ObservableFuture

controller.dart

import 'package:mobx/mobx.dart';
part 'mainpage.controller.g.dart';

class ExampleModel {
  final String observacao;

  ExampleModel(this.observacao);
}

class MainPageController = _MainPageControllerBase with _$MainPageController;

abstract class _MainPageControllerBase with Store {
  _MainPageControllerBase() {
    reaction<List<ExampleModel>>(
      (_) => future.value,
      (value) => setObservacao(value?.elementAt(0)?.observacao ?? ""),
    );

    when((_) => future.value != null, () => print("nao esta nulo"));
  }

  @observable
  ObservableFuture<List<ExampleModel>> future;

  @observable
  String observacao = "";

  @action
  Future<void> getModel() async {
    future = Future.delayed(
      Duration(seconds: 1),
      () => [ExampleModel("example observacao")],
    ).asObservable();
  }

  @action
  void outro() {
    future = Future.delayed(
      Duration(seconds: 1),
      () => [ExampleModel("example observacao")],
    ).asObservable();
  }

  @action
  void setObservacao(String value) {
    observacao = value;
  }
}

main.dart

import 'package:flutter/material.dart';
import 'package:flutter_mobx/flutter_mobx.dart';
import 'package:mobx/mobx.dart';
import 'package:mobx_example/mainpage.controller.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key}) : super(key: key);

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  final controller = MainPageController();
  final textController = TextEditingController();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("Example"),
      ),
      body: Column(
        children: [
          Observer(
            builder: (_) => Container(
              color: Colors.red,
              child: Column(children: [
                for (var model in (controller.future?.value ?? []))
                  Text(model.observacao)
              ]),
            ),
          ),
          Observer(builder: (_) {
            if (textController.text != controller.observacao) {
              textController.text = controller.observacao;
            }

            return Column(
              children: [
                Text(controller.observacao ?? ""),
                TextField(
                  onChanged: controller.setObservacao,
                  controller: textController,
                ),
                RaisedButton(
                  onPressed: () => controller.getModel(),
                  child: Text("Fetch"),
                ),
              ],
            );
          }),
        ],
      ),
    );
  }
}