I'm using callbackFlow to adapt a callback from Fi...

# coroutines

[Ale]

I'm using callbackFlow to adapt a callback from Firestore into a flow, and then store the received data locally, as a "fire and forget" call. I can do this:

@ExperimentalCoroutinesApi
private fun retrieveFromFirestore(porteroId: String): Flow<Portero> = callbackFlow {
    val porteroCallback = EventListener<QuerySnapshot> { querySnapshot, e ->
        val element = //extract data from snapshot etc.
             launch { //save to local sqllite }
        offer(element)
    }
}

This works well.
However, when I tried to refactor the code a bit, and extract the snapshot manipulation to another method:

@ExperimentalCoroutinesApi
private fun retrieveFromFirestore(porteroId: String): Flow<Portero> = callbackFlow {
    val porteroCallback = EventListener<QuerySnapshot> { querySnapshot, e ->
        val element = buildData(querySnapshot, e)
        offer(element)
    }
}

private buildData(querySnapshot: QuerySnapshot?, e: FirebaseFirestoreException?): Element {
    //extract data from snapshot etc.
    launch { //save to local sqllite }  // <--- this doesn't compiles
    return element
}

The launch function is no longer available, with the error Unresolved reference.
I can fix it using CoroutineScope(Dispatchers.Default).launch { ... } but why is this happening? How come launch is available only in the calling function but not in the called one?

[Manuel Vivo]

You have to make it a suspend function, and in order to create new coroutines inside a suspend function, you need a scope (e.g. coroutineScope or supervisorScope depending on whether you want a Job or SupervisorJob as a parent)

[wasyl]

And more specifically to answer your question why launch is not accessible anymore: launch is an extension function on CoroutineScope. The `callbackFlow`’s parameter (the lambda) is defined as block: suspend ProducerScope<T>.() -> Unit, and ProducerScope implements `CoroutineScope`: interface ProducerScope<in E> : CoroutineScope.

So when calling launch inside callbackFlow lambda you simply have a CoroutineScope in scope. When you extract a function like this, you need to follow what Manuel said: make the extracted function `suspend`able and start a new scope using coroutineScope { } builder.

[Ale]

Ah, I see, the key part is that callbackFlow is a suspended block! Now it makes sense, thank you!