provider_select_example.dart

import 'package:meta/meta.dart';
import 'package:riverpod/riverpod.dart';

@immutable
class Count {
  final int countA;
  final int countB;

  Count(this.countA, this.countB);

  @override
  int get hashCode => runtimeType.hashCode ^ countA.hashCode ^ countB.hashCode;

  @override
  bool operator ==(dynamic other) =>
      identical(this, other) ||
      (other is Count &&
          identical(countA, other.countA) &&
          identical(countB, other.countB));
}

class CountVM extends StateNotifier<Count> {
  final Ref ref;

  CountVM(this.ref) : super(Count(0, 0)) {
    print('${state.countA} ${state.countB}');

    ref.listen(count, (_, Count value) {
      state = value;
    });
  }

  int get countA => state.countA;
  int get countB => state.countB;
}

final count = StateProvider((_) => Count(0, 0));

final countVM = StateNotifierProvider<CountVM, Count>((ref) {
  return CountVM(ref);
});

final watchCountA = Provider((ref) {
  final a = ref.watch(countVM.select((value) => value.countA));
  print('Detected change countA from watchCountA : $a');

  ref.onDispose(() => print('d a'));

  return a;
});

final watchCountB = Provider((ref) {
  final b = ref.watch(countVM.select((value) => value.countB));
  print('Detected change countB from watchCountB : $b');

  ref.onDispose(() => print('d b'));

  return b;
});

void main() async {
  final container = ProviderContainer();

  container.listen<int>(watchCountA, (_, int a) {
    print(a);
  });

  container.listen<int>(watchCountB, (_, int b) {
    print(b);
  });

  final _count = container.read(count.notifier);

  print('change a to 1');
  _count.update((_) => Count(1, 0));
  
  await Future.delayed(const Duration(milliseconds: 500));
  
  print('change a to 2');
  _count.update((_) => Count(2, 0));
  
    await Future.delayed(const Duration(milliseconds: 500));

  print('change b to 1');
  _count.update((_) => Count(2, 1));
}