exception - setState() 或 markNeedsBuild() 被调用 > 当 widget 树被锁定时。改变状态时

Question

请看我的代码:

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
  }

}

所以我有抽屉导航。一项(“选择城市”)不关闭抽屉导航，它显示选择城市对话框:

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));
    });
  }

所以，基本上它的目的是显示繁忙的表单，加载城市，然后隐藏繁忙的表单并显示城市列表对话框。它似乎有效，但我遇到了异常:

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)

如何正确地做我想做的事？

Answer 1

此错误意味着您在构建阶段调用了 setState，您不能这样做。