Adding a category method to NSObject but receiving warnings because it is not in the <NSObject> protocol when I call it

Question

(I found some questions discussing this idea, but I did not see a solution for my problem.)

I added this convenience method as a category in NSObject. (I added other methods, so I am still interested in the answer, even if you do not agree with this particular convenience method.)

@implementation NSObject (MyCategory) - (void)performInvocationOnMainThread:(NSInvocation *)invocation waitUntilDone:(BOOL)waitForMainThread; @end 

Then I have a protocol that I defined:

 @protocol MyDelegateProtocol <NSObject> - (void)myDelegateProtocolMethod; @end 

Then I declare the delegate as a property of my class that implements the specified protocol.

 @property (nonatomic, assign) id <MyDelegateProtocol> delegate; 

But when I try to call the NSObject method, I added to my category like this:

 NSInvocation *invocation = [self.delegate invocationForSelector:@selector(someSelector:withArg:)]; 

I get this warning

 '-performInvocationOnMainThread:waitUntilDone:' not found in protocol(s) 

If I pass my delegate as (NSObject *) , then I will not receive a warning. What am I doing wrong? It seems I could not (or should I?) Add methods to an existing protocol without creating an “auxiliary protocol” and use it from now on. (What kind of hit is the meaning of adding methods to NSObject).

 NSInvocation *invocation = [(NSObject *)self.delegate invocationForSelector:@selector(someSelector:withArg:)]; 

Answer 1

Your category extends the NSObject class , not the NSObject protocol . Although the class now has this method, it is not defined as part of the protocol, so there is a warning.

Why does casting to a pointer type NSObject * ; you are executing an NSObject class NSObject , not something like id<NSObject> , which means an arbitrary Objective-C object that conforms to the NSObject protocol.

You will need to make an intermediate protocol (or "auxiliary protocol") that extends the NSObject protocol:

 @protocol ExtendedNSObject <NSObject> - (void)performInvocationOnMainThread:(NSInvocation *)invocation waitUntilDone:(BOOL)waitForMainThread; @end 

Then pass your delegate protocol instead:

 @protocol MyDelegateProtocol <ExtendedNSObject> - (void)myDelegateProtocolMethod; @end 

If I'm not mistaken, you can keep the existing implementation of NSObject (MyCategory) , and they will play well together.

Answer 2

when pass / expect this type, qualify it like this:

 - (void)race:(NSObject<MyDelegateProtocol>*)arg;