How to recieve a message in the bloc getter changeMessage (which is actualy works as a setter)?

Question

There is a bloc

  final _message = BehaviorSubject<String>();
  Stream<String> get message => _message.stream;

  Function(String) get changeMessage {
    if (value == 'error') { // value is undefined here - how to get it?
      _message.sink.addError('error message');
    }

    return _message.sink.add;
  }

and in a service method I run changeMessage on the bloc

  ... } catch (e) {
      aBloc.changeMessage('error');
      return null;
  }

Is it possible to get passed value error in the bloc getter changeMessage? It will give a condition to put error in the sink.

Later it the view it give the ability to obtaine snapshot.hasError condition.