Stateful widget

stateful_widget.dart

import 'package:flutter/material.dart';

void main() {
  runApp(MaterialApp(
    title: 'Flutter Tutorial',
    home: TutorialHome(),
  ));
}

class TutorialHome extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(child: Counter()),
    );
  }
}

class Counter extends StatefulWidget {
  
  // Creates State
  @override
  _CounterState createState() => _CounterState();
}

class _CounterState extends State<Counter> {
  int _counter = 0;

  void _increment() {
    // triggers build() after assigning new values 
    setState(() {
      _counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Row(
      mainAxisAlignment: MainAxisAlignment.center,
      children: <Widget>[
        RaisedButton(
          onPressed: _increment,
          child: Text('Increment'),
        ),
        SizedBox(width: 16.0),
        Text(
          'Count: $_counter',
          style: TextStyle(fontSize: 16.0),
        ),
      ],
    );
  }
}