QueryPublisher unsubscribes after one event

[Feuerwerk]

Is there an existing issue?

• I have searched existing issues

Build info

• ObjectBox version: 4.0.0

Steps to reproduce

let query: Query<Person> = ......
query.publisher
                    .replaceError(with: [])
                    /*.assign(to: \.persons, on: viewModel)*/
                    .sink(receiveValue: { persons in
                        viewModel.persons = persons
                    })
                    .store(in: &cancellables)

Expected behavior

I expected that viewModel.persons will update continuously when I add / change a record that will match my query.

Actual behavior

After the initial query result is consumed by the Subscriber, QueryPublisher will unsubscribe because various Subscriber implementations like sink(), assign(to:) or onReceive() of a SwiftUI view will return Subscriber.Demand.none which causes QueryPublisher to signal completion to the Subscriber and shut down.

[kemkriszt]

I'm also curious of how to handle this, I ended up creating a custom subscriber that after every receive requests unlimited but it seems like an ugly hack to me

[greenrobot-team]

Thanks for reporting. The current publisher implementation indeed does send the finished event once it receives a none demand (so demand for 0) from the subscriber.

I guess the underlying issue is that it uses Query.subscribe() behind the scenes which does not have a concept for demand. We should probably have a look at improving this, or maybe someone has an idea for an improvement?

And so I think the current workaround is to use the subscribe APIs (like in the example) instead.

(There is also an internal issue to improve SwiftUI support at objectbox/objectbox-swift#134.)