Flutter progress indicator full working example, used here: https://www.flutterclutter.dev/flutter/tutorials/displaying-a-prorgess-indicator/2020/37/

progess_indicator_05.dart

import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: MyHomePage(title: 'Flutterclutter: Loading indicator'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);
  final String title;

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

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;

  Future _incrementCounter() async {
    return Future.delayed(Duration(seconds: 2), () {
      setState(() {
        _counter++;
      });
    });
  }

  @override
  Widget build(BuildContext context) {
      return Scaffold(
        appBar: AppBar(
        title: Text(widget.title),
      ),
        body: Center(
          child: Text('Current counter is $_counter')
        ),
        floatingActionButton: FloatingActionButton(
        onPressed: () {
          _onPressed(context);
        },
        child: Icon(Icons.add),
      ),
      );
    }

  void _onPressed(BuildContext context) async {
    DialogBuilder(context).showLoadingIndicator('Calculating $_counter + 1');
    await _incrementCounter();
    DialogBuilder(context).hideOpenDialog();
  }
}

class DialogBuilder {
  DialogBuilder(this.context);

  final BuildContext context;

  void showLoadingIndicator([String text]) {
    showDialog(
      context: context,
      barrierDismissible: false,
      builder: (BuildContext context) {
        return WillPopScope(
            onWillPop: () async => false,
            child: AlertDialog(
              shape: RoundedRectangleBorder(
                  borderRadius: BorderRadius.all(Radius.circular(8.0))
              ),
              backgroundColor: Colors.black87,
              content: LoadingIndicator(
                  text: text
              ),
            )
        );
      },
    );
  }

  void hideOpenDialog() {
    Navigator.of(context).pop();
  }
}

class LoadingIndicator extends StatelessWidget{
  LoadingIndicator({this.text = ''});

  final String text;

  @override
  Widget build(BuildContext context) {
    var displayedText = text;

    return Container(
        padding: EdgeInsets.all(16),
        color: Colors.black87,
        child: Column(
            mainAxisAlignment: MainAxisAlignment.center,
            mainAxisSize: MainAxisSize.min,
            children: [
              _getLoadingIndicator(),
              _getHeading(context),
              _getText(displayedText)
            ]
        )
    );
  }

  Padding _getLoadingIndicator() {
    return Padding(
        child: Container(
            child: CircularProgressIndicator(
                strokeWidth: 3
            ),
            width: 32,
            height: 32
        ),
        padding: EdgeInsets.only(bottom: 16)
    );
  }

  Widget _getHeading(context) {
    return
      Padding(
          child: Text(
            'Please wait …',
            style: TextStyle(
                color: Colors.white,
                fontSize: 16
            ),
            textAlign: TextAlign.center,
          ),
          padding: EdgeInsets.only(bottom: 4)
      );
  }

  Text _getText(String displayedText) {
    return Text(
      displayedText,
      style: TextStyle(
          color: Colors.white,
          fontSize: 14
      ),
      textAlign: TextAlign.center,
    );
  }
}