How to add a method to an existing protocol in Cocoa?

Question

I want to extend or add another method to an existing protocol. Although the protocol in particular is not important, this is what I am trying to do.

@protocol NSMatrixDelegate
- (void)myNewMethod:(id)sender;
@end

The compiler warns that I have a duplicate declaration of the same protocol. How would I do this properly?

Thanks.

Answer 1

You can't define categories for protocols. There are 2 ways around this:

• use a new formal protocol
• use an informal protocol and runtime checking

Formal Protocol

Defining a new formal protocol would look like this:

@protocol MyCustomMatrixDelegate <NSMatrixDelegate>

- (void) myNewMethod:(id)sender;

@end

Then you would make your custom class conform to <MyCustomMatrixDelegate> instead of <NSMatrixDelegate>. If you use this approach, there's something to be aware of: [self delegate] will likely be declared as id<NSMatrixDelegate>. This means that you can't do [[self delegate] myNewMethod:obj], because <NSMatrixDelegate> does not declare the myNewMethod: method.

The way around this is to retype the delegate object via casting. Maybe something like:

- (id<MyCustomMatrixDelegate>) customDelegate {
  return (id<MyCustomMatrixDelegate>)[self delegate];
}

(However, you might want to do some type checking first, like:

if ([[self delegate] conformsToProtocol:@protocol(MyCustomMatrixDelegate)]) {
  return (id<MyCustomMatrixDelegate>)[self delegate];
}
return nil;

)

And then you'd do:

[[self customDelegate] myNewMethod:obj];

Informal Protocol

This is really a fancy name for a category on NSObject:

@interface NSObject (MyCustomMatrixDelegate)

- (void) myNewMethod:(id)sender;

@end

Then you just don't implement the method. In your class that would send the method, you'd do:

if ([[self delegate] respondsToSelector:@selector(myNewMethod:)]) {
  [[self delegate] myNewMethod:someSenderValue];
}