state_simple.dart

import 'package:flutter/widgets.dart';

void main() => runApp(TopWidget());

class TopWidget extends StatefulWidget {
  @override
  _TopWidgetState createState() => _TopWidgetState();
}

class _TopWidgetState extends State<TopWidget> {
  int counter = 0;

  void increment() {
    setState(() {
      counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: MiddleWidget(counter),
    );
  }
}

class MiddleWidget extends StatelessWidget {
  MiddleWidget(this.counter);
  final int counter;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: BottomWidget(counter),
    );
  }
}

class BottomWidget extends StatelessWidget {
  BottomWidget(this.counter);
  final int counter;

  @override
  Widget build(BuildContext context) {
    return Container(
      child: Text(counter.toString()),
    );
  }
}