state_example_provider.dart

import 'package:flutter/material.dart';
import 'package:provider/provider.dart';

class ReactiveWidget extends StatefulWidget {
  @override
  State<StatefulWidget> createState() => ReactiveWidgetState();
}

class ReactiveWidgetState extends State<ReactiveWidget> {
  @override
  Widget build(BuildContext context) {
    return ChangeNotifierProvider<ProviderState>(
      builder: (_) => ProviderState(),
      child: ReactiveWidgetChild(),
    );
  }
}

class ReactiveWidgetChild extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Consumer<ProviderState>(
      builder: (_, state, child) {
        return Center(
          child: Text(state.value.toString()),
        );
      }
    );
  }
}

class ProviderState with ChangeNotifier {
  int _value;

  ProviderState() : _value = 0;

  int get value => _value;
  set value(int v) {
    _value = v;
    notifyListeners();
  }
}