Flutter Stateful Widget

main.dart

import 'package:flutter/material.dart';

void main() {
  runApp(new Application());
}

class Counter extends StatefulWidget {
  final String name;

  Counter(this.name);

  _CounterState createState() => _CounterState();
}

class _CounterState extends State<Counter> {

  int counterValue = 0;

  void incrementCounter() {
    setState(() => counterValue++);
    print('(' + MediaQuery.of(context).size.width.toString() + ', ' + MediaQuery.of(context).size.height.toString() + ')');
  }

  @override
  Widget build(BuildContext context) {
    return(
      Container(
        padding: EdgeInsets.all(10),
        margin: EdgeInsets.all(10),
        decoration: BoxDecoration(border: Border.all(width: 0.5, color: Colors.blueAccent)),
        alignment: Alignment.center,
        child: Column(
          crossAxisAlignment: CrossAxisAlignment.center,
          mainAxisSize: MainAxisSize.max,
          mainAxisAlignment: MainAxisAlignment.spaceEvenly,
          children: <Widget>[
            Text('${widget.name} : $counterValue', style: TextStyle(fontSize: 18)),
            IconButton(enableFeedback: true, icon: Icon(Icons.add), onPressed: incrementCounter, color: Colors.blueAccent)
          ],
        ),
      )
    );
  }
}

class Application extends StatelessWidget {

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: "Hello World!",
      theme: ThemeData(primarySwatch: Colors.blue),
      home: Scaffold(
        appBar: AppBar(title: Text("Hello World!")),
        body: Counter("The current counter value")
      ),
    );
  }
}