Why is only `flow2` being called in `flow1.map{ flow2(it) }.flattenConcat()`? `flow1` is a `Mutabl...

# flow

[Sam Stone]

Why is only flow2 being called in flow1.map{ flow2(it) }.flattenConcat()?

flow1 is a MutableStateFlow<MutableSet<Int>> of db entity IDs, flow2 is a Flow of lists of entities, which is updated when any of the flow's entities' properties change. I am updating the properties and then calling flow1.emit(set), but only flow2.onEach is being called, not flow1.map{}. Any suggestions?

[julian]

You mention flow2.onEach , but it's not clear how this is being used by flow1.map{ flow2(it) }.flattenConcat().
Can you provide a more detailed/complete code sample?

[Sam Stone]

onEach is just for logging purposes.

[julian]

If flow2.onEach is called, then flow1.map must have been called at least once. Are you saying that you don't see execution of flow1.map when you call flow1.emit after the first time? If so, is it possible that flow2 is exerting backpressure? If so, this from the MutableStateFlow docs may be relevant:

Updates to the value are always conflated. So a slow collector skips fast updates, but always collects the most recently emitted value.

This would cause `emit`s to be ignored, and map would not execute for those dropped values.

[wasyl]

I think it would be helpful if you provided some small code sample that demonstrates the issue, ideally one that’d run on https://play.kotlinlang.org