main.dart

import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  final GlobalKey<_CounterWidgetState> _key = GlobalKey();
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Desktop',
      home: Scaffold(
        backgroundColor: Colors.blueGrey,
        appBar: AppBar(
          title: Text('I am Apollo'),
          centerTitle: true,
          backgroundColor: Colors.blueGrey[900],
        ),
        body: Center(
          child: Column(mainAxisAlignment: MainAxisAlignment.center, children: [
            Text(
              'Tap "-" to decrement',
              style: TextStyle(color: Colors.white54),
            ),
            Card(
              color: Colors.white54,
              child: Row(
                mainAxisAlignment: MainAxisAlignment.center,
                mainAxisSize: MainAxisSize.min,
                children: <Widget>[
                  IconButton(
                      padding: const EdgeInsets.all(0.0),
                      icon: Icon(Icons.remove),
                      onPressed: () {
                        _key.currentState.pressRemove();
                      }),
                  CounterWidget(
                    key: _key,
                  ),
                  IconButton(
                      padding: const EdgeInsets.all(0.0),
                      icon: Icon(Icons.add),
                      onPressed: () {
                        _key.currentState.pressPlus();
                      }),
                ],
              ),
            ),
            Text('Tap "+" to increment',
                style: TextStyle(color: Colors.white54)),
          ]),
        ),
      ),
    );
  }
}

class CounterWidget extends StatefulWidget {
  CounterWidget({Key key}) : super(key: key);
  @override
  _CounterWidgetState createState() => _CounterWidgetState();
}

class _CounterWidgetState extends State<CounterWidget> {
  int counter = 50;

  @override
  Widget build(BuildContext context) {
    return Text('$counter');
  }

  pressRemove() => setState(() {
        counter--;
      });
  pressPlus() => setState(() {
        counter++;
      });
}