Should you use "extends" or "with" keyword for ChangeNotifier? - Flutter

You can use either extends (to inherit) or with (as a mixin). Both ways give you access to the notifyListeners() method in ChangeNotifier.

Inheritance

Extending ChangeNotifier means that ChangeNotifier is the super class.

class MyModel extends ChangeNotifier {
  
  String someValue = 'Hello';
  
  void doSomething(String value) {
    someValue = value;
    notifyListeners();
  }
}

If your model class is already extending another class, then you can't extend ChangeNotifier because Dart does not allow multiple inheritance. In this case you must use a mixin.

Mixin

A mixin allows you to use the concrete methods of the mixin class (ie, notifyListeners()).

class MyModel with ChangeNotifier {
  
  String someValue = 'Hello';
  
  void doSomething(String value) {
    someValue = value;
    notifyListeners();
  }
}

So even if your model already extends from another class, you can still "mix in" ChangeNotifier.

class MyModel extends SomeOtherClass with ChangeNotifier {
  
  String someValue = 'Hello';
  
  void doSomething(String value) {
    someValue = value;
    notifyListeners();
  }
}

Here are some good reads about mixins:

• Dart: What are mixins?
• Dart for Flutter : Mixins in Dart

extends is used to inherit a class
with is used to use class as a mixin

Refer here to understand difference between mixin and inheritence: https://stackoverflow.com/a/860312/10471480

Refering to ChangeNotifier, the docs says

A class that can be extended or mixed in that provides a change notification API using VoidCallback for notifications.

Hence you can inherit it as well as use it as a mixin