class ExampleState extends State<Example> {

  bool condition = false;

  @override
  Widget build(BuildContext context) {
    Widget myWidget;
    if (condition) {
      myWidget = Center(child: CircularProgressIndicator());
    } else {
      myWidget = Center(child: Text("Un testo"));
    }
    return Scaffold(
      body: myWidget,
      floatingActionButton: FloatingActionButton(
        onPressed: () {
          setState(() {
            condition = true;
          });
        },
      ),
    );
  }

}