[firebase_messaging] Android: Memory Leak

[ened]

Currently, the background handling in FlutterFirebaseMessagingService is leaking the backgroundChannel property. This get's evident by activating LeakCanary and moving the App from FG to BG:

D  ┬
D  ├─ android.net.ConnectivityThread
D  │    Leaking: NO (PathClassLoader↓ is not leaking)
D  │    Thread name: 'ConnectivityThread'
D  │    GC Root: Thread object
D  │    ↓ thread ConnectivityThread.contextClassLoader
D  ├─ dalvik.system.PathClassLoader
D  │    Leaking: NO (Object[]↓ is not leaking and A ClassLoader is never leaking)
D  │    ↓ PathClassLoader.runtimeInternalObjects
D  ├─ java.lang.Object[]
D  │    Leaking: NO (FlutterFirebaseMessagingService↓ is not leaking)
D  │    ↓ array Object[].[1679]
D  ├─ io.flutter.plugins.firebasemessaging.FlutterFirebaseMessagingService
D  │    Leaking: NO (a class is never leaking)
D  │    ↓ static FlutterFirebaseMessagingService.backgroundChannel
D  │                                             ~~~~~~~~~~~~~~~~~
D  ├─ io.flutter.plugin.common.MethodChannel
D  │    Leaking: UNKNOWN
D  │    ↓ MethodChannel.messenger
D  │                    ~~~~~~~~~
D  ├─ io.flutter.view.FlutterNativeView
D  │    Leaking: UNKNOWN
D  │    ↓ FlutterNativeView.mPluginRegistry
D  │                        ~~~~~~~~~~~~~~~
D  ├─ io.flutter.app.FlutterPluginRegistry
D  │    Leaking: UNKNOWN
D  │    ↓ FlutterPluginRegistry.mActivity
D  │                            ~~~~~~~~~
D  ╰→ com.company.app.MainActivity
D  ​     Leaking: YES (Activity#mDestroyed is true and ObjectWatcher was watching this)
D  ​     key = 1fd3d04b-7f8d-4bbf-99ae-440d0fb8146c
D  ​     watchDurationMillis = 5307
D  ​     retainedDurationMillis = 306
D  , retainedHeapByteSize=160758)], libraryLeaks=[])

Expected behavior
No memory leaks.

Additional context
As @kroikie put it in the comments within service class, the isolate/instance stuff must not be static.

[ghivert]

I got regular memory leaks from Flutter on Android since I added Firebase Messaging. Is there any way to work around?

[ened]

@ghivert the plugin is currently being reworked and the memory issues will be addressed. I'm sure @Ehesp and @Salakar will come back to this issue.

[Salakar]

Hey would you be able to try out the reworked version and see if this is now sorted for you - currently a dev release (8.0.0-dev.5) - PR #4012 for reference, migration guide has also been updated: https://firebase.flutter.dev/docs/migration/

For discussion please see #4023