Using ChangeNotifier as a Singleton

main.dart

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

void main() {
  runApp(const SampleApp());
}

class SampleApp extends StatelessWidget {
  const SampleApp({super.key});
  @override
  Widget build(BuildContext context) {
    return ChangeNotifierProvider(
      create: (context) => NotificationHandler(),
      child: const MaterialApp(
        home: ListOfContactsPage(),
      ),
    );
  }
}

class ListOfContactsPage extends StatefulWidget {
  const ListOfContactsPage({super.key});

  @override
  State<ListOfContactsPage> createState() => _ListOfContactsPageState();
}

class _ListOfContactsPageState extends State<ListOfContactsPage> {
  List<Contact> contacts = [
    Contact(
      name: 'John',
      userId: 'john@gmail.com',
    ),
    Contact(
      name: 'Jane',
      userId: 'jane@gmail.com',
    ),
    Contact(
      name: 'Bob',
      userId: 'bob@gmail.com',
    ),
  ];

  @override
  void initState() {
    super.initState();

    Provider.of<NotificationHandler>(context, listen: false)
        .addListener(_onNotification);
  }

  void _onNotification() {
    // Deep-copy the existing list of contacts.
    List<Contact> newContacts = List.from(contacts);

    // This is another way to access the ChangeNotifier.
    // This is synatically idential to
    // Provider.of<NotificationHandler>(context, listen: false)
    final userId = context.read<NotificationHandler>().userId;

    // Check to see which member in the list this email is
    // identical to. This is the one who will get the alert
    // badge
    for (var contact in contacts) {
      if (contact.userId == userId) {
        contact.isAlert = true;
      }
    }

    setState(() {
      // we are assigning a new value to contacts,
      // which will trigger a rebuild of the widget tree.
      // Just modifying the list will not trigger a rebuild.
      contacts = List.from(newContacts);
    });
  }

  void _simulateNotification() {
    /// access the notification handler singleton
    NotificationHandler().setUnreadForUserId('bob@gmail.com');
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('Contacts'),
      ),
      body: Center(
        child: ListView.builder(
          itemCount: contacts.length,
          itemBuilder: (context, index) {
            Contact contact = contacts[index];
            return ContactListCard(contact);
          },
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _simulateNotification,
        tooltip: 'Increment',
        child: const Icon(Icons.add),
      ),
    );
  }
}

/// A simple contact with name and email, as well as a boolean
/// to indicate if there is an unread alert for this user
class Contact {
  final String name;
  final String userId;
  bool isAlert = false;

  Contact({required this.name, required this.userId});
}

class ContactListCard extends StatelessWidget {
  final Contact contact;

  const ContactListCard(this.contact, {super.key});

  @override
  Widget build(BuildContext context) {
    return Container(
      color: Colors.white,
      child: ListTile(
        trailing: Stack(
          children: [
            const Icon(Icons.add_ic_call),

            // only if this contact has an alert, display the alert indicator
            if (contact.isAlert) const AlertIndicator(),
          ],
        ),
        title: Text(contact.name),
        subtitle: Text(contact.userId),
      ),
    );
  }
}

/// A simple alert indicator that will be displayed
class AlertIndicator extends StatelessWidget {
  const AlertIndicator({
    Key? key,
  }) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Positioned(
      right: 0,
      child: Container(
        padding: const EdgeInsets.all(1),
        decoration: const BoxDecoration(
          color: Colors.red,
          borderRadius: BorderRadius.all(Radius.circular(8)),
        ),
        constraints: const BoxConstraints(
          minWidth: 16,
          minHeight: 16,
        ),
        child: const Text(
          '1',
          style: TextStyle(
            color: Colors.white,
            fontSize: 8,
          ),
          textAlign: TextAlign.center,
        ),
      ),
    );
  }
}

/// A simple notification handler, but modified
/// to be a singleton. You can access the singleton
/// to set the message and email outside of the widget tree.
class NotificationHandler extends ChangeNotifier {
  static final NotificationHandler _notificationHandler =
      NotificationHandler._internal();

  factory NotificationHandler() {
    return _notificationHandler;
  }

  NotificationHandler._internal();
  // the above code creates this as a singleton, when accessing
  // like a standard class, eg, NotificationHandler()

  String _userId = '';
  String get userId => _userId;

  void setUnreadForUserId(String userId) {
    _userId = userId;
    notifyListeners(); // important, to trigger listeners
  }
}