androidkotlinkotlin-coroutineskotlin-flow

flatMapLatest does not trigger firstOrNull with empty flow

I'm having a curious issue when dealing with flatMapLatest and emptyFlow as well as firstOrNull.

Scenario A:

val dataFlow: Flow<Boolean> = emptyFlow()

launch {
    val data = dataFlow.firstOrNull()  // returns data = null
}

Scenario B:

val rootFlow: MutableStateFlow<Boolean?> = MutableStateFlow(null)
val dataFlow: Flow<Boolean> = rootFlow.flatMapLatest {
    emptyFlow()
}

launch {
    val data = dataFlow.firstOrNull()  // blocks and does not return any value
}

So my question would be, why does firstOrNull not return any value in Scenario B. I'm seeing that flatMapLatest is being called with the initial null value of the mutable state flow.

Solution

  • rootFlow is a StateFlow which means it never ends.

    Your dataFlow is a transformation of rootFlow so it will not end until source flow ends.

    That means in this case dataFlow.firstOrNull() will suspend forever as it keeps transforming each value of rootFlow into empty flows.