Testing with the PersistentContainer Dependencies Addition

[Nomad00]

Greetings all 👋

I've run into an issue with an unimplemented dependancy and I am unsure if I'm using it wrong or if something needs to be tweaked with PersistentContainer's testValue.

Given this Reducer:

struct ItemListFeature: Reducer {
    @Dependency(\.persistentContainer)
    var persistentContainer
    
    struct State: Equatable {
        var items: Item.FetchedResults = .empty
    }
    
    enum Action: Equatable { }
    
    func reduce(into state: inout State, action: Action) -> Effect<Action> {
        .none
    }
}

I created the following test:

@MainActor
class ItemListFeatureTests: XCTestCase {
   func testBasics() async {
        let store = TestStore(
            initialState: ItemListFeature.State(),
            reducer: ItemListFeature())
        
        store.dependencies.persistentContainer = .default(
            inMemory: true
        )
    }
}

I get the following error:

testBasics(): Unimplemented: @dependency(.persistentContainer.newBackgroundContext) …
Defined at:
PersistentContainerDependency/PersistentContainer.swift:28

I don't expect to have to override newBackgroundContext. If I am, it is unclear to me how to properly override it myself. Is anyone able to set me straight here? Thanks!

[tgrapperon]

Hey @Nomad00!
I just tried your code in a standalone project, and I'm not able to reproduce the issue.
It is maybe possible that your host application is hitting this dependency while testing. This kind of issue can also happen if you're statically linking Dependencies in your test target.
Have you checked this section of the documentation?
In any case, you shouldn't have to implement individual endpoints, .default(inMemory:) is a fully fledged persistent store.

[Nomad00]

That you so much for the response @tgrapperon ! I'm pretty sure I have the linking right, but that is exactly the type of problem I was expecting. I'm going to make a fresh project and see what happens.

[Nomad00]

And did not happen in a fresh project. Thanks again @tgrapperon , I must have something funky with my host application. Thanks for listening / reading!

[tgrapperon]

One thing you can try is using the trailing closure to set your dependencies up in TestStore:

let store = TestStore(
  initialState: ItemListFeature.State(),
  reducer: ItemListFeature()) {
  $0.persistentContainer = .default(inMemory: true)
}

This is slightly different from setting it later because the dependency will be set when you create your state (but it would be rather odd to touch a newBackgroundContext there).
Please let us know your findings, it is also possible that something can be improved in this area.

[Nomad00]

Hey @tgrapperon ! Apologies that I'm only getting to this sporadically, but I started a repo where I'm hoping to use PersistentContainer with TCA and have test coverage of basic operations. If you'd like to take a look, feel free https://github.com/Nomad00/TestingPersistentContainer.