如何在颤动中使用共享首选项保存开关小部件状态？

有一个 ChangeTheme 类，它有一个切换小部件，可以改变应用程序的主题，但是如果你转到另一个屏幕，然后再次返回带有开关的屏幕，它将不会再次处于活动状态。此外，如果您关闭应用程序，则由开关设置的主题将被重置。如何解决？

更改主题.dart

import 'package:flutter/material.dart';
import 'package:flutter_local_notifications/flutter_local_notifications.dart';
import 'package:shared_preferences/shared_preferences.dart';
import 'package:todo/theme.dart';
import 'package:provider/provider.dart';



class ChangeTheme extends StatefulWidget{

  ChangeTheme({Key key, this.title}): super(key: key);
  final String title;
  @override
  MyState createState() => new MyState ();

}


class MyState extends State<ChangeTheme>{
  bool val = false;
  String message = "смена темы";
  FlutterLocalNotificationsPlugin flutterLocalNotificationsPlugin;

  @override
  Widget build(BuildContext context){
    set();
    return new Scaffold(

        appBar: AppBar(
          title: Text('Настройки'),
        ),
        body: Container(
            child: Column(
              children: <Widget>[
               new Switch(
                 value: val,
                 onChanged: (bool e) => setState(() => something(e)),
                ),
                new Text (message),

                ],
              ),
      ),
    );
  }

 void something(bool e) {
   ThemeChanger _themeChanger = Provider.of<ThemeChanger>(context);
    setState(() {

      if (e){
        _themeChanger.setTheme(ThemeData.dark());
        message = "Темнаяя тема";
        val = e;
        get();

      } else {
        _themeChanger.setTheme(ThemeData.light());
        message = "Светлая тема";
        val = e;
        get();
      }
    });
 }
  get() async {
    SharedPreferences prefs = await SharedPreferences.getInstance();
    final myBool = prefs.getBool('my_bool_key') ?? false;
  }
  set() async {
    SharedPreferences prefs = await SharedPreferences.getInstance();
    prefs.setBool('my_bool_key', true);
  }

}

主要.dart

import 'package:flutter/material.dart';
import 'package:provider/provider.dart';
import 'package:shared_preferences/shared_preferences.dart';
import 'theme.dart';
import 'ChangeTheme.dart';
import 'Constants.dart';

void main() async => runApp(new TodoApp());

class TodoApp extends StatelessWidget {

  @override
  Widget build(BuildContext context) {
    return ChangeNotifierProvider<ThemeChanger>(
      builder: (_) => ThemeChanger(ThemeData.light()),
      child: new MaterialAppWithTheme (
      ),
    );
  }
}

class MaterialAppWithTheme extends StatelessWidget {

  @override
  Widget build(BuildContext context) {

    final theme = Provider.of<ThemeChanger>(context);

    return MaterialApp(
      debugShowCheckedModeBanner: false,
      home: new TodoList(),
      theme: theme.getTheme(),
    );
  }

}

class TodoList extends StatefulWidget {
  @override
  createState() => new TodoListState();
}

class TodoListState extends State<TodoList> {

  List<String> _todoItems = [];



  @override
  Widget build(BuildContext context) {
    _getPrefs();
    return new Scaffold(
      appBar: new AppBar(title: new Text('Список задач'),
      actions: <Widget>[
      PopupMenuButton<String>(
       onSelected: choiceAction ,
        itemBuilder: (BuildContext context){
         return Constants.choices.map((String choice){
            return PopupMenuItem<String>(
            value: choice,
              child: Text(choice),
            );
         }).toList();
        },
      )
      ]),
      body: _buildTodoList(),

      floatingActionButton: new FloatingActionButton(
          onPressed: _addTodoItem,
          tooltip: 'Add task',
          child: new Icon(Icons.add)),
    );
  }

  Widget _buildTodoList() {
    return new ListView.builder(itemBuilder: (context, index) {
      if (index < _todoItems.length) {
        return _buildTodoItem(_todoItems[index], index);
      }
    });
  }


 void choiceAction(String choice){
    if (choice == Constants.ChangeTheme){
      _changeTheme();


