The plugin stops recieveing messages from unity if a plugin "flutter_pushed_messaging" is added to the dependency.

[EvgenFree45]

As soon as I add the "flutter_pushed_messaging" plugin to the project dependencies and initialize it in the code, the "flutter_embed_unity" plugin stops receiving messages from Unity.

flutter_pushed_messaging is a plugin for receiving push messages. I tried to figure out on my own what the conflict might be, but all I could find out was that this plugin also uses the Platform Channel.

[jamesncl]

Thanks for the bug report. Not sure why this would be. Is this on Android, iOS or both?

[EvgenFree45]

Thanks for the bug report. Not sure why this would be. Is this on Android, iOS or both?

So far, we can only talk about Android. There was no testing on iOS.

[zilijonas]

Not sure if this is related, but I have "firebase_messaging" and "flutter_local_notifications" libs installed in my project, and my Android flutter app is not receiving any messages from Unity even though I've followed all instructions step by step. Everything works fine on iOS.

[timbotimbo]

Just linking this issue to this closed one where zilijonas added more useful info.

[timbotimbo]

@jamesncl
Here are some easy reproduction steps:

1. Download and run the example.
   Tapping on the Flutter logo increments the number in the text "has been touched x times".
   The onAttachedToEngine log occurs once in the debug console.
2. Add the following plugin to pubspec.yaml
   android_alarm_manager_plus: ^4.0.7
3. Modify main.dart to initialize this plugin.

import 'package:android_alarm_manager_plus/android_alarm_manager_plus.dart';

void main() async {
  WidgetsFlutterBinding.ensureInitialized();

  await AndroidAlarmManager.initialize(); // comment or uncomment this line to disable or enable the bug.
  runApp(const ExampleApp());
}

4. Now the plugin runs, but tapping the Flutter logo does NOT increment the number.
   The debug console contains 3 logs of onAttachedToEngine.

Just a quick look suggests several plugins that use background processes can trigger this:
android_alarm_manager_plus , firebase_messaging, flutter_pushed_messaging, flutter_local_notifications.
And likely more.

I used the alarm manager plugin above because reproduction requires hardly any setup.

[timbotimbo]

Here is a quick and dirty fix that shows that the bug is indeed onAttachedToEngine being called multiple times.
Change the methodchannel reference in SendToFlutter.java to a hashset and use add and remove in onAttachedToEngine and onDetachedFromEngine

Unity messages now get passed to all Flutter engines, which fixes the bug.

// SendToFlutter.java

import java.util.HashSet;

public class SendToFlutter {
   public static final HashSet<MethodChannel> methodChannels = new HashSet<>();

   // IMPORTANT: Do not remove @Keep annotations, they tell the minifier not to remove or rename
   // these elements during compilation so that refection from Unity C# does not break on release build
   @Keep
   public static void sendToFlutter(String data) {
      new Handler(Looper.getMainLooper()).post(() -> {
         for (MethodChannel methodChannel : methodChannels) {
            if(methodChannel != null) {
               methodChannel.invokeMethod(methodNameSendToFlutter, data);
            }
            else {
               Log.e(logTag, "Couldn't send message from Android to Flutter: method channel not registered");
            }
         }
      });
   }
}

// FlutterEmbedUnityAndroidPlugin.kt

override fun onAttachedToEngine(flutterPluginBinding: FlutterPlugin.FlutterPluginBinding) {
       ...
        SendToFlutter.methodChannels.add(channel);
}

override fun onDetachedFromEngine(binding: FlutterPlugin.FlutterPluginBinding) {
     ...
    SendToFlutter.methodChannels.remove(channel);
}

A better fix would be to send the message only to the right Flutter engine instead of all.
But you would have to find a reliable way to identify it, even in edge cases like the app being resurrected from an alarm or push notification.

[zilijonas]

Here is a quick and dirty fix that shows that the bug is indeed onAttachedToEngine being called multiple times. Change the methodchannel reference in SendToFlutter.java to a hashset and use add and remove in onAttachedToEngine and onDetachedFromEngine

Unity messages now get passed to all Flutter engines, which fixes the bug.

// SendToFlutter.java

import java.util.HashSet;

public class SendToFlutter {
public static final HashSet methodChannels = new HashSet<>();

// IMPORTANT: Do not remove @keep annotations, they tell the minifier not to remove or rename
// these elements during compilation so that refection from Unity C# does not break on release build
@keep
public static void sendToFlutter(String data) {
new Handler(Looper.getMainLooper()).post(() -> {
for (MethodChannel methodChannel : methodChannels) {
if(methodChannel != null) {
methodChannel.invokeMethod(methodNameSendToFlutter, data);
}
else {
Log.e(logTag, "Couldn't send message from Android to Flutter: method channel not registered");
}
}
});
}
}
// FlutterEmbedUnityAndroidPlugin.kt

override fun onAttachedToEngine(flutterPluginBinding: FlutterPlugin.FlutterPluginBinding) {
...
SendToFlutter.methodChannels.add(channel);
}

override fun onDetachedFromEngine(binding: FlutterPlugin.FlutterPluginBinding) {
...
SendToFlutter.methodChannels.remove(channel);
}
A better fix would be to send the message only to the right Flutter engine instead of all. But you would have to find a reliably way to identify it, even in edge cases like the app being resurrected from an alarm or push notification.

Dirty fix works! Will create a local plugin from this until it gets fixed. Thank you!

[timbotimbo]

I submitted a different fix in #46.

Should be fewer redundant messages being passed compared to the dirty fix.