Query stream is not updated in UI isolate when changes made in another isolate

[Tumist76]

Hello! My app has a feature which sends queued data from database table in foreground service in separate isolate via flutter_foreground_task.

I'm using drift with NativeDatabase.createInBackground called on UI isolate. So the app can have 3 isolates at some moment:

• UI isolate
• Drift isolate
• Foreground service isolate

The foreground service isolate is started when some data was queued to send on ui isolate. I'm using ports to establish connection to Drift isolate in foreground service isolate like this:

UI isolate registers port:

  static const driftIsolatePortName = 'drift-isolate-port';
  static Future<void> startService(AppDBService database) async {
    final connection = await database.serializableConnection();
    IsolateNameServer.removePortNameMapping(driftIsolatePortName);
    IsolateNameServer.registerPortWithName(
      connection.connectPort,
      driftIsolatePortName,
    );
  }

Then I'm starting foreground service isolate and then I'm creating database attached to Drift isolate like this:

Future<ScoutDBService> _initDatabase() async {
  final sendPort = IsolateNameServer.lookupPortByName(
    BackgroundQueueServiceManager.driftIsolatePortName,
  )!;
  final driftIsolate = DriftIsolate.fromConnectPort(sendPort);
  final dbConnection = await driftIsolate.connect();
  final database = LazyDatabase(() => dbConnection);
  return AppDBService(database: database);
}

This isolate send data from database and deletes sent entries. It's all fine, but I want to update data in UI from database. I've subscribed to changes on UI thread, but the stream sends only one event when I subscribe and then nothing:

  void init() {
    _requestsSubscription =
        _requestDao.getAllRequestsWithTasksStream().listen(_updateRequestsList);
  }

It works fine if send data on UI isolate, but with current implementation it doesn't update the stream. As I see in documentation, when I use same drift isolate on both my isolates, query streams should work correctly when data is updated from either isolate, but that's not the case here.

Have I done something wrong?

[simolus3]

The LazyDatabase wrapper on the foreground service isolate stands out. In drift, the stream query store responsible for distributing table updates is part of the DatabaseConnection class (which essentially wraps the QueryExecutor for running statements and the StreamQueryStore).

DriftIsolate.connect() gives you a DatabaseConnection, but using a LazyDatabase (which is only a standalone QueryExecutor) will essentially set up a local stream query store instead of using the one from the drift isolate.
In your case, the LazyDatabase wrapper isn't necessary at all since _initDatabase() is already async, just pass the dbConnection to the AppDBService constructor directly.

If you need _initDatabase to return an instance synchronously, use DatabaseConnection.delayed instead:

ScoutDBService _initDatabase() {
  final sendPort = IsolateNameServer.lookupPortByName(
    BackgroundQueueServiceManager.driftIsolatePortName,
  )!;
  return AppDBService(database: DatabaseConnection.delayed(Future(() async {
    final driftIsolate = DriftIsolate.fromConnectPort(sendPort);
    return await driftIsolate.connect();
  })));
}

[Tumist76]

Yes, that worked, thank you! Should this nuance be reflected on this doc's page?

[simolus3]

Good point, I'll mention it explicitly.