The issue is that you are trying to access _count in a separate class without providing context to the Provider. To access the value of _count using Provider, you need to wrap the _HamburgerState widget in a ChangeNotifierProvider and expose _count as a listenable property.
This is an example:
import 'package:flutter/widgets.dart';
import 'package:provider/provider.dart';
class Hamburger extends StatefulWidget {
const Hamburger({Key key}) : super(key: key);
@override
State<Hamburger> createState() => _HamburgerState();
}
class _HamburgerState extends State<Hamburger> {
int _count = 1;
void incrementCount() {
setState(() {
_count++;
});
}
@override
Widget build(BuildContext context) {
return ChangeNotifierProvider.value(
value: this,
child: Container(),
);
}
}
class CountNotifier extends ChangeNotifier {
int _count;
int get count => _count;
set count(int value) {
_count = value;
notifyListeners();
}
}