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androidrx-javarx-kotlin

Single concatArrayEager doesnt subscribe to all sources

I am writing some repository logic for android application, I have this logic for retrieving data from database and updating the database with data from api immediately after that.

   fun fetchData(): Single<Data> {
    return Single.concatArrayEager(
            fetchFromDb(),
            fetchFromApi())
            .firstOrError()
}

By the way - database is Room and api is Retrofit. Now what happens is that room emits almost instantly, but for some reason api request is not fired - the second Single doesn't start.

I read the documentation on the eager operator and I am not sure if this is correct behavior. I even tried to delay the database fetch, by like 20 miliseconds - which resulted in that second single being actually fired

Solution

  • The concatArrayEager should subscribe to all Observables. But your API call might take some time to actually start. This might be more time than your DB call needs to return.

    Now we see the effect of firstOrError. When the first value is received, the stream is terminated and all active or pending subscriptions of the concatenation are canceled. Also the API result won't ever be used.