Emit coroutine Flow from Room while backfilling via network request

user_1947601 0
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I have my architecture like so:

  1. Dao methods returning Flow<T>: 

    @Query("SELECT * FROM table WHERE id = :id")
fun itemById(id: Int): Flow<Item>

  2. Repository layer returning items from DB but also backfilling from network:

    (* Need help here -- this is not working as intended **)

    fun items(): Flow<Item> = flow {
    // Immediately emit values from DB
    emitAll(itemDao.itemById(1))

    // Backfill DB via network request without blocking coroutine
    itemApi.makeRequest()
        .also { insert(it) }
}

  3. ViewModel layer taking the flow, applying any transformations, and converting it into a LiveData using .asLiveData():

    fun observeItem(): LiveData<Item> = itemRepository.getItemFlow()
    .map { // apply transformation to view model }
    .asLiveData()

  4. Fragment observing LiveData emissions and updating UI: 

    viewModel.item().observeNotNull(viewLifecycleOwner) {
    renderUI(it)
}

The issue I'm having is at step 2. I can't seem to figure out a way to structure the logic so that I can emit the items from Flow immediately, but also perform the network fetch without waiting.

Since the fetch from network logic is in the same suspend function it'll wait for the network request to finish before emitting the results downstream. But I just want to fire that request independently since I'm not interested in waiting for a result (when it comes back, it'll update Room and I'll get the results naturally).

Any thoughts?

EDIT

Marko's solution works well for me, but I did attempt a similar approach like so:

suspend fun items(): Flow<List<Cryptocurrency>> = coroutineScope {
    launch {
        itemApi.makeRequest().also { insert(it) }
    }

    itemDao.itemById(1)
}
android-room kotlin-coroutines android
1 Answers

It sounds like you're describing a background task that you want to launch. For that you need access to your coroutine scope, so items() should be an extension function on CoroutineScope:

fun CoroutineScope.items(): Flow<Item> {
    launch {
        itemApi.makeRequest().also { insert(it) }
    }
    return flow {
        emitAll(itemDao.itemById(1))
    }
}

On the other hand, if you'd like to start a remote fetch whose result will also become a part of the response, you can do it as follows:

fun items(): Flow<Item> = flow {
    coroutineScope {
        val lateItem = async { itemApi.makeRequest().also { insert(it) } }
        emitAll(itemDao.itemById(1))
        emit(lateItem.await())
    }
}
user_11038720
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