main.dart

import 'package:flutter/material.dart';

void main() => runApp(MaterialApp(home: Scaffold(body: Center(child: MyWidget()))));

Future<String> callAsyncFetch() => Future.delayed(Duration(seconds: 2), () => "hi");

class MyWidget extends StatelessWidget {
  @override
  Widget build(context) {
    return FutureBuilder<String>(
      future: callAsyncFetch(),
      builder: (context, AsyncSnapshot<String> snapshot) {
        if (snapshot.hasData) {
          return Text(snapshot.data);
        } else {
          return CircularProgressIndicator();
        }
      }
    );
  }
}