Need some pointers using MoorIsolate

[frankvollebregt]

Hello, I'm trying to use isolates in my app to use the database, but something is going wrong.

For a bit of context, the app uses a Moor database, from which it streams several values into the front-end (watch with StreamBuilders). Now there's two issues. Firstly, the CircularProgressIndicator starts to lag heavily when Moor is busy clearing/inserting/updating the database (quite a bit of data). As a result, I want to use a separate isolate to perform these database operations. Secondly (at a later point) I want the app to be able to load data from the server in the background, and store it in the database. In the page about isolates in the documentation a lot of info is mentioned, but I somehow can't get it to work the way I want it to.

Both when loading from my main isolate or from another isolate I spawned using the FlutterIsolate package, when I execute the code below no errors/exceptions occur, but the database seems to be empty.

// function from the documentation
DatabaseConnection _backgroundConnection() {
  // construct the database. You can also wrap the VmDatabase in a "LazyDatabase" if you need to run
  // work before the database opens.
  final database = VmDatabase.memory();
  return DatabaseConnection.fromExecutor(database);
}

doThingsInIsolate() async {
    // create the isolate with the database
    MoorIsolate isolate = await MoorIsolate.spawn(_backgroundConnection);
    DatabaseConnection connection = await isolate.connect();
    final MyDatabase db = MyDatabase.connect(connection);
    print('database is $db');    // instance of 'MyDatabase'
    print('the items: ${await db.allItems().get()}');    // expected to return [Item<>], but returns []

    // the actual DB stuff I want to do goes here
    // intermediate prints happen fine, but the database remains untouched
    await doActualDatabaseInteractions();

    db.close();
    isolate.shutdownAll();
}

If there's anything clearly wrong, I would love to know. If there is some working example code I can take a look at, which shows how to work with these, that would also be very helpful.

[simolus3]

Are you inserting data in your database's migration? If you're using a VmDatabase.memory() you'll get a clean in-memory database for each MoorIsolate.spawn, it would not get persisted anywhere. So it's not entirely unexpected that allItems() would return an empty list after opening a fresh database. You can re-use the existing VmDatabase you were using before isolates in _backgroundConnection(). When using things like path_provider, you'll also need to call FlutterWidgetsBinding.ensureInitialized() in the background isolate to setup platform channels.

Apart from that, the code looks correct to me though.

[davidmartos96]

@simolus3 Is it a new thing that the isolates work with Platform channels? As far as I know that is still a work in progress in the Flutter team

[simolus3]

Oh good point actually. If I remember correctly path_provider worked on a background isolate, but the docs also mention the other approach of computing the right path on the main isolate and sending that.

[frankvollebregt]

It works fine for me right now. I'm setting up an isolate for Moor in my main.dart, and when I want to do networking to retrieve new data, I create a new isolate using the flutter_isolate package (which does allow me to use other packages), and I use the SendPort to connect to the database isolate.

As an aside, I currently pass this SendPort down the widget tree to share it, but is there a recommended way to do this? Would the provider package be a good approach here?

[simolus3]

I don't have any preferences here, using provider sounds reasonable. I'd probably wrap the port then, though:

class MoorConnectPort {
  final SendPort connect;
  MoorConnectPort(this.connect);
}

Because if you just do Provider.of<SendPort>(context) it's not clear what kind of send port we're talking about :)

[frankvollebregt]

Thank you very much for all the help!