TestStore Changing Environment Variable

[natemann]

I have an effect that attempts to download data every 60 seconds

public struct Environment {
        var backgroundQueue: AnySchedulerOf<DispatchQueue>
        var download: () -> AnyPublisher<Data, APIError>
        var logError: (Error) -> Effect<Never, Never>
        var now: () -> Date
        var runLoop: AnySchedulerOf<RunLoop>
        var userDefaults: UserDefaults
}

case .start:
Effect.timer(id: CancelTimer(), every: .seconds(60), on: environment.runLoop)
                .prepend(.init(environment.now()))
                .assertNoFailure()
                .flatMap(maxPublishers: .max(1), { _ -> Effect<Result<Data, APIError>, Never> in
                    environment.download()
                        .handleEvents(receiveOutput: { _ in print("Downloading data") })
                        .catchToEffect()
                })
                .map(Action.save)
                .eraseToEffect()
                .cancellable(id: CancelDownload())

because the download can take some time, I limit the download to 1 in progress with the flatMap(maxPublishers. Everything works fine in production, but I am trying to test that when a download error is received, and then 60 seconds later, data is received, everything continues to work. Here is my test

func test_download_failure_then_success() {
        let runLoop = RunLoop.test
        var errors: [Error] = []
        let store = TestStore(
            initialState: Downloader.State(),
            reducer: Downloader.reducer,
            environment: Downloader.Environment(
                backgroundQueue: DispatchQueue.test.eraseToAnyScheduler(),
                download: { Fail(error: APIError.invalidBody).eraseToAnyPublisher() },
                logError: { error in .fireAndForget { errors.append(error) } },
                now: Date.init,
                runLoop: runLoop.eraseToAnyScheduler(),
                userDefaults: UserDefaults.mock()))

        store.assert(
            .send(.start),
            .receive(.save(.failure(.invalidBody))),
            .environment {
                $0.download = { Just(Data()).mapError(APIError.unknown).eraseToAnyPublisher() }
            },

            .do { runLoop.advance(by: .seconds(60)) },
            .receive(.save(.success(.init()))),
            .send(.cancel)
        )

The environment.download success is never called. I imagine this has something to do with the maxPublishers, but I cannot figure it out. Any help would be appreciated.

[mbrandonw]

I think I see what is going on here, and it's tricky.

The change you make to the environment to replace the $0.download endpoint doesn't actually update the environment in the flatMap. The flatMap will capture the environment at the time of sending the .start action, and will never change after that. In fact, that kinda makes using environment inside .flatMaps a little dangerous, in particular when flat mapping from a long living effect like a timer. We should add some documentation/case studies for this behavior.

One thing you should be able to do to work around is use a PassthroughSubject for environment.download in your test and then send it values to emulate the download effect firing, rather than replacing the environment.

Wanna give that a shot?

There's another work around for this but it won't work for you because you specifically need the maxPublishers behavior. But the idea is to create a new action that can be sent from the effect in the .flatMap and then do your work inside that action. That extra little ping-pong with actions gives you back the ability to update the environment while the timer is running.

[natemann]

@mbrandonw thanks for the help. Using a PassThroughSubject worked after I changed download to Effect<Result<Data, APIError>, Never>