Unity Message not coming to Flutter app

[6fried]

Is there an existing issue for this?

• I have searched the existing issues

Unity version

• I am using Unity 2022.3 LTS

Common issues

• I have checked the common issues

Flutter version

3.13.9

Description

When sending messages from flutter, everything's doing well. But when it comes to send a message from Unity, my Flutter app doesn't receive the message. I've created a new project and tried the configuratino again and everything worked perfectly. But when I go back to my existing app, I have the same problème. I'm sure it is a problem with my project but actually, I can't figure out what exactly is the proble.

Minimum reproducible example (MRE)

I've created an empty Unity project, only purposed to test the mesaging feature

Unity project

C# code

Flutter code

What platforms are you seeing the problem on?

Android

Devices

No response

Anything else?

No response

[jamesncl]

Hi! This sounds like a Unity support question rather than an issue with this package. But here's a few things I can suggest you try to help debug the problem:

• Are you sure OnSceneLoaded is being called? (Try adding a Console.Log and check in Unity editor that it gets called)
• Are you sure that SceneLoadMessenger is attached to a Game Object in the scene?
• In Build Settings, is this scene the first scene in the list of Active Scenes?
• It might be possible that OnSceneLoaded is being called before Flutter is ready to receive it somehow - what happens if you add a delay before using SendToFlutter?
• Have you tried sending a message on Awake or on Start?

[6fried]

Hello !

I've tried every steps suggested, but the problem is still there. And as mentionned earlier, I used the same project with the same configuration for an empty flutter project and everything worked fine. I'm then sure that the problem is in my app. As it's an advanced app, I can't figure out what could have caused the problem. That's why I'm asking for hints about what could have caused the problem.

[jamesncl]

The only other thing I can think of is to make sure you only have one EmbedUnity widget in your Flutter app, because by default messages will only go to the last EmbedUnity widget in the stack - see using more than one instance of EmbedUnity.

Other than that, it's difficult to know what to suggest. Here's some pointers to learn more about how the message passing works from Unity to Flutter. It varies depending on if you are using Android or iOS:

• SendToFlutter.cs is where the message sending starts
• For iOS, it then goes to SendToFlutter.swift
• For Android, it goes to SendToFlutter.kt
• In both cases, Android and iOS send the message via Flutter's Platform channels system to UnityMessageListeners.dart, which creates a platform channel listener for every EmbedUnity widget

You could fork the package and add some of your own logging statements, or consult the wiki on how to run the debugger against the examples (except you could replace the example Unity project with your own)

I'll close this issue as it doesn't seem to be an issue with the package but feel free to comment further

[zilijonas]

@jamesncl I am having the same issue.

Everything works fine on iOS, all messages received from Unity without any issues.
But on Android, I don't get any messages from Unity. It feels like SendToFlutter.Send fails silently.

Everything is installed whilst following instructions.

Sadly, I cannot fork my project.

I also have installed "firebase_messaging" and "flutter_local_notifications". I guess that the issue might be with the event channels.

Here's what AI suggests after extensive hours spent on debugging:

	1.	flutter_embed_unity_android keeps its MethodChannel in a static field.
That means one channel instance per JVM process, not per FlutterEngine.
	2.	Firebase Messaging starts a head-less FlutterEngine on Android.
As soon as FirebaseMessagingPlugin initialises, it spins up an engine
called something like FlutterFirebaseMessagingBackgroundExecutor.
When that engine attaches, all plugins (including FlutterEmbedUnityAndroidPlugin) are also attached to it.
	3.	Whichever engine attaches last overwrites the static channel.
After Firebase’s engine is alive, the static channel variable now points at
the background engine’s BinaryMessenger.
SendToFlutter.send() dutifully calls channel.invokeMethod( … ), but the
message goes to an isolate that has no Dart code listening for it — so your
UI never sees it.

That’s why:
	•	✅ Works on iOS (no extra engine).
	•	❌ Disappears on Android the moment Firebase is included.

[timbotimbo]

Yeah, this is probably onAttachedToEngine getting called multiple times, because some of these messaging plugins spawn additional Flutter engines for background processes.

This plugin doesn't expect that to happen and the message from unity likely ends up in a different Flutter engine than expected.

Nothing a plugin user can do to fix it, the plugin would need to be modified.

There is an issue in the Flutter repo that mentions that this needs documentation.
flutter/flutter#150855.

This article (not sure how reliable) mentions some possible workarounds.