Error when I try to show a SnackBar in a builder - flutter

Question

This is my main.dart:
class MyApp extends StatelessWidget {
var login = new Login("xxxxxxx", "xxxxxxxxx");
final scaffoldKey = new GlobalKey<ScaffoldState>();
#override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Fetch Data Example',
theme: ThemeData(
primarySwatch: Colors.blue,
),
home: Scaffold(
key: scaffoldKey,
appBar: AppBar(
title: Text('Fetch Data Example'),
),
body: Center(
child: FutureBuilder(
future: login.main(),
builder: (context, snapshot) {
if (snapshot.hasData) {
_showSnackBar(snapshot.data);
return new Container();
} else if (snapshot.hasError) {
return Text("${snapshot.error}");
}
// By default, show a loading spinner
return CircularProgressIndicator();
},
),
),
),
);
}
_showSnackBar(var text) {
scaffoldKey.currentState
.showSnackBar(new SnackBar(content: new Text(text)));
}
}
I'd like to show a SnackBar in FutureBuilder, but when I run, something wrong occurs:
I/flutter ( 8918): ══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY
╞═══════════════════════════════════════════════════════════
I/flutter ( 8918): The following assertion was thrown building
FutureBuilder<dynamic>(dirty, state:
I/flutter ( 8918): _FutureBuilderState<dynamic>#9d4af):
I/flutter ( 8918): setState() or markNeedsBuild() called during build.
I/flutter ( 8918): This Scaffold widget cannot be marked as needing to
build because the framework is already in the
I/flutter ( 8918): process of building widgets. A widget can be marked
as needing to be built during the build phase
I/flutter ( 8918): only if one of its ancestors is currently building.
This exception is allowed because the framework
I/flutter ( 8918): builds parent widgets before children, which means
a dirty descendant will always be built.
I/flutter ( 8918): Otherwise, the framework might not visit this
widget during this build phase.
I/flutter ( 8918): The widget on which setState() or markNeedsBuild()
was called was:
I/flutter ( 8918): Scaffold-[LabeledGlobalKey<ScaffoldState>#e4a3f]
(state: ScaffoldState#cf908(tickers: tracking 2
I/flutter ( 8918): tickers))
I/flutter ( 8918): The widget which was currently being built when the
offending call was made was:
I/flutter ( 8918): FutureBuilder<dynamic>(dirty, state: _
FutureBuilderState<dynamic>#9d4af)
I/flutter ( 8918):
I/flutter ( 8918): When the exception was thrown, this was the stack:
I/flutter ( 8918): #0 Element.markNeedsBuild.<anonymous closure>
(package:flutter/src/widgets/framework.dart:3472:11)
I/flutter ( 8918): #1 Element.markNeedsBuild
(package:flutter/src/widgets/framework.dart:3498:6)
I/flutter ( 8918): #2 State.setState
(package:flutter/src/widgets/framework.dart:1141:14)
I/flutter ( 8918): #3 ScaffoldState.showSnackBar
(package:flutter/src/material/scaffold.dart:1110:5)
I/flutter ( 8918): #4 MyApp._showSnackBar
I/flutter ( 8918): #5 MyApp.build.<anonymous closure>
I/flutter ( 8918): #6 _FutureBuilderState.build
(package:flutter/src/widgets/async.dart)
I/flutter ( 8918): #7 StatefulElement.build
(package:flutter/src/widgets/framework.dart:3766:27)
I/flutter ( 8918): #8 ComponentElement.performRebuild
(package:flutter/src/widgets/framework.dart:3678:15)
I/flutter ( 8918): #9 Element.rebuild
(package:flutter/src/widgets/framework.dart:3531:5)
I/flutter ( 8918): #10 BuildOwner.buildScope
(package:flutter/src/widgets/framework.dart:2273:33)
I/flutter ( 8918): #11
My purpose is that it shows a loading spinner before it gets the data, and once it got the data, it shows a SnackBar. So what should I do to fix the problem? Login.main is a function that GET data from the database.

Answer 1

FutureBuilder or StreamBuilder should only used to build widgets, not to execute logic like navigation. Use instead initState like
#override
void initState() {
super.initState();
_checkLogin();
}
Future<void> _checkLogin() async {
try {
var data = await login.main();
setState(() {
_waitForLogin = false;
});
_showSnackBar(data);
} catch(e) {
setState(() {
_waitForLogin = false;
_loginErrorMessage = '$e';
});
}
}
bool _isWaitForLogin = true;
String _loginErrorMessage;
#override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Fetch Data Example',
theme: ThemeData(
primarySwatch: Colors.blue,
),
home: Scaffold(
key: scaffoldKey,
appBar: AppBar(
title: Text('Fetch Data Example'),
),
body: Center(
child: _isWaitForLogin ? CircularProgressIndicator() :
(_loginErrorMessage != null ? Text(_loginErrorMessage : Container())
),
),
);
}

Answer 2

The solution that #thought7878 recommended suggests wrapping the breaking logic in WidgetsBinding.instance.addPostFrameCallback((_) { }. For example:
builder: (context, AsyncSnapshot<Object> snapshot) {
if (snapshot.hasError) {
WidgetsBinding.instance.addPostFrameCallback((_) {
Scaffold.of(context)
..hideCurrentSnackBar()
..showSnackBar(
// SnackBar
);
});
return Container();
}
// Rest ...
}

Answer 3

Could you try like this: modify _showSnackBar and pass the context
_showSnackBar(var text, BuildContext context) {
ScaffoldKey.of(context)
.showSnackBar(new SnackBar(content: new Text(text)));
}