Flutter reactive UI without class based widgets (...but don't do this)

main.dart

import 'package:flutter/material.dart';

final Color darkBlue = Color.fromARGB(255, 18, 32, 47);

void main() {
  MyAppModel myAppModel = MyAppModel();
  myAppModel.addListener(() => runApp(myApp(myAppModel)));
  myAppModel.updateUI(1);
}

Widget myApp(myAppModel) {
  return MaterialApp(
    theme: ThemeData.dark().copyWith(scaffoldBackgroundColor: darkBlue),
    debugShowCheckedModeBanner: false,
    home: Scaffold(
      body: Center(
        child: myWidget(myAppModel),
      ),
    ),
  );
}

Widget myWidget(myAppModel) {
  return RaisedButton(
    onPressed: () {
      myAppModel.updateUI(3);
    },
    child:  Text(myAppModel.n.toString(), style: TextStyle(fontSize: 20)),
  );
}

class MyAppModel with ChangeNotifier {
  int _n = 1;
  updateUI(int newN) {
    _n = newN;
    notifyListeners();
  }

  int get n => _n;
}