Extend `StateNotifier` from a generic class

[om-ha]

Describe the bug Cannot extend StateNotifier from a generic class

To Reproduce

• This works:

  class DataHolder<T> {
  final T data;
  
  DataHolder(this.data);
  }

• This does not work (does not compile):

  class DataHolderStateNotifier<T> extends StateNotifier<T> {
  final T data;
  
  DataHolderStateNotifier(this.data) : super(null);
  }

Expected behavior To be able to use generic type with a class, and still extend StateNotifier.

Problem I am trying to solve

• Using DataHolder as a part of Riverpod's provider (access through Ref)
• Being able to listen to the generic value of type T inside DataHolder

• Moving to StateNotifier implementation (attempted one above) to ChangeNotifier/ValueNotifier implementation, see below:

  class DataHolderValueNotifier<T> {
  final ValueNotifier<T?> data;
  
  DataHolderValueNotifier(this.data);
  }

[om-ha]

Note: bug label might not be appropriate here, apologies.

[om-ha]

Looks like I got this to work:

• StateNotifier Definition

  class DataHolderStateNotifier<T> extends StateNotifier<T?> {
  final String stringField;
  
  final bool boolField;
  
  DataHolderStateNotifier({
  required this.stringField,
  required this.boolField,
  }) : super(null);
  
  void set({T? newValue}) {
  state = newValue;
  }
  }

• Provider Definition

  
  final counterProvider =
  StateNotifierProvider<DataHolderStateNotifier<int>, int?>((Ref ref) {
  return DataHolderStateNotifier(
  stringField: 'counter',
  boolField: false,
  );
  });

final temperatureProvider = StateNotifierProvider<DataHolderStateNotifier, double?>((Ref ref) { return DataHolderStateNotifier( stringField: 'temperature', boolField: true, ); });

- Provider Usage
```dart
// read
int? counterValue = ref.watch(counterProvider);
double? temperatureValue = ref.watch(temperatureProvider);

// write
ref.watch(counterProvider.notifier).set(newValue: 100);
ref.watch(temperatureProvider.notifier).set(newValue: 24.42);

I hope this proves useful to others.