Sample Bloc

main.dart

import 'dart:async';

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'BloC Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

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

  final String title;

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

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

  void _incrementCounter() {
    bloc.fetchUpdatedNumber('${++_counter}');
    // setState(() {
    //   _counter++;
    // });
  }

  @override
  Widget build(BuildContext context) {
    return StreamBuilder<bool>(
        stream: bloc.showProgress,
        builder: (BuildContext context, AsyncSnapshot<bool> progressBarData) {
          /// To display/hide LinearProgressBar
          /// call bloc.updateShowProgress(bool)
          var showProgress = false;
          if (progressBarData.hasData && progressBarData.data != null)
            showProgress = progressBarData.data!;
          return Scaffold(
            appBar: AppBar(
              title: Text(widget.title),
              bottom: showProgress
                  ? PreferredSize(
                      preferredSize: Size(double.infinity, 4.0),
                      child: LinearProgressIndicator())
                  : null,
            ),
            body: StreamBuilder<String>(
                stream: bloc.updatedNumber,
                builder: (BuildContext context,
                    AsyncSnapshot<String> numberSnapshot) {
                  var number = '0';
                  if (numberSnapshot.hasData && numberSnapshot.data != null)
                    number = numberSnapshot.data!;
                  return Center(
                    child: Column(
                      mainAxisAlignment: MainAxisAlignment.center,
                      children: <Widget>[
                        Text(
                          'You have pushed the button this many times:',
                        ),
                        Text(
                          '$number',
                          style: Theme.of(context).textTheme.headline4,
                        ),
                      ],
                    ),
                  );
                }),
            floatingActionButton: FloatingActionButton(
              onPressed: _incrementCounter,
              tooltip: 'Increment',
              child: Icon(Icons.add),
            ),
          );
        });
  }
}

class Bloc {
  /// UI updates can be handled in Bloc
  final _repository = Repository();
  final _progressIndicator = StreamController<bool>.broadcast();
  final _updatedNumber = StreamController<String>.broadcast();

  /// StreamBuilder listens to [showProgress] to update UI
  Stream<bool> get showProgress => _progressIndicator.stream;

  Stream<String> get updatedNumber => _updatedNumber.stream;

  updateShowProgress(bool showProgress) {
    _progressIndicator.sink.add(showProgress);
  }

  /// Updates the List<UserThreads> Stream
  fetchUpdatedNumber(String number) async {
    bloc.updateShowProgress(true); // Show ProgressBar

    /// Timer mocks an instance where we're waiting for
    /// a response from the HTTP request
    Timer(Duration(seconds: 2), () async {
      // delay for 4 seconds to display LinearProgressBar
      var updatedNumber = await _repository.fetchUpdatedNumber(number);
      _updatedNumber.sink.add(updatedNumber); // Update Stream
      bloc.updateShowProgress(false); // Hide ProgressBar
    });
  }

  dispose() {
    _updatedNumber.close();
  }

  disposeProgressIndicator() {
    _progressIndicator.close();
  }
}

/// this enables Bloc to be globally accessible
final bloc = Bloc();

/// Class where we can keep Repositories that can be accessed in Bloc class
class Repository {
  final provider = Provider();

  Future<String> fetchUpdatedNumber(String number) =>
      provider.updateNumber(number);
}

/// Class where all backend tasks can be handled
class Provider {
  Future<String> updateNumber(String number) async {
    /// HTTP requests can be done here
    return number;
  }
}