https://dartpad.dev/embed-flutter.html?theme=light&run=false&split=60&ga_id=false

indicatorSample.dart

import 'package:flutter/material.dart';
import 'package:flutter/cupertino.dart';

void main() async {
  runApp(
    MaterialApp(
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        body: MyApp(),
      ),
    ),
  );
}

class MyApp extends StatelessWidget {
  static const String _title = 'Flutter Code Sample';

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: _title,
      home: MyStatefulWidget(),
    );
  }
}

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

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

class _MyStatefulWidgetState extends State<MyStatefulWidget> {
  Future<String> _calculation = Future<String>.delayed(
    Duration(seconds: 2),
    () => 'Data Loaded',
  );

  Widget build(BuildContext context) {
    return FutureBuilder<String>(
      future: _calculation, // a previously-obtained Future<String> or null
      builder: (BuildContext context, AsyncSnapshot<String> snapshot) {
        List<Widget> children;

        if (snapshot.hasData) {
          children = <Widget>[
            Icon(
              Icons.check_circle_outline,
              color: Colors.green,
              size: 60,
            ),
            Padding(
              padding: const EdgeInsets.only(top: 16),
              child: Text('Result: ${snapshot.data}'),
            )
          ];
        } else if (snapshot.hasError) {
          children = <Widget>[
            Icon(
              Icons.error_outline,
              color: Colors.red,
              size: 60,
            ),
            Padding(
              padding: const EdgeInsets.only(top: 16),
              child: Text('Error: ${snapshot.error}'),
            )
          ];
        } else {
          children = <Widget>[
//             const Padding(
//               padding: EdgeInsets.only(top: 300),
//               child: CupertinoActivityIndicator(
                 CupertinoActivityIndicator(
//                 radius: 50
                   animating: false
//               ),
            )
          ];
        }
        return Center(
          child: Column(
            mainAxisAlignment: MainAxisAlignment.center,
            crossAxisAlignment: CrossAxisAlignment.center,
            children: children,
          ),
        );
      },
    );
  }
}