    }

    else if (choice == Constants.OffAd){
      showDialog(
          context: context,
          builder: (BuildContext context) {
            return new AlertDialog(
                title: new Text('Хочешь отключить рекламу?\n\n'
                    'Разработчку и дизайнеру тоже хочеца кушатб, '
                    'поэтому для отключения рекламы отправь деняк. '),
                actions: <Widget>[
                  new FlatButton(
                      child: new Text('подробнее'),
                      onPressed: () => Navigator.of(context).pop()),
                  new FlatButton(
                      child: new Text('нет я жмот'),
                      onPressed: () => Navigator.of(context).pop()),
                ]);
          });


    } else if (choice == Constants.info){
      showDialog(
          context: context,
          builder: (BuildContext context) {
            return new AlertDialog(
                title: new Text('Todo\n'
                                'Разбработчик:  \n'
                                'Дизайнер:  \n'
                                ),
                actions: <Widget>[
                  new FlatButton(
                      child: new Text('оценить'),
                      onPressed: () => Navigator.of(context).pop()),
                  new FlatButton(
                      child: new Text('отмена'),
                      onPressed: () => Navigator.of(context).pop()),
                ]);
          });

    }
 }

 void _changeTheme(){
   Navigator.push(context,
       new MaterialPageRoute(
           builder: (context) => new ChangeTheme()));
 }



  Widget _buildTodoItem(String todoText, int index) {
    return new ListTile(
        title: new Text(todoText), onTap: () => _removeTodoItem(index));
  }

  void _addTodoItem() {
    // Push this page onto the stack
    Navigator.of(context).push(new MaterialPageRoute(builder: (context) {
      return new Scaffold(
          appBar: new AppBar(title: new Text('добавление задачи')),
          body: new TextField(
            autofocus: true,
            onSubmitted: (val) {
              _addItem(val);
              Navigator.pop(context); // Close the add todo screen
            },
            decoration: new InputDecoration(
                hintText: 'Введите вашу задачу',
                contentPadding: const EdgeInsets.all(16.0)),
          ));
    }));
  }

  void _addItem(String task) {
    if (task.length > 0) {
      setState(() => _todoItems.add(task));
      _setPrefs();
    }
  }

  void _removeTodoItem(int index) {
    showDialog(
        context: context,
        builder: (BuildContext context) {
          return new AlertDialog(
              title: new Text('Задача "${_todoItems[index]}" выполнена?'),
              actions: <Widget>[
                new FlatButton(
                    child: new Text('отмена'),
                    onPressed: () => Navigator.of(context).pop()),
                new FlatButton(
                    child: new Text('выполнена'),
                    onPressed: () {
                      _removeItem(index);
                      Navigator.of(context).pop();
                    })
              ]);
        });
  }

  void _removeItem(int index) {
    setState(() => _todoItems.removeAt(index));
    _setPrefs();
  }

  void _setPrefs() async {
    SharedPreferences prefs = await SharedPreferences.getInstance();
    prefs.setStringList('TodoList', _todoItems);
  }

  void _getPrefs() async {
    SharedPreferences prefs = await SharedPreferences.getInstance();
    if (prefs.getStringList('TodoList') != null)
      _todoItems = prefs.getStringList('TodoList');
  }

}

主题.dart

import 'package:flutter/material.dart';

class ThemeChanger with ChangeNotifier {
  ThemeData _themeData;

  ThemeChanger(this._themeData);

  getTheme() => _themeData;
  setTheme(ThemeData theme) {
    _themeData = theme;

    notifyListeners();
  }
}

常量.dart

 class Constants {
  static const String ChangeTheme = 'Настройки';
  static const String info = 'Информация';
  static const String OffAd = 'Отключить рекламу';

  static const List<String> choices = <String>[
    ChangeTheme,
    info,
    OffAd
  ];

}