeDelegate <NSObject>
  - (void)someDelegateMessage;
@end

Example category:

@interface NSObject (SomeCategory) <SomeDelegate>
  - (void)someDelegateMessage;    
@end

And with an otherwise typical implementation

@implement NSObject (SomeCategory)
  - (void)someDelegateMessage {}
@end

When I actually try this, I get a warning for each NSObject method:

warning: incomplete implementation of category 'SomeCategory'

warning: method definition for '-description' not found

...

warning: method definition for '-isEqual:' not found

warning: category 'SomeCategory' does not fully implement the 'NSObject' protocol

Works fine if I remove <SomeDelegate> from the declaration, but of course NSObject isn't recognized as a SomeDelegate

Answer 1

Any chance your protocol declaration includes the NSObject protocol? Like this:

@protocol SomeDelegate <NSObject>
...

That's where the warnings are coming from because now your category does not implement the full protocol. In the test code I just typed up, removing NSObject from the protocol removes the compiler warnings.

Answer 2

If you want the compiler to shut up about sending <NSObject> messages (and its important that you remember that thats the protocol name, not the class name) then just use 'id' variables, not 'id' since thats you explicitly telling the compiler "This is an object which only implements the SomeDelegate protocol".

Alternately, use NSObject as your variable type instead.

Answer 3

A workaround is to declare the protocol on a category with no implementation, and implement the method in a different category, e.g.:

@interface NSObject (SomeCategory) <SomeDelegate>
  - (void)someDelegateMessage;    
@end
...
@implement NSObject (SomeCategory_Impl)
  - (void)someDelegateMessage {}
@end

If you do this, NSObject will be considered to conform to <SomeCategory> at compile time, and runtime checks for someDelegateMessage will succeed. However, conformsToProtocol: runtime checks will fail.

Of course, you should file a bug requesting that methods declared on the core class don’t generate warnings.