Why do we call doesNotRecognizeSelector: method?

Question

I am working with socket programming.I just wanted to clear a doubt related with a code i downloaded from -mobileorchard.com - Chatty. While R&D , I saw a function call in ChatRoomViewController.m file

 [chatRoom broadcastChatMessage:input.text fromUser:[AppConfig getInstance].name];

when I saw in Room.m file, for the implementation of above call; it was

- (void)broadcastChatMessage:(NSString*)message fromUser:(NSString*)name
{
    // Crude way to emulate an "abstract" class
    [self doesNotRecognizeSelector:_cmd];
}

i googled for "doesNotRecognizeSelector:" , according to Apple its for error handling, stating "The runtime system invokes this method whenever an object receives an aSelector message it can’t respond to or forward." my question is why does the developer call the broadcastChatMessage:fromUser: function if its none of use there and to handle which method's "selector not found" exception ?

According to Stackovrflow, its used to create abstract class , according to this Question, its to avoid "Incomplete implementation" warning.

I am still not getting why that method is used in that Chatty Code, Kindly help me to understand the reason why that method is used.

Answer 1

This is the method that exists on every NSObject derived object that triggers the path to an exception when a method isn't recognized in a runtime call. For example, if you try to send a message to an NSString called -foo, it'll end up there since that's not a valid method on NSString.

In this case, the Chatty class Room is a base class that is never used directly. LocalRoom and RemoteRoom derive from it, and both of those classes provide an overriding implementation of -broadcastChatMessage:fromUser. Nobody ever calls that base class version, but for "completeness" the programmer has guaranteed that a subclasser must override this by implementing the method, but then turning around and calling this to trigger an exception.

Thing is, this isn't specifically idiomatic Objective-C. An "abstract" class is a concept from C++ and other languages; it's base class that exists only as a "pattern" from which to subclass. (In ObjC, this is often done by creating a formal @protocol when there isn't meaningful state, as there (mostly) isn't here).

Note that the call to -doesNotRecognizeSelector: is arbitrary. It's not necessary to avoid compiler warnings here (since the method is in fact implemented) and the original writer could have easily just thrown an exception directly, or done nothing instead.

Answer 2

It seems to me that you already answered your own question. There is no method to make abstract classes in Objective-C, so the closest thing to do it to have the methods that you need to override throw exceptions. If you override this method in a subclass, then doesNotRecognizeSelector: will no longer be called. Basically it is a way to get a developer to promise to implement this method in their subclass.

Also, as you mentioned, if you don't put this in then the compiler will issue a warning because no implementation exists for a method defined in the header. This will perform the same behavior as not implementing it, but the compiler will realize that you are doing it on purpose.