main_stateful_widget.dart

class MySecondWidget extends StatefulWidget {
  MySecondWidget({Key key}) : super(key: key);

  @override
  _MySecondWidgetState createState() => 
                _MySecondWidgetState();
}

class _MySecondWidgetState extends State<MySecondWidget> {
  int _clickCount = 0;

  void incrementClickCount() {
    setState(() {
      this._clickCount++;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("My First App"),
      ),
      body: Container(
        padding: EdgeInsets.all(8.0),
        child: Text(
          "Hello! Times clicked: " 
                + this._clickCount.toString(),
          style: TextStyle(
            color: Colors.green, 
            fontSize: 20.0),
        ),
      ),
      floatingActionButton: FloatingActionButton(
        child: Icon(Icons.add),
        onPressed: this.incrementClickCount,
      ),
    );
  }
}