future.dart

Future<String> _calculation = Future<String>.delayed(
  Duration(seconds: 2),
  () => 'Data Loaded',
);

Widget build(BuildContext context) {
  return Container(
    child: FutureBuilder<String>(
        future: _calculation,
        builder: (BuildContext context, AsyncSnapshot<String> snapshot) {
          if (snapshot.hasError) {
            return Center(child: Text(snapshot.error.toString()));
          }
          if (snapshot.hasData) {
            return Center(child: Text(snapshot.data));
          }
          return CircularProgressIndicator();
        }
    ),
  );
}