global.dart

import 'package:flutter/material.dart';

// Global reactive value
final counter = ValueNotifier<int>(0);

class MyWidget extends StatefulWidget {
  @override
  _MyWidgetState createState() => _MyWidgetState();
}

class _MyWidgetState extends State<MyWidget> {
  @override
  Widget build(BuildContext context) => Column(
    children: [
      ElevatedButton(
        onPressed: () => counter.value++,
        child: Text('Increase'),
      ),
      // ValueListenableBuilder allows the
      // state change in the child widget
      // in this case the child is just a Text
      ValueListenableBuilder(
        valueListenable: counter,
        builder: (context, value, widget) {
          return Text(counter.toString());
        },
      ),
      ElevatedButton(
        onPressed: () => counter.value--,
        child: Text('Decrease'),
      ),
    ],
  );
}