A simple question, but I don't know how to solve this. - I have a `Flow<List<Foo>>` whe...

# coroutines

[Arjan van Wieringen]

A simple question, but I don't know how to solve this.
• I have a Flow<List<Foo>> where Foo also contains a Flow
• interface Foo { val name: String; val flow: Flow<Long> }
How can I transform Flow<List<Foo> to something like Flow<List<Pair<String, Long>>> where the pair is based on the name anf the inner flow value?

[Sam]

I guess you’re looking for flatMap

[Arjan van Wieringen]

I guess, but these are concattenated or merged? I have hard time understanding how that works.

[Arjan van Wieringen]

I made a typo

[Arjan van Wieringen]

I think I need combine probably?

[Sam]

Flow has two versions of `flatMap`: flatMapMerge and flatMapConcat. Which one you want is going to depend on your specific case.

[Arjan van Wieringen]

But it returns a Flow of R, I need a Flow of List<R>

[Sam]

Oh… I guess that was the typo that you fixed after I replied 😄

[Arjan van Wieringen]

Yes 😄

[Sam]

Should the new flow emit just one value for each Foo, or should it emit a new value each time Foo.flow emits? 🤔

[Arjan van Wieringen]

A new value each time a Foo.flow emits. So I want to receive a whole new List

[Sam]

When the Flow<List<Foo>> emits a new list, should that replace all of the previous Foo flows?

[Arjan van Wieringen]

I have this now which seems to work:

val state = fooListFlow.flatMapLatest { fooFlows: List<Foo> ->
        combine(
            fooFlows.map { foo: Foo ->
                foo.flow.map { inner: Long ->
                    Pair(foo.name, inner)
                }
            }
        ) {
           it.toList()
        }

[Arjan van Wieringen]

Don't know if it is very efficient

[Sam]

That looks like about what I would expect, based on what you described 👍

[Arjan van Wieringen]

flatMapLatest / flatMapMerged / flatMapCombined doesn't make any difference though

[Sam]

The difference would probably not be apparent unless one of the ‘old’ inner flows emits a new value after it has been superseded by a new emission from the outer flow.

[Sam]

flatMapLatest will stop collecting the existing flows each time it gets new ones, but flatMapMerge and flatMapConcat will just keep on collecting all of them them until they terminate.

[Arjan van Wieringen]

FlatmapLatest makes the most sense then I guess. Thanks!