Ggtcf

Please look at my code:

class HomePageState extends State<HomePage> 
 bool _isLoading = false;

.....

 @override
 Widget build(BuildContext context) 

 var drawerOptions = <Widget>[];
 ......

 drawerOptions.add(new ListTile(
 leading: new Icon(Icons.account_balance),
 title: new Text(Strings.menu_change_city),
 onTap: () => createDialog()
 ));


 if(_isLoading) return buildBusyForm();

 return Scaffold( .... //window content
 

So I have navigation drawer. One item ("Select city") does not close navigation drawer, it shows select city dialog:

createDialog() 

 setState(() _isLoading = true;);

 fetchCities().then((response) 

 setState(() _isLoading = false;);

 showDialog(
 context: context,
 builder: (context) => CityChoiceDialog<City>(
 title: Text(Strings.menu_change_city),
 items: response,
 initialValue: response.firstWhere((c) => c.id == globals.cityId, orElse: () => new City()),
 itemBuilder: (City city) => Text(city.name),
 onSelected: _onSelected,
 onSubmitted: _onSubmitted));
 );
 

So, basically it's intended to show busy form, load cities, then hide busy form and show cities list dialog. As it seems to work, I'm getting exception:

I/flutter (10662): ══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY
╞═══════════════════════════════════════════════════════════ I/flutter
(10662): The following assertion was thrown while finalizing the
widget tree: I/flutter (10662): setState() or markNeedsBuild() called
when widget tree was locked. I/flutter (10662): This
_ModalScope widget cannot be marked as needing to build because the framework is I/flutter (10662): locked. I/flutter (10662):
The widget on which setState() or markNeedsBuild() was called was:
I/flutter (10662):

_ModalScope-[LabeledGlobalKey<_ModalScopeState>#1f222](state:
I/flutter (10662): _ModalScopeState#6c40b)

How to do what I want correctly?

This error means you are calling setState during the build phase, which you cannot do.