Switch theme mode (system, light, dark) dynamically

theme_mode_app.dart

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

// Wrap your application instance with this class.
//
//   class MyApp extends StatelessWidget {
//     // This widget is the root of your application.
//     @override
//     Widget build(BuildContext context) {
//       return ThemeModeApp((BuildContext context, ThemeMode themeMode) {
//         return MaterialApp(
//           title: 'MyApp',
//           theme: ThemeData.light(),
//           darkTheme: ThemeData.dark(),
//           home: HomeView(),
//           themeMode: themeMode,
//         );
//       },
//     }
//   }
//
// And then somewhere in your app (e.g., settings view):
//
//   ThemeModeApp.of(context).themeMode = ThemeMode.dark; // or ThemeMode.light or ThemeMode.system
//
// This is similar to dynamic_theme package but this focuses only on the theme mode.
class ThemeModeApp extends StatefulWidget {
  final Function(BuildContext context, ThemeMode themeMode) appBuilder;
  final String preferencesKeyPrefix;

  ThemeModeApp(this.appBuilder, {this.preferencesKeyPrefix});

  static _ThemeModeAppState of(BuildContext context, {bool root = false}) =>
      root
          ? context.findRootAncestorStateOfType<_ThemeModeAppState>()
          : context.findAncestorStateOfType<_ThemeModeAppState>();

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

class _ThemeModeAppState extends State<ThemeModeApp> {
  _ThemeModePreferences _preferences;

  @override
  Widget build(BuildContext context) {
    return widget.appBuilder(context, _preferences.themeMode);
  }

  @override
  void initState() {
    super.initState();

    _preferences = _ThemeModePreferences(widget.preferencesKeyPrefix);

    _preferences.addListener(_onPreferencesUpdate);
    _preferences.loadState();
  }

  @override
  void dispose() {
    _preferences.removeListener(_onPreferencesUpdate);

    super.dispose();
  }

  set themeMode(ThemeMode mode) {
    _preferences.themeMode = mode;
  }

  ThemeMode get themeMode => _preferences.themeMode;

  void _onPreferencesUpdate() {
    setState(() {});
  }
}

// Do NOT use this in a Provider
class _ThemeModePreferences extends ChangeNotifier {
  final String preferencesKeyPrefix;

  _ThemeModePreferences(this.preferencesKeyPrefix);

  bool _stateLoaded = false;

  ThemeMode _themeMode = ThemeMode.system;

  ThemeMode get themeMode => _themeMode;

  set themeMode(ThemeMode value) {
    _storeThemeMode(value);
  }

  void _storeThemeMode(ThemeMode value) async {
    if (_themeMode == value) {
      return;
    }

    SharedPreferences prefs = await SharedPreferences.getInstance();
    _themeMode = value;
    await prefs.setInt(
        _preferenceKey(_themeModePreferenceFieldName), _themeMode.index);
    notifyListeners();
  }

  Future loadState() async {
    if (_stateLoaded) {
      return;
    }

    _stateLoaded = true;

    SharedPreferences prefs = await SharedPreferences.getInstance();
    final themeModeIndex =
        prefs.getInt(_preferenceKey(_themeModePreferenceFieldName));
    if (themeModeIndex != null) {
      final themeMode = ThemeMode.values[themeModeIndex];
      if (_themeMode != themeMode) {
        _themeMode = themeMode;
        notifyListeners();
      }
    }
  }

  static const _themeModePreferenceFieldName = 'themeMode';

  String _preferenceKey(String fieldName) {
    if (preferencesKeyPrefix?.isNotEmpty == true) {
      if (preferencesKeyPrefix.endsWith('.')) {
        return preferencesKeyPrefix + fieldName;
      }
      return preferencesKeyPrefix + '.' + fieldName;
    }
    if (preferencesKeyPrefix == null) {
      return 'app.' + fieldName;
    }
    return fieldName;
  }
}