Flutter：通过共享首选项加载主题

Question

我有一个保存在共享首选项中的主题，该应用程序下次打开时应立即更新。我在main.dart中有一个ChangeNotifierProvider，它等待更新它，但是Wrapper.dart中出现错误：“在构建期间调用了setState（）或markNeedsBuild（）”。 我不确定如何在“包装器”中重新格式化我的代码以解决此问题。有什么想法吗？

Wrapper.dart

class Wrapper extends StatelessWidget {
  static const id = "wrapper";
  @override
  Widget build(BuildContext context) {

    final user = Provider.of<User>(context);
    print(user);

    bool checkedSharedPref = false;

    if(checkedSharedPref == false){
      SharedPref sharedPref = SharedPref();
      ThemeOptions dropdownValue;

      loadSharedPrefs() async {
        try {
          ThemeOptions currentTheme = ThemeOptions.fromJson(
              await sharedPref.readThemeFromSharedPref('currentTheme'));
          print("is being copyed");
          dropdownValue = currentTheme;
        } catch(e){
          print(e);
        }
      }


      checkedSharedPref = true;
      loadSharedPrefs();
       Provider.of<ThemeModel>(context, listen: false).toggleTheme();
    }

      return MainScreen();
}

MainScreen.dart

class MainScreen extends StatefulWidget {
  static const id = "main_screen";

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


class ThemeOptions{
   Color themeColor;
   ThemeType enumTheme;

  ThemeOptions({this.themeColor, this.enumTheme});

   ThemeOptions.fromJson(Map<String, dynamic> json)
       : themeColor = Color(json['themeColor']),
         enumTheme = json['enumTheme'];

  Map<String, dynamic> toJson()=>{
    'themeColor' : themeColor.toString(),
    'enumType' :  EnumToString.parse(enumTheme),
  };
}

class SharedPref{
  saveThemeToSharedPref(String key, ThemeOptions value) async{
    final prefs = await SharedPreferences.getInstance();
    prefs.setString(key, json.encode(value.toJson()));
    print('dataSaved');
    print(prefs.getString(key));
  }

  readThemeFromSharedPref(String key) async{
    final prefs = await SharedPreferences.getInstance();
    print('dataLoaded');
    return json.decode((prefs.getString(key)));
  }
}



class _MainScreenState extends State<MainScreen> {

  ThemeOptions dropdownValue;
  SharedPref sharedPref = SharedPref();

  List<ThemeOptions> themes = [
    ThemeOptions(themeColor: Colors.teal, enumTheme: ThemeType.Teal),
    ThemeOptions(themeColor: Colors.green, enumTheme: ThemeType.Green),
    /*    ...
   the other cases
    ...*/
  ];



  loadSharedPrefs() async {
    try{
      ThemeOptions currentTheme = ThemeOptions.fromJson(await sharedPref.readThemeFromSharedPref('currentTheme'));
      setState(() {
        print("is being copyed");
        dropdownValue = currentTheme;
      });
    }catch(e){
      print(e);
    }
  }

  @override
  void initState() {
    SharedPref sharedPref = SharedPref();
    dropdownValue =themes[0];
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('MainScreen'),
      ),
      body: Column(
       children: <Widget>[
         Container(
           child: DropdownButton<ThemeOptions>(
             value: dropdownValue,
             icon: Icon(Icons.arrow_downward),
             iconSize: 24,
             elevation: 16,
             style: TextStyle(
                 color: Colors.deepPurple
             ),
             underline: Container(
               height: 0.0,
               color: Colors.deepPurpleAccent,
             ),
             onChanged: (ThemeOptions newValue) {
               setState(() {
                 dropdownValue = newValue;
                 sharedPref.saveThemeToSharedPref("currentTheme", dropdownValue);
                 Provider.of<ThemeModel>(context, listen: false).changeEnumValue(dropdownValue.enumTheme);
               });
             },
             items: themes.map((ThemeOptions colorThemeInstance) {
               return DropdownMenuItem<ThemeOptions>(
                 value: colorThemeInstance,
                 child: CircleAvatar(
                   backgroundColor: colorThemeInstance.themeColor,
                 ),
               );
             })
                 .toList(),
           ),
         ),
         SizedBox(height: 20.0,),
       ],
      ),
    );
  }
}

Themes.dart

class ThemeModel extends ChangeNotifier {
  ThemeData currentTheme = tealTheme;
  ThemeType _themeType = ThemeType.Teal;

  toggleTheme() {
    if (_themeType == ThemeType.Teal) {
      currentTheme = tealTheme;
      _themeType = ThemeType.Teal;
      print('Teal');
       notifyListeners();
    }
    /*    ...
   the other cases
    ...*/
 }

  ThemeType getEnumValue(){
    return _themeType;
  }

  void changeEnumValue(ThemeType newThemeType){
   _themeType = newThemeType;
   toggleTheme();
  }

}