state_persisting_change_notifier.dart

import 'dart:async';

import 'package:flutter/cupertino.dart';
import 'package:flutter/foundation.dart';
import 'package:fritter_for_reddit/exports.dart';
import 'package:hive/hive.dart';

/// Every time notifyListeners is called, the state is persisted to allow easy app persistence.
/// Persistence is based on Hive with a box opened with the class name.
/// If this is not desired, supply an alternative [boxName] in the constructor.
///
/// Listeners are automatically notified whenever [state] changes.
abstract class StatePersistingChangeNotifier<T extends SerializableClass>
    extends ChangeNotifier {
  final Completer<bool> isInitializedCompleter = Completer();

  Future<bool> get initializationFuture => isInitializedCompleter.future;

  Box _storage;
  T _state;

  T get state => _state;

  set state(T state) {
    _state = state;
    writeState(state);
    notifyListeners();
  }

  StatePersistingChangeNotifier({String boxName}) {
    _state = initialState;
    Hive.openBox(boxName ?? runtimeType.toString()).then((box) {
      _storage = box;
      isInitializedCompleter.complete(true);
      state = fromJson(_storage.get('state')) ?? initialState;
      notifyListeners();
    });
  }

  T fromJson(Map json);

  @mustCallSuper
  void writeState(T state) async {
    await initializationFuture;
    debugPrint('Writing state: $state');
    _storage.put('state', state.toJson());
  }

  T get initialState;
}

abstract class SerializableClass<T> {
  Map<String, dynamic> toJson();
}



class SettingsNotifier extends StatePersistingChangeNotifier<SettingsState> {
  @override
  SettingsState get initialState => SettingsState.initial();

  static SettingsNotifier of(BuildContext context, {bool listen = false}) =>
      Provider.of<SettingsNotifier>(context, listen: listen);

  void changeViewMode(ViewMode viewMode) =>
      state = state.copyWith(viewMode: viewMode);

  @override
  SettingsState fromJson(Map<dynamic, dynamic> json) {
    return SettingsState.fromJson(json);
  }
}

@immutable
class SettingsState extends SerializableClass {
  final ViewMode viewMode;

  SettingsState({@required this.viewMode});

  @override
  String toString() {
    return 'SettingsState{viewMode: $viewMode}';
  }

  SettingsState.initial() : viewMode = ViewMode.card;

  @override
  factory SettingsState.fromJson(Map json) {
    if (json == null) {
      return SettingsState.initial();
    }
    return SettingsState(
      viewMode: viewModeFromString(json['viewMode']),
    );
  }

  @override
  Map<String, dynamic> toJson() => {'viewMode': viewMode.toString()};

  static ViewMode viewModeFromString(String viewModeString) {
    switch (viewModeString) {
      case 'ViewMode.card':
        return ViewMode.card;
      case 'ViewMode.gallery':
        return ViewMode.gallery;
      default:
        {
          debugPrint(
              '$viewModeString is not a supported ViewMode setting. Returning ViewMode.card');
          return ViewMode.card;
        }
    }
  }

  SettingsState copyWith({
    ViewMode viewMode,
  }) =>
      SettingsState(
        viewMode: viewMode ?? this.viewMode,
      );
}

@loicgeek, yeah, I use copyWith as well.