0_today.dart

import 'package:flutter/material.dart';

void main() {
  runApp(
    MaterialApp(
      home: Scaffold(
        body: CounterPage(),
      ),
    ),
  );
}

class CounterPage extends StatefulWidget {
  @override
  State<CounterPage> createState() => _CounterPageState();
}

class _CounterPageState extends State<CounterPage> {
  int _counter = 0;

  @override
  Widget build(BuildContext context) {
    return Column(children: [
      Text('Count: $_counter'),
      
      ElevatedButton(
        onPressed: () {
          setState(() {
            _counter++;
          });
        },
        child: Text('Increment'),
      ),
    ]);
  }
}

1_tomorrow.dart

import 'package:flutter/material.dart';

void main() {
  runApp(
    MaterialApp(
      home: Scaffold(
        body: CounterPage(),
      ),
    ),
  );
}

@observable
class Counter({int count = 0});

final counter = Counter();

class CounterPage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    context.watchListenable(counter);
 
    return Column(children: [
      Text('Count: ${counter.count}'),
 
      ElevatedButton(
        onPressed: () {
          counter.count++;
        },
        child: Text('Increment'),
      ),
    ]);
  }
}

2_tomorrow_without_globals.dart

import 'package:flutter/material.dart';

void main() {
  final counter = Counter();

  runApp(
    MaterialApp(
      home: Scaffold(
        body: CounterPage(counter: counter),
      ),
    ),
  );
}

@observable
class Counter({int count = 0});

class CounterPage extends StatelessWidget({
  Counter counter,
}) {
  
  @override
  Widget build(BuildContext context) {
    context.watchListenable(counter);
 
    return Column(children: [
      Text('Count: ${counter.count}'),
 
      ElevatedButton(
        onPressed: () {
          counter.count++;
        },
        child: Text('Increment'),
      ),
    ]);
  }
}