import 'package:flutter/material.dart';

​

void main() {

  runApp(MyApp());

}

​

final darkNotifier = ValueNotifier<bool>(false);

​

class MyApp extends StatelessWidget {

  @override

  Widget build(BuildContext context) {

    return ValueListenableBuilder<bool>(

        valueListenable: darkNotifier,

        builder: (BuildContext context, bool isDark, Widget? child) {

          return MaterialApp(

            title: 'Flutter Demo',

            themeMode: isDark ? ThemeMode.dark : ThemeMode.light,

            theme: ThemeData(primaryColor: Colors.blue),

            darkTheme: ThemeData.dark(),

            home: MyHomePage(

              title: 'Homepage',

            ),

          );

        });

  }

}

​

class MyHomePage extends StatefulWidget {

  MyHomePage({Key? key, required this.title}) : super(key: key);

​

  final String title;

​

  @override

  _MyHomePageState createState() => _MyHomePageState();

}

​

class _MyHomePageState extends State<MyHomePage> {

  @override

  void dispose() {

    // TODO: implement dispose

    darkNotifier.dispose();

    super.dispose();

  }

​

  @override

  Widget build(BuildContext context) {

    bool isDark = darkNotifier.value;

    return Scaffold(

      appBar: AppBar(

        title: Text(widget.title),

      ),

      body: Center(

        child: Column(

          mainAxisAlignment: MainAxisAlignment.center,

          children: <Widget>[

            Text(

              'You have pushed the button this many times:',

            ),

            Text(

              _darkNotifier.value ? 'DarkMode' : 'LightMode',

              style: Theme.of(context).textTheme.headline4,

            ),

          ],

        ),

      ),

      floatingActionButton: FloatingActionButton(

        onPressed: () {

          isDark = !isDark;

          darkNotifier.value = isDark;

        },

        tooltip: 'Increment',

        child: Icon(isDark ? Icons.wb_sunny_outlined : Icons.bubble_chart),

      ),

    );

  }

}