ChangeNotifier is a Flutter class that holds state and notifies its listeners with notifyListeners() when it changes. Combined with Provider or ListenableBuilder, it rebuilds only the widgets concerned. It is a simple way to share state between several screens.
Example
class Panier extends ChangeNotifier {
final _items = <String>[];
void ajouter(String s) { _items.add(s); notifyListeners(); }
}