Empty combine <Iterable<Flow<T>>> not emitting anything

I just debugged an issue where I did not get any events from the flows combine function.

I had a code where I combined the latest values of a List<Observable<Recipe>> into an Observable<List<Recipe>> by using combineLatest.

I refactored that code to use flows’'s combine to convert a List<Flow<Recipe>> into a Flow<List<Recipe>>. However this never emitted an item if the data sources were empty.

I expect that the function:

@ExperimentalCoroutinesApi
public inline fun <reified T, R> combine(
    flows: Iterable<Flow<T>>,
    crossinline transform: suspend (Array<T>) -> R
): Flow<R> {
    val flowArray = flows.toList().toTypedArray()
    return flow {
        combineInternal(
            flowArray,
            arrayFactory = { arrayOfNulls(flowArray.size) },
            transform = { emit(transform(it)) })
    }
}

gives me an empty array in the transform method but it just hangs forever. (coroutines 1.3.2)

[psteiger]

@mtrakal consider this more expressive code:

val currencies = currencyDao.getCurrencies()
    .onEmpty { emit(emptyList()) }
    .single()

I do agree that empty flows should not be confused with a flow with a default ‘empty value’. Maybe this should be Room-configurable, but I think Flow behavior here should not change.

Edit: I’m not sure why your Room flow is not emitting an empty list. Room docs suggest that should be the behavior:

Keep nullability in mind when choosing a return type, as it affects how the query method handles empty tables:

When the return type is Flow<T>, querying an empty table throws a null pointer exception.

When the return type is Flow<T?>, querying an empty table emits a null value.

When the return type is Flow<List<T>>, querying an empty table emits an empty list.

https://developer.android.com/reference/androidx/room/Query

[niqueco]

It’s wrong to close this issue. I’ve just debugged my code and reached to this unexpected behavior (I was about to report this as a new bug). A “combine” that is passed an empty list of flows should emit once the current result status, which is an empty list. The current behavior will hit many users.