Subclassing, protocols, and delegates - not calling

Question

I have 2 classes:

@protocol MainProtocol
-(void) method1;
@end

@interface ClassA : NSObject
@property id <MainProtocol> delegate;
@end

and

@protocol SubProtocol <MainProtocol>
-(void) method2
@end

@interface ClassB : ClassA
@end

@implementation ClassB

-(void) foo {
    [self.delegate method1]; // works fine
    [self.delegate method2]; // error
}

@end

I am not sure why I am unable to call method2 with self.delegate. Is it because delegate is declared in the parent class? If so, how do I localize delegate in ClassB?

Answer 1

Inside -[ClassB foo], self.delegate is declared as type id<MainProtocol> – that is, it conforms to MainProtocol, but not necessarily to SubProtocol. As such, ClassB instances aren't sure that delegate responds to -method2. You can redeclare delegate in ClassB as an object of type id<SubProtocol> if you want to send -method2 to it.