How to build a hierarchical state-notifier structure?

Question

I am trying to split my state notifiers into a parent/child hierarchy but I am not sure how to approach that.

//I want this to be a parent controller that keeps the "index" for its child in its own state.
// "ParentState" is a freezed dataclass so inheriting from it is not possible.
class ParentController extends StateNotifier<ParentState>{
   ParentController(ParentState state) : super(state);
   
   //Every "ChildController" needs to have this method.
   void setIndex(int index){
      state = state.copyWith(index: index);
   }
}

// "ChildState" is a freezed dataclass.
class ChildController extends StateNotifier<ChildState> implements ChildController {
   ChildController (ChildState state) : super(state);
   
   //this controller should have access to the parents index and update 
   //if the parents "setIndex" is called.
}

So a successful implementation would look like this:

final childController = ChildController();

childControllerOne.setIndex(2);
childControllerOne.index == 2 //true;

Because I have multiple ChildControllers, my goal is to put as much shared code in this parent controller and state, so that I don't have to repeat myself.

My current solution is a mixin like this:

mixin SetIndexMixin {
 void setIndex(int index){
    onSetIndex(index);
 }

 void onIndexSet(int index);
}

which I implement like so :

class ChildController extends StateNotifier<ChildState> with SetIndexMixin {
   ChildController (ChildState state) : super(state);

    @override
    onIndexSet(int index){
      state = state.copyWith(index);
    }
}

But I feel like there has to be a more elegant and scalable solution.

Thanks!