[Flutter] Show progress bar based on the response of a method using streams and stream builder

main.dart

import 'package:flutter/material.dart';

final Color darkBlue = Color.fromARGB(255, 18, 32, 47);

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData.dark().copyWith(scaffoldBackgroundColor: darkBlue),
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        body: Center(
          child: MyWidget(),
        ),
      ),
    );
  }
}

class MyWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return baseBuilder(context);
  }

  /// This method shows how your code should be framed for showing a progress bar based
  /// on the return value of a normal method
  Widget baseBuilder(context) {
    return StreamBuilder<bool>(
      initialData: true,
      stream: _longOperation(),
      builder: (context, snapshot) {
        final isLoading = snapshot?.data ?? true;
        return Stack(
          children: <Widget>[
            Container(
              child: Center(
                child: Text('Base Widget : $isLoading'),
              ),
            ),
            Visibility(
              visible: isLoading,
              child: Center(
                child: CircularProgressIndicator(),
              ),
            ),
          ],
        );
      },
    );
  }

  /// you can use any return type but should be able to distinguish when to start
  /// and when to stop the progress bar
  /// like, `null` for showing  and `not null` for hiding progress bar
  Stream<bool> _longOperation() async* {
    yield true;
    //do some long operation instead of wait
    await Future.delayed(
      Duration(
        seconds: 5,
      ),
    );
    yield false;
  }
}