[firebase_messaging] Why background handler needs to be top-level func. or static method?

[comatory]

Why does this limitation exist?

1. Is it a pattern to force developers not to do expensive operations in the background?
2. Is it a technical decision?

I'm kind of having hard time understanding how to do anything useful within the handler beyond some simple app.
Lot of times Firebase setup will be happening within Flutter widget or some service class.

My project uses dependency injection so I can create (mostly) singletons for various services and provide dependencies for them. This DI happens within class AppContainer and it's where I control how various services are set up.
This AppContainer is instantiated outside of runApp because the services do not use Flutter, mostly it's services that deal with storing and retrieving information (Database & API fetching). Flutter is then notified from these services via exposed streams about any changes happening.
This AppContainer is provided to application via widget AppProvider that extends InheritedWidget. This way I can choose which services to expose to Flutter and retrieve them anywhere in the tree.

You can probably guess what my problem is. When all my logic for doing network calls and storing data is encapsulated like this (and I have a good reason to do it), I can't access it in top-level function or static method.

I'm not sure what other folks are doing. I can imagine that in some projects you might be doing writes to DB like this:

void backgroundHandler(RemoteMessage message) async {
  final data = message.data;
  final box = await Hive.openBox('myBox');
  box.put(data['id'], data['payload']);
 }

Which works OK I guess but for me, it's just not very scalable. I would basically need to at least access my AppContainer instance on which I'd expose service for writing to DB.

I think the only option that is left for me is to create factory constructor for AppContainer and then do something like this:

void backgroundHandler(RemoteMessage message) async {
  final data = message.data;
  // 😝
  await AppContainer()..database.writeData(data['id'], data['payload']);
 }

Which will probably work but I just hate it. It's kind of breaking my architecture where the AppContainer should not be used directly like this, now I have two places where it's accessed and it can lead to potential trouble.
My ideal scenario is to register FirebaseMessaging inside some class and provide any services to this class via dependency injection. But as I explained, it does not seem to be possible.

Thank you for any answers.

[designervoid]

Did you high-level in main only background handler function?

[comatory]

What do you mean by that?