combiningProvider.dart

// STATE
class ProductNotifier extends ChangeNotifier{
  
  FormzStatus _status;
  
  NameInput _name;
  PriceInput _price;
  DescriptionInput _description;
  
  FormzStatus get status => this._status;

  NameInput get name => this._name;
  PriceInput get price => this._price;
  DescriptionInput get description => this._description;

  bool get completed => name.valid && price.valid && description.valid;

  void onChangeName(String val){
    this._name = NameInput.dirty(value: val);
    notifyListeners();
  }
 ...
}

//PROVIDERS
final productState = ChangeNotifierProvider.autoDispose<ProductNotifier>((ref){
  return ProductNotifier();
});

final nameInputState = Provider.autoDispose((ref) {
    return ref.watch(productState).name;
});
final priceInputState = Provider.autoDispose((ref) {
    return ref.watch(productState).price;
});

// WIDGETS BUILDERS

Consumer(
  builder: (_, watch, __) {
    print("nameInputState");
    final field = watch(nameInputState);
    return Container(
    child: TextFormField(
      decoration: InputDecoration(
        errorText: field?.error?.asString,
      ),
      onChanged: (value) {
        context.read(productState).onChangePrice(value);
      },
    ),
  );
},),

Consumer(
  builder: (_, watch, __) {
    print("PriceInputState");
    final field = watch(priceInputState);
    return Container(
    child: TextFormField(
      decoration: InputDecoration(
        errorText: field?.error?.asString,
      ),
      onChanged: (value) {
        context.read(productState).onChangeName(value);
      },
    ),
  );
},),