riverpod/website/docs/concepts/why_immutability/codegen.dart

import 'package:flutter/material.dart';
import 'package:freezed_annotation/freezed_annotation.dart';
import 'package:riverpod_annotation/riverpod_annotation.dart';

part 'codegen.freezed.dart';
part 'codegen.g.dart';

/* SNIPPET START */

@riverpod
class ThemeNotifier extends _$ThemeNotifier {
  @override
  ThemeSettings build() => const ThemeSettings(
        mode: ThemeMode.light,
        primaryColor: Colors.blue,
      );

  void toggle() {
    state = state.copyWith(mode: state.mode.toggle);
  }

  void setDarkTheme() {
    state = state.copyWith(mode: ThemeMode.dark);
  }

  void setLightTheme() {
    state = state.copyWith(mode: ThemeMode.light);
  }

  void setSystemTheme() {
    state = state.copyWith(mode: ThemeMode.system);
  }

  void setPrimaryColor(Color color) {
    state = state.copyWith(primaryColor: color);
  }
}

@freezed
sealed class ThemeSettings with _$ThemeSettings {
  const factory ThemeSettings({
    required ThemeMode mode,
    required Color primaryColor,
  }) = _ThemeSettings;
}

extension ToggleTheme on ThemeMode {
  ThemeMode get toggle {
    switch (this) {
      case ThemeMode.dark:
        return ThemeMode.light;
      case ThemeMode.light:
        return ThemeMode.dark;
      case ThemeMode.system:
        return ThemeMode.system;
    }
  }
}