Objective C protocols requiring the implementation of at least one of a set of methods

Question

In Objective C, protocol methods can be required or optional:

@protocol AProtocol

@required
-(void) aRequiredMethod;

@optional
-(void) anOptionalMethod;

@end

Is there an elegant way to say the object conforming to the protocol has to respond to at least one of a set of methods, my dream would be something like

@protocol AProtocol

@anyof
-(void) onePossibleMethod;
-(void) anotherPossibleMethod;

@optional
-(void) anOptionalMethod;

@end

EDIT:

This being (as far as I know), impossible, would there be a way to raise a compile time warning if a class declared as conforming to a given protocol made entirely of optional methods

@protocol AProtocol

@optional
-(void) onePossibleMethod;
-(void) anotherPossibleMethod;

@end

failed to implement at least one of them.

Answer 1

There isn't a great way to express this in Objective-C. If you must do this, IMO the most idiomatic and least code-bloating way is something along these lines:

@protocol AProtocol

@required
- (SEL)methodToUse; // returns one of "onePossibleMethod" or "anotherPossibleMethod"

@optional
-(void) onePossibleMethod;
-(void) anotherPossibleMethod;
-(void) anOptionalMethod;

@en

Answer 2

Well, you are working with ObjC, so you will need to exercise some restraint yourself:

@protocol MONAbstractDataProvider
@optional
- (void)anOptionalMethod;
@end

@protocol MONDataProviderA < MONAbstractProvider >
@required
- (void)onePossibleMethod;
@end

@protocol MONDataProviderB < MONAbstractProvider >
@required
- (void)anotherPossibleMethod;
@end

In that case, you'll have to do a confromsToProtocol: test at the callsite, rather than respondsToSelector: test for onePossibleMethod and anotherPossibleMethod. Then you pass MONAbstractDataProvider around. That can introduce some type safety, as long as you remember the rules, but it is really only slightly better than the customary approach.

So the client side would look like this:

- (void)update:(NSObject<MONAbstractDataProvider>*)provider
{
    if ([provider conformsToProtocol:@protocol(MONDataProviderA)]) {
        [(NSObject<MONDataProviderA>*)protocol onePossibleMethod];
    }
    else if ([provider conformsToProtocol:@protocol(MONDataProviderB)]) {
        [(NSObject<MONDataProviderB>*)provider anotherPossibleMethod];
    }
    else {
        assert(0 && "rule broken");
    }
}

That of course assumes the client knows about all of the derivatives.

You may instead prefer the simpler singular approach if they are both void:

@protocol MONAbstractDataProvider
@required
- (void)performPossibleMethod;
@optional
- (void)anOptionalMethod;
@end