I have 2 flows, one of them is cold, another is hot. The goal is to `merge` them, and collect all va...

# flow

[rcd27]

I have 2 flows, one of them is cold, another is hot. The goal is to merge them, and collect all values they produce(and complete them all when the resulting flow completes). Issue: no items emitted, when hot one is idle. What is the nice way to combine two flows, making the resulting one - cold?

where:
• banEvents: MutableStateFlow
• chatMessagesOutput: Flow

[rcd27]

image.png

[CLOVIS]

What do you mean merge them?

• pair them together (one emission everytime both of them emit): https://kotlinlang.org/api/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines.flow/zip.html
• all values one after the other (unknown order): https://kotlinlang.org/api/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.corout[…]%20%3CT%3E%20Iterable%3CFlow%3CT%3E%3E.merge():%20Flow%3CT%3E

[rcd27]

I mean all values from both without any order

[CLOVIS]

Then you can use the second link

[rcd27]

I've tried that out at first place, but my Turbine tests fails after using that operator. Says "no values produced in 10s"(our timeout for flow testing)

[CLOVIS]

You can add .onEach { println("some message: $it") } to both of your flows (before the merge) to check that they do emit values

onEach prints whatever goes through the flow without impacting it

[rcd27]

Yes, Ivan, that's the point, the StateFlow emits nothing in this particular case

[Kevin Worth]

From the screen shot, is chatMessages the StateFlow?

[rcd27]

@Kevin Worth no, it is cold Flow

[Kevin Worth]

banEvents, then?

[rcd27]

yes

[rcd27]

banEvents with filterNotNull becomes Flow as well

[rcd27]

Let's do this one: I'll write some tests using different cases and then return here with my results

[Kevin Worth]

And filterNotNull().onEach { println(it) } prints nothing? Are you sure there’s an emit somewhere on banEvents?

[rcd27]

Seems like I'm missing something

[rcd27]

@Kevin Worth that's the case! It is IDLE, so nothing emitted there.

[rcd27]

Is it MUST that it emits?

[Kevin Worth]

I think it would help to see more of the code, to try to get to an answer.

[rcd27]

OK, guys, seems like the resulting flow (after merge) just never completes, that's why I'm catching the error in Turbine test(yes, I still receive chat messages, no bans since they are null)