🐛 [firebase_database] OnDisconnect not called upon internet connection lost

Tom3652 commented 2 years ago

Bug report

Describe the bug

I am not sure that this is a bug, but the OnDisconnect is never called in my case when :

I loose internet connection
I kill my app

I am building an online presence system and this feature is highly required in my case.

Expected behavior

I have followed this documentation and setup a onDisconnect() method on a specific ref.

I expect that when loosing connection to the database (by crashes, no internet, etc..), the FirebaseDatabase.instance.ref("any/ref").onDisconnect().set(updates) is triggered.

Also, if I may ask, how I am suppose to "listen" the "regain" of connectivity to do the opposite query of OnDisconnect ? Is there a onConnect() ?

Additional context

Maybe i simply don't understand how to use the OnDisconnect or didn't understand well the documentation ( i don't understand why there is a await actually.

But in my initState() method of a specific widget that is alive, i simply call the above method (FirebaseDatabase.instance.ref("any/ref").onDisconnect().set(updates)) once and assume that it will be called at the right moment.

Flutter doctor

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``` Doctor summary (to see all details, run flutter doctor -v): [✓] Flutter (Channel stable, 2.8.1, on macOS 12.1 21C52 darwin-x64, locale fr-FR) [✓] Android toolchain - develop for Android devices (Android SDK version 31.0.0) [✓] Xcode - develop for iOS and macOS (Xcode 13.2.1) [✓] Chrome - develop for the web [✓] Android Studio (version 2020.3) [✓] VS Code (version 1.63.2) [✓] Connected device (3 available) • No issues found! ```

Flutter dependencies

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``` Dart SDK 2.15.1 Flutter SDK 2.8.1 myappname 1.0.0+1 dependencies: - app_settings 4.1.1 [flutter] - app_tracking_transparency 2.0.2+1 [flutter] - apple_maps_flutter 1.0.1 [flutter] - auto_size_text 3.0.0 [flutter] - badges 2.0.2 [flutter] - cached_network_image 3.2.0 [flutter flutter_cache_manager octo_image cached_network_image_platform_interface cached_network_image_web] - camera 0.9.4+5 [camera_platform_interface camera_web flutter pedantic quiver flutter_plugin_android_lifecycle] - clipboard 0.1.3 [flutter] - cloud_functions 3.2.4 [cloud_functions_platform_interface cloud_functions_web firebase_core firebase_core_platform_interface flutter] - connectivity_plus 2.1.0 [flutter connectivity_plus_platform_interface connectivity_plus_linux connectivity_plus_macos connectivity_plus_web connectivity_plus_windows] - country_code_picker 2.0.2 [flutter modal_bottom_sheet collection universal_platform] - crypto 3.0.1 [collection typed_data] - cupertino_back_gesture 0.1.0 [flutter] - cupertino_icons 1.0.4 - 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darshankawar commented 2 years ago

@Tom3652 Although I haven't used onDisconnect() yet, but check this article and see if it helps to understand the method and it's usage a bit better.

Tom3652 commented 2 years ago

Thanks for the nice article especially on the Cloud Function part. However, this is a very similar logic that i have already implemented and the onDisconnect is never called in my case :/

I have tried using update() and set() to see if one of these methods would work but both were not triggered.

darshankawar commented 2 years ago

@Tom3652 Can you provide us a minimal code sample that we can directly use along with expected and actual behavior you are seeing ?

Tom3652 commented 2 years ago

Hi @darshankawar sorry i have not provided it before. Here is the code sample :

import 'package:firebase_core/firebase_core.dart';
import 'package:firebase_database/firebase_database.dart';
import 'package:flutter/material.dart';

Future<void> main() async {
  WidgetsFlutterBinding.ensureInitialized();
  await Firebase.initializeApp();
  runApp(const MyApp());
}

class MyApp extends StatefulWidget {
  const MyApp({Key? key}) : super(key: key);

  @override
  State<MyApp> createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {

  void _registerOnDisconnect() {
    FirebaseDatabase.instance.ref().child("test").onDisconnect().update({"online": false});
  }

  void _saveOnlineTrue() {
    FirebaseDatabase.instance.ref().child("test").update({"online": true});
  }

  @override
  void initState() {
    super.initState();
    _saveOnlineTrue();
    _registerOnDisconnect();
  }

  @override
  Widget build(BuildContext context) {
    return const MaterialApp(
      home: Scaffold()
    );
  }
}

What i have noticed :

The onDisconnect is indeed called when i kill my app with an active internet connection.
The onDisconnect is not called when loosing internet connection.

Maybe there are more cases in which this method should be called, but these 2 are the most important and useful in my app.

Step to reproduce :

Run the sample code
Make sure your phone is only WIFI-connected (not 4G)
Turn off WIFI
See that the online: true remains true in the Firebase Console.

Correct me if i am wrong, but it should call the onDisconnect upon losing internet connection (thus to the Database) right ?

darshankawar commented 2 years ago

it should call the onDisconnect upon losing internet connection (thus to the Database) right ?

Ideally yes, I mean if you lose internet connection, onDisconnect() should be called. I am not sure if there if the method takes a bit time to comeback or get triggered since it need to communicate with the server side and hence you may be seeing online: true still.

Keeping it open for further insvestigation.

russellwheatley commented 2 years ago

Hey @Tom3652, if you wait a couple of minutes it does actually make the update. Not much we can do about the delay, it's up to the Firebase server to make the change once the connection is lost. Might be worth contacting Firebase support to make them aware of the delay.

Tom3652 commented 2 years ago

Hi @russellwheatley, thanks for the explanation ! I have made a ticket to Firebase support explaining this, i will post the answer here once i have it.

Tom3652 commented 2 years ago

So you were right @russellwheatley here is the answer from Firebase support :

The onDisconnect() function is called when the client disconnects (we can use some examples such as Inactivity, or closing the app manually). In that case the client will send a signal to the server that it is disconnecting and the server will immediately execute the onDisconnect callback. When you switch off wifi, your app doesn't get a chance to tell the server that it is disconnecting. In that case the onDisconnect() callback will fire once the server detects that the client is gone. This may take a few minutes, so in that case you just have to be a bit more patient.

I have also asked how can i do then to set a user offline when he is not connected to the database, so that users don't see him online when he is not ?

But if you have an answer i would be extremely thankful.

But OnDisconnect works as expected :)

russellwheatley commented 2 years ago

I'm glad you got an answer, @Tom3652. I don't know how you would set it up using Firebase to be honest. I read that the OnDisconnect function can take up to 10 minutes to execute on the Firebase server.

Either way, you'll probably need a server to track connections. But that also sounds like a lot of work.

In the past, I've used socketsio Node server for web projects for chat apps which will also indicate with minimum latency when a client goes offline. A quick scan reveals iOS & android client setup. But then you'll probably have a problem with scale.

There might be a much easier and simple solution so take the above with a pinch of salt.

Tom3652 commented 2 years ago

Thanks for your comments and suggestion ! I will take some time to find a way good way that scales :)

firebase / flutterfire