What is the fancy way to use SnackBar in StreamBuilder?

Question

I'm implementing Bloc pattern for my application and I have to show SnackBar which shows error message when login is unauthenticated.

But I cannot show SnackBar during building phase of widget. I looked for lots of solutions, but I couldn't found.

What is the most efficient way to use this function?

My code

import 'package:chat_app/auth/auth_bloc.dart';
import 'package:chat_app/auth/auth_state.dart';
import 'package:chat_app/main_page.dart';
import 'package:flutter/material.dart';

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

class App extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Chat App',
      home: MyApp(),
      debugShowCheckedModeBanner: false,
    );
  }
}

class MyApp extends StatefulWidget {
  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {

  final GlobalKey<FormState> _formKey = GlobalKey<FormState>();
  final GlobalKey<ScaffoldState> _scaffoldKey = GlobalKey<ScaffoldState>();
  final AuthBloc _bloc = AuthBloc();

  final _emailController = TextEditingController();
  final _passwordController = TextEditingController();

  @override
    void dispose() {
      _emailController.dispose();
      _passwordController.dispose();
      _bloc.dispose();
      super.dispose();
    }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      key: _scaffoldKey,
      appBar: AppBar(title: Text('Chat Example')),
      body: StreamBuilder(
          initialData: AuthInitializing(),
          stream: _bloc.authStream,
          builder: (BuildContext context, AsyncSnapshot<AuthState> snapshot){
            AuthState state = snapshot.data;
            if(state is AuthUnauthenticated){
              _showErrorMessage(state.errorMessage);
            }
            if(state is AuthAuthenticated){
              _moveNextPage(context);
            }
            return Form(
              key: _formKey,
              child: Padding(
                padding: const EdgeInsets.symmetric(horizontal: 20.0),
                child: Column(
                  children: <Widget>[
                    TextFormField(
                      controller: _emailController,
                      keyboardType: TextInputType.emailAddress,
                      decoration: InputDecoration(
                        border: OutlineInputBorder(),
                        labelText: '이메일',
                      ),
                    ),
                    SizedBox(height: 20.0),
                    TextFormField(
                      controller: _passwordController,
                      keyboardType: TextInputType.text,
                      obscureText: true,
                      decoration: InputDecoration(
                        border: OutlineInputBorder(),
                        labelText: '비밀번호'
                      ),
                    ),
                    SizedBox(height: 20.0),
                    RaisedButton(
                      child: Text('로그인',style: TextStyle(color: Colors.white),),
                      onPressed: () => _bloc.addLoginData(_emailController.text, _passwordController.text),
                      color: Theme.of(context).primaryColor,
                    ),
                    SizedBox(height: 15.0),
                    state is AuthLoading ? _progressBar() : Container()
                  ],
                ),
              ),
            );
          },
        ),
    );
  }

  void _showErrorMessage(String message){
    _scaffoldKey.currentState.showSnackBar(SnackBar(
      content: Text(message),
    ));
  }

  void _moveNextPage(BuildContext context) {
    Navigator.pushReplacement(context, MaterialPageRoute(
      builder: (_) => MainPage()
    ));
  }

  Widget _progressBar() {
    return Center(
      child: CircularProgressIndicator(),
    );
  }
}

StackTrace

I/flutter (30505): ══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════ I/flutter (30505): The following assertion was thrown building StreamBuilder(dirty, state: I/flutter (30505): _StreamBuilderBaseState>#bd8b2): I/flutter (30505): setState() or markNeedsBuild() called during build. I/flutter (30505): This Scaffold widget cannot be marked as needing to build because the framework is already in the I/flutter (30505): process of building widgets. A widget can be marked as needing to be built during the build phase I/flutter (30505): only if one of its ancestors is currently building. This exception is allowed because the framework I/flutter (30505): builds parent widgets before children, which means a dirty descendant will always be built. I/flutter (30505): Otherwise, the framework might not visit this widget during this build phase. I/flutter (30505): The widget on which setState() or markNeedsBuild() was called was: I/flutter (30505):
Scaffold-[LabeledGlobalKey#5bdc5](state: ScaffoldState#61be4(tickers: tracking 2 I/flutter (30505): tickers))

Answer 1

First, you must ensure that you are always returning a widget

and then you can schedule the SnackBar for the end of the frame

if(state is AuthUnauthenticated){
  WidgetsBinding.instance.addPostFrameCallback((_) => _showErrorMessage(state.errorMessage));
  return Container();
}

You should also check if the data is null o the snapshot has data.