Lets say I have a sealed class `A` with 2 subclass...

# coroutines

[Florian]

Lets say I have a sealed class A with 2 subclasses B and C. Can I combine a Flow<B> and a Flow<C> to a Flow<A>?

[magnumrocha]

combine(flowA, flowB) { some logic here }

[louiscad]

@Florian Did you fire up a Google search with the obvious keywords before asking? If not, can you guess what would be the right keywords?

[Florian]

I already tried combine but I don't see how I can return a Flow of the superclass

[magnumrocha]

combine<A>(flowB, flowC)

[magnumrocha]

well, I think the kotlin compiler inference can resolve it for you

[Florian]

but I have to return a single value from the block

[louiscad]

Now I understand what you can better. It seems you want to merge these two flows, am I right?

[Florian]

yes

[Florian]

I can only return something like a Pair from combine

[Florian]

but I want a Flow that emits each value separately as its supertype

[louiscad]

So the right keywords are "kotlin flow merge", not combine as combine is designed to combine the last value of each flow.

[Florian]

Thanks, but this was not an obvious keyword

[louiscad]

Yep, I understand how combine was more natural to think of

[Florian]

but thank you very much for your help

[louiscad]

Did you find the right operator to use?

[Florian]

merge seems to work

[louiscad]

Yep, looks to be just what you need 🙂

[louiscad]

Before asking here, did you look at the KDoc of combine? I'm aksing to understand your thought process, and see if the doc should mention the merge operator to help folks in the future.

[Florian]

I already knew combine, I'm using it in my project

[Florian]

I thought I'll have to use combine but I didn't know with what transform block

[Florian]

I will google more thoroughly next time

[magnumrocha]

you can learn a lot with the Rx diagrams that explain the work of each operator...

[magnumrocha]

https://rxmarbles.com/

[magnumrocha]

the concept of Flow transformation operators are pretty the same thing

[Florian]

thank you

[magnumrocha]

oh, I didn't realize there is one for Flows already:
https://flowmarbles.com/

[Florian]

wow 🤩 I'm gonna bookmark that right away

[Florian]

looks like combine was actually the correct operator, because with merge I don't get all the values into a single Flow

[Florian]

I just get separate emits from both Flows

[Florian]

instead I now merge the 2 lists inside the combine operator like this:

val combinedChatFlow: Flow<List<Chat>> = combine(directChatsFlow, groupChatsFlow) { directChats, groupChats ->
    directChats + groupChats
}

[magnumrocha]

this is the difference between the two operators