[Flutter] Creating & Updating State in a Flutter Application

To create a Stateful widget:

1. Create a StatefulWidget

2. Create a State

class SGreeting extends StatefulWidget {
  @override
  SGreetingState createState() => SGreetingState()
}

class SGreetingState extends State {

  String greeting = "Hello State";

  void _toggleState () {
    var next = greeting == "Hello State" ? "Hello World" : "Hello State";
    setState(() {
      greeting = next;
    });
  }

  @override
  Widget build(BuildContext context) {
    // TODO: implement build
    return GestureDetector(
      onTap: this._toggleState,
      child: Text(
          this.greeting,
          style: TextStyle(
              fontSize: 32
          )
      )
    );
  }

}