Light / Dark Mode in flutter

Question

I want to put the light/dark mode switch inside the application. Switch is okay. But it isn't any changing in application. Just I see the light theme. And, when I clicked the switch, I get this error or warning (idk). : Another exception was thrown: Tried to listen to a value exposed with provider, from outside the widget tree.

This is the main.dart:

    import 'package:provider/provider.dart';
    import 'package:bankingapp/widget/themestate.dart';
    
    void main() {
      runApp(ChangeNotifierProvider<ThemeState>(
        create: (context) => ThemeState(),
        child: MyApp(),
      ));
    }

    class MyApp extends StatelessWidget {
      // This widget is the root of your application.
      @override
      Widget build(BuildContext context) {
        return MaterialApp(
          theme: Provider.of<ThemeState>(context).theme == ThemeType.DARK
              ? ThemeData.dark()
              : ThemeData.light(),
          debugShowCheckedModeBanner: false,
          home: MySplash(),
        );
      }
    }

This is the homescreen.dart:

Container(
  child: Switch(
    value: Provider.of<ThemeState>(context).theme == ThemeType.DARK,
    onChanged: (value) {
      Provider.of<ThemeState>(context).theme =
          value ? ThemeType.DARK : ThemeType.LIGHT;
      setState(() {});
    },
  ),
),

This is the themestate.dart:

import 'package:flutter/material.dart';

enum ThemeType { DARK, LIGHT }

class ThemeState extends ChangeNotifier {
  bool _isDarkTheme = false;

  ThemeState() {
    getTheme().then((type) {
      _isDarkTheme = type == ThemeType.DARK;
      notifyListeners();
    });
  }
  ThemeType get theme => _isDarkTheme ? ThemeType.DARK : ThemeType.LIGHT;
  set theme(ThemeType type) => setTheme(type);

  void setTheme(ThemeType type) async {
    _isDarkTheme = type == ThemeType.DARK;
    notifyListeners();
  }

  Future<ThemeType> getTheme() async {
    return _isDarkTheme ? ThemeType.DARK : ThemeType.LIGHT;
  }
}

Note: This code snippet is the part of the project about ThemeS. Because code is consisted of very long line.

https://stackoverflow.com/questions/60232070/how-to-implement-dark-mode-in-flutter — sh.seo, Sep 08 '21 at 08:48

score 0 · Accepted Answer · answered Sep 08 '21 at 12:49

When calling Provider.of on a button, you should always pass listen: false, like so:

onChanged: (value) {
  Provider.of<ThemeState>(context, listen: false).theme =
    value ? ThemeType.DARK : ThemeType.LIGHT;
    setState(() {});
})

To be completely honest, I'm not sure if this will fix your code, however when I tried to replicate your error, I got the following error message:

════════ Exception caught by gesture ═══════════════════════════════════════════
Tried to listen to a value exposed with provider, from outside of the widget tree.

This is likely caused by an event handler (like a button's onPressed) that called
Provider.of without passing `listen: false`.

Here's my minimal example, which worked after adding listen: false

import 'package:flutter/material.dart';
import 'package:provider/provider.dart';

void main() {
  runApp(ChangeNotifierProvider<MyValue>(
    create: (context) => MyValue(true),
    child: MyApp(),
  ));
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key? key}) : super(key: key);

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

class _MyHomePageState extends State<MyHomePage> {
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: Checkbox(
          value: Provider.of<MyValue>(context).value,
          onChanged: (val) {
            Provider.of<MyValue>(context, listen: false).value = val!;
            setState(() {});
          },
        ),
      ),
    );
  }
}

class MyValue extends ChangeNotifier {
  MyValue(this.value);
  bool value;
}

So I hope adding listen: false will solve your problem

Light / Dark Mode in flutter

1 Answers1