Use AsyncValue.guard rather than try/catch inside your StateNotifier subclasses

When writing your own StateNotifier subclasses, it's quite common to use try/catch blocks to deal with Futures that can fail:

class SignOutButtonController extends StateNotifier<AsyncValue> {
  SignOutButtonController({required this.authRepository})
      : super(const AsyncValue.data(null));
  final AuthRepository authRepository;

  Future<void> signOut() async {
    try {
      state = const AsyncValue.loading();
      await authRepository.signOut(); // this future can fail
      state = const AsyncValue.data(null);
    } catch (e) {
      state = AsyncValue.error(e);
    }
  }
}

In such cases, AsyncValue.guard is a convenient alternative that does all the heavy-lifting for us:

Future<void> signOut() async {
  state = const AsyncValue.loading();
  state = await AsyncValue.guard(() => authRepository.signOut());
}

Here's how this method is implemented in the the AsyncValue class:

abstract class AsyncValue<T> {
  static Future<AsyncValue<T>> guard<T>(Future<T> Function() future) async {
    try {
      return AsyncValue.data(await future());
    } catch (err, stack) {
      return AsyncValue.error(err, stackTrace: stack);
    }
  }
}