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blog-gist-2020-7-flutter-app-state-0.dart

import 'package:flutter/material.dart';

class Update {
  Update({required this.description});

  String description;
}

class MyStatefulWidget extends StatefulWidget {
  @override
  State<StatefulWidget> createState() => MyStatefulWidgetState();
}

class MyStatefulWidgetState extends State<MyStatefulWidget> {
  List<Update> updates = [];

  @override
  Widget build(BuildContext context) {
    print("Updates: $updates");
    return Column(
      children: [
        ElevatedButton(
          onPressed: () {
            fetchUpdates().then((newUpdates) {
              setState(() {
                updates = newUpdates;
              });
            });
          },
          child: const Text("Fetch notifications"),
        ),
        Flexible(
          child: ListView.builder(
            itemCount: updates.length,
            itemBuilder: (BuildContext context, int index) {
              return ListTile(
                title: Text(updates[index].description),
              );
            },
          ),
        )
      ],
    );
  }
}

blog-gist-2020-7-flutter-app-state-1.dart

import 'package:flutter/material.dart';

class NotificationIcon extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    var notificationsCount = 0;
    return Stack(
        children: <Widget>[
        const Icon(Icons.notifications),
        Positioned(
            right: 0,
            child: Container(
            padding: EdgeInsets.all(1),
            decoration: BoxDecoration(
                color: Colors.red,
                borderRadius: BorderRadius.circular(6),
            ),
            constraints: const BoxConstraints(
                minWidth: 12,
                minHeight: 12,
            ),
            child: Text(
                '$notificationsCount',
                style: const TextStyle(
                color: Colors.white,
                fontSize: 8,
                ),
                textAlign: TextAlign.center,
            ),
            ),
        )
        ],
    );
  }
}

blog-gist-2020-7-flutter-app-state-2.undefined

flutter pub add rxdart

blog-gist-2020-7-flutter-app-state-3.dart

import 'package:rxdart/rxdart.dart';

import 'MyStatefulWidget.dart';

class NotificationsService {
  final BehaviorSubject<List<Update>> notificationSubject = BehaviorSubject.seeded([]);

  ValueStream get stream$ => notificationSubject.stream;
  
  updateNotifications(List<Update> updates) {
    notificationSubject.add(updates);
  }
}

blog-gist-2020-7-flutter-app-state-4.js

flutter pub add get_it

blog-gist-2020-7-flutter-app-state-5.dart

void main() {
  GetIt getIt = GetIt.I;
  getIt.registerSingleton<NotificationsService>(NotificationsService());
  runApp(const MyApp());
}

blog-gist-2020-7-flutter-app-state-6.dart

GetIt getIt = GetIt.I;
var service = getIt.get<NotificationsService>();

blog-gist-2020-7-flutter-app-state-7.dart

fetchUpdates().then((newUpdates) {
    GetIt getIt = GetIt.I;
    var service = getIt.get<NotificationsService>();
    service.updateNotifications(newUpdates);
    ...

blog-gist-2020-7-flutter-app-state-8.dart

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

import 'MyStatefulWidget.dart';
import 'NotificationsService.dart';

class NotificationIcon extends StatelessWidget {
  late NotificationsService service;

  NotificationIcon() {
    GetIt getIt = GetIt.I;
    service = getIt.get<NotificationsService>();
  }

  @override
  Widget build(BuildContext context) {
    return StreamBuilder(
      stream: service.stream$,
      builder: (BuildContext context, AsyncSnapshot<dynamic> snapshot) {
        if (!snapshot.hasData || snapshot.data == null) {
          return const CircularProgressIndicator();
        }

        List<Update> updates = snapshot.data;

        if (updates.isEmpty) {
          return Container();
        }
        var notificationsCount = updates.length;

        return Stack(
          children: <Widget>[
            const Icon(Icons.notifications),
            Positioned(
              right: 0,
              child: Container(
                padding: EdgeInsets.all(1),
                decoration: BoxDecoration(
                  color: Colors.red,
                  borderRadius: BorderRadius.circular(6),
                ),
                constraints: const BoxConstraints(
                  minWidth: 12,
                  minHeight: 12,
                ),
                child: Text(
                  '$notificationsCount',
                  style: const TextStyle(
                    color: Colors.white,
                    fontSize: 8,
                  ),
                  textAlign: TextAlign.center,
                ),
              ),
            )
          ],
        );
      },
    );
  }
}

blog-gist-2020-7-flutter-app-state-9.dart

 return StreamBuilder(
      stream: service.stream$,
      builder: (BuildContext context, AsyncSnapshot<dynamic> snapshot) {
        if (!snapshot.hasData || snapshot.data == null) {
          return const CircularProgressIndicator();
        }

        List<Update> updates = snapshot.data;

        if (updates.isEmpty) {
          return Container();
        }
        var notificationsCount = updates.length;
        // ... render the notification icon