```override suspend fun requestBooks Flow lt List lt Book gt kotlinlang #coroutines

override suspend fun requestBooks(): Flow<List<Book>> {
  return callbackFlow {
    val listener =
        FirebaseFirestore.getInstance().collection("books").addSnapshotListener { value, error
          ->
          trySend(value!!.toObjects())
        }
    awaitClose { listener.remove() }
  }
}

How do I move the

trySend(value!!.toObjects())

to a background thread. My issue is that

value!!.toObjects()

is a deserialization operation that can take a long time. Wrapping

trySend

in a launch with Dispatchers.IO seems to help my lag issue in my app, but is it really that easy? Or am i shooting myself in the foot.

launch(<http://Dispatchers.IO|Dispatchers.IO>) { 
  trySend(value!!.toObjects())
}

ephemient

05/11/2022, 1:40 AM

that has the potential of being reordered - two callbacks may result in sends in the opposite order

if that's tolerable then great, if it's not then you need a queue, which Channel itself effectively is (with appropriate buffering)

Colton Idle

05/11/2022, 1:42 AM

I believe it's tolerable yes, but just out of curiosity... I can use a queue here? Is that a just kotlin data structure, or is queue + channel some kind of flowable primitive?

ephemient

05/11/2022, 1:43 AM

I mean that anything

blockingSend

to a

channel

, and then collected from the

channel.collectAsFlow().buffer()

, is effectively queued

as long as the buffer isn't full (or is unlimited), you can get that with

trySend().onFailure { throw }

oh right, if you wanted to move

toObjects

to another thread though…

I believe

callbackFlow {
    addListener { value, error ->
        trySend(value!!).onFailure { throw it }
    }
}.map { value.toObjects() }.flowOn(<http://Dispatchers.IO|Dispatchers.IO>).buffer()

would do that

Colton Idle

05/11/2022, 1:58 AM

hm. i'll play around with variations of that since that doesn't seem to compile.

ephemient

05/11/2022, 11:01 AM

I had to move and adjust the buffer but this should work: https://pl.kotl.in/Bx4HXwYWb

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