exemplo reaction dialog

List<ReactionDisposer> _disposers;


 @override
  void initState() {
    super.initState();
    
    _disposers ??= [
      reaction(
        // Tell the reaction which observable to observe
        (_) => _controller.msg,
        // Run some logic with the content of the observed field

        (String message) {
          if (message.isNotEmpty) {
            showDialog(
                context: context,
                builder: (BuildContext context) {
                  return Dialog(
                    shape: RoundedRectangleBorder(
                        borderRadius:
                            BorderRadius.circular(20.0)), //this right here
                    child: Container(
                      height: 100,
                      child: Column(
                        mainAxisAlignment: MainAxisAlignment.spaceEvenly,
                        crossAxisAlignment: CrossAxisAlignment.start,
                        children: [
                          Center(child: Text(message)),
                          Center(
                            child: RaisedButton(
                              onPressed: () {
                                Navigator.pop(context);
                              },
                              child: Text(
                                "Ok",
                                style: TextStyle(color: Colors.white),
                              ),
                              color: Color.fromRGBO(109, 207, 246, 0.9),
                            ),
                          ),
                        ],
                      ),
                    ),
                  );
                });
          }
        },
      ),
    ];
  }
////// fim widget



//controller



@observable
String msg;


@action
  alterarEndereco(String valor) async {
    loading = true;
    msg = null;
    try {
      await _repository.alterarEndereco(valor);
      msg = "Alterado com sucesso.";
    } on DioError catch (e) {
      print(e?.response?.data);
      //pegar do response.data
      msg = "Ocorreu um erro";
    } finally {
      loading = false;
      Modular.to.pop();
    }
  }