ChageNotifier with Undo/Redo functionality

undo_change_notifier.dart

import 'package:flutter/cupertino.dart';

class UndoChangeNotifier<T> extends ChangeNotifier {
  List<T> _history;
  int _historyIndex;

  UndoChangeNotifier([T initialState]) {
    _history = initialState != null ? [initialState] : [];
    _historyIndex = initialState != null ? 0 : -1;
  }

  T get state => hasState ? _history[_historyIndex] : null;

  bool get hasState => _history.isNotEmpty;

  void updateState(T text) {
    _history.add(text);
    _historyIndex++;
    notifyListeners();
  }

  void jumpTo(int index) {
    _historyIndex = index;
    notifyListeners();
  }

  void undo() {
    _historyIndex--;
    notifyListeners();
  }

  void redo() {
    _historyIndex++;
    notifyListeners();
  }

  void clear() {
    _historyIndex = -1;
    _history.clear();
    notifyListeners();
  }
}

undo_change_notifier_test.dart

import 'package:flutter_test/flutter_test.dart';
import 'package:untitled24/undo_change_notifier.dart';

void main() {
  group('UndoChangeNotifier', () {
    test('should start with an initial state', () {
      final cn = UndoChangeNotifier('I');

      expect(cn.state, 'I');
    });

    test('can update the state', () {
      final cn = UndoChangeNotifier('I');

      cn.updateState('N');

      expect(cn.state, 'N');
    });

    test('can undo a change', () {
      final cn = UndoChangeNotifier('I');

      cn.updateState('N');
      cn.undo();

      expect(cn.state, 'I');
    });

    test('can redo a change', () {
      final cn = UndoChangeNotifier('I');

      cn.updateState('N');
      cn.undo();
      cn.redo();

      expect(cn.state, 'N');
    });

    test('can jump to a state', () {
      final cn = UndoChangeNotifier('I');

      cn.updateState('N');
      cn.jumpTo(0);

      expect(cn.state, 'I');
    });

    test('can clear the history', () {
      final cn = UndoChangeNotifier('I');

      cn.clear();

      expect(cn.state, isNull);
    });

    test('can clear the history', () {
      final cn = UndoChangeNotifier('I');

      cn.clear();

      expect(cn.state, isNull);
    });
  });
}