swift - @objc keyword extension subclass behaviour

Question

Can someone explain why @objc keyword is needed here to compile the code?

As I understood this keyword is used in order to work ObjC message method dispatch. But this is not an NSObject instance.

 class MyClass {
 }

 extension MyClass {
     @objc func extensionMethod() { /// THIS LINE
         print("A")
     }
 }

 class SubClass: MyClass {
     override func extensionMethod() {
         print("B")
     }
 }

Does @objc keyword enable message dispatch as well as dynamic? Or not?

Answer 1

Does @objc keyword enable message dispatch as well as dynamic?

Not usually. Usually, the @objc attribute on its own just exposes a given class member to Objective-C – Swift is still free to dispatch to it either using table or static dispatch. You would need to mark the member as dynamic if you wanted Swift to use message dispatch when calling it.

However, for a non-final @objc class extension member, Swift will automatically infer it to be dynamic. Why? Because for interoperability reasons, Swift allows @objc extension members to override and be overridden (much like how you can override an Obj-C method in a subclass category). In order to achieve this behaviour, Swift relies on Obj-C message dispatch.

Therefore, in an extension, @objc infers dynamic. You cannot override an extension member without exposing it to the Obj-C runtime because extension members cannot currently be added to Swift class vtables (as Swift vtables currently cannot have members dynamically added to them at runtime).

But this is not an NSObject instance.

On Apple platforms (i.e those with Obj-C interop), all Swift classes are exposed to the Obj-C runtime, and all implicitly inherit from a special Obj-C base class called _SwiftObject, which conforms to NSObjectProtocol. So Swift classes are able to take advantage of message dispatch without having to inherit from NSObject.