[Flutter] `openStore()` is not working across multiple FlutterEngines

ℹ️ Quick links:

• There is an explanation
• There is a workaround

(Edit by ObjectBox)

I have a use case where I have a Flutter app that does two things.

• a void main(){} entrypoint that runs a regular Flutter app (i.e. runApp(MaterialApp()))
  This app uses the database normally. It also schedules tasks using the workmanager plugin
• the workmanager plugin that executes the tasks that the app schedules

The problem

The void main(){} entrypoint is executed from the default FlutterEngine (created by the Activity/AppDelegate)
The workmanager plugin creates a new FlutterEngine for each task it needs to run. This is because the DartExecutor from the original FlutterEngine is executing the void main(){} entrypoint. A DartExecutor can only run one entrypoint at a time.

Because there are two FlutterEngines (each with their own Isolate pool and such), the Dart code is not in sync between the Engines. That is, one engine might have a Store _myStore that is not null, but the other engine still has one that is null (because it does not have the same memory pool allocated).

This results in the following code failing on the FlutterEngine that didn't open the store:

class MyDbService {
  static Store _myStore;

  Future<void> init() async {
   // The store is null in the other entrypoint, since that is an entire new block of memory, owned by another engine.
   // This results in the error
   // Bad state: failed to create store: 10001 Cannot open store: another store is still open using the same path 
   _store ??= await openStore();
  }
}

Describe the solution you'd like

I'd like to be able to use a Store across different FlutterEngines.
If openStore() would return a new connection, regardless of the FlutterEngine, that would be sufficient I think. (I.e. using a connection pool)
I'd expect the ObjectBox API to work as-is through this connection. I.e. CRUD / observable queries should work on this new connection.

Then I could do something like this to fix my problem:

main.dart

void main() async {
   await DbService().initialize(); // Open the database service in the first FlutterEngine

  runApp(MyApp()); // Does call to `WorkManager.registerTask()` which invokes the task runner function
}

my_app.dart

class MyApp extends StatefulWidget {
 // ...
}

class _MyAppState extends State<MyApp> {
 @override
 Widget build(BuildContext context){
    return Scaffold(
      appBar: AppBar(title: Text('My app')),
      body: Center(
        child: ElevatedButton(
           child: Text('Schedule task'),
           onPressed: (){
             WorkManager().registerTask('MyTask', {'input': 'foo'});
           }
        ),  
      ),
    );
  }


  @override
  void dispose() async {
   await DbService().close(); //Close the database service in the first FlutterEngine
   super.dispose();
  }
}

task_runner.dart

@pragma("vm:entry-point")
void _taskRunner(){
   WidgetsFlutterBinding.ensureInitialized();

    Workmanager().executeTask((taskName, inputData) async {
      await DbService().initialize(); // open a new connection in this FlutterEngine

      switch(taskName){
        case 'MyTask':
          // do work
          break;
      }

      await DbService().close(); // open the connection that was closed in this FlutterEngine
    }
}

Additional note

This could also benefit the add-to-app use case where people use a FlutterEngine per view they embed into an existing native app.