indexNotifier.dart

import 'package:flutter_riverpod/flutter_riverpod.dart';

final indexNotifier = StateNotifierProvider<IndexNotifier>((ref) {
  return IndexNotifier();
});

class IndexNotifier extends StateNotifier<int> {
  IndexNotifier() : super(0);

  void changeIndex(int index) {
    state = index;
  }
}

// ====== ERROR ======
// The type 'StateNotifierProvider' is declared with 2 type parameters, 
// but 1 type arguments were given. (Documentation)  
// Try adjusting the number of type arguments to match the number of type parameters.