Hey yall I m having a combining flow issue I have two flows

Question

Hey yall I m having a combining flow issue I have two flows of different types that depend on an id I need to combine them in such a way that if flow B receives an emission the entire flow needs to emit again the current state I tried `combine` `combineTransform` `zip` and `merge` with no success The first emission occurs correctly but once only B emits it doesn t get propagated downstream Any ideas on what I could try ```private val state = MutableStateFlow lt State gt State Loading fun loadData collectionId String viewModelScope launch val collectionFlow = repository getCollection collectionId val itemsFlow = repository getCollectionItems collectionId combine collectionFlow itemsFlow Pair flatMapConcat collection items gt val map = maps the data modelUseCase map sends to another flow collect model gt if model items isEmpty state value = State Empty else state value = State Content model ```

kevin.cianfarini · Answer

```fun flowFromId id String Flow lt Pair lt A B gt gt = combine flowAFromId id flowBFromId id Pair fun flowAFromId id String Flow lt A gt = flow omitted fun flowBFromId id String Flow lt B gt = flow omitted ```

kevin.cianfarini · Answer

Without seeing your code it s impossible to know what s going wrong but the above sample should work to emit on every constituent flow emission

Leo Delon · Answer

Will try

Leo Delon · Answer

I tried but the emission is not being propagated disappointed

Leo Delon · Answer

< kevin cianfarini> code above

Leo Delon · Answer

collectionFlow emits but combine doesn t receive anything

kevin.cianfarini · Answer

To debug you should simplify the above code to see if anything is happening at all You ve got quite a bit of moving parts in there I d suggest ``` val collectionFlow = repository getCollection collectionId val itemsFlow = repository getCollectionItems collectionId combine collectionFlow itemsFlow Pair collect collection items gt error Got an item ``` If that produces an error then you know the combine is working

kevin.cianfarini · Answer

I am suspicious of your flatmapConcat there being the bug

kevin.cianfarini · Answer

I doubt that it s combine that isn t emitting

Leo Delon · Answer

I added onEach for each flow Both are emitting fine

Leo Delon · Answer

GPT says

Leo Delon · Answer

The `combine` operator of Kotlin Flow waits until all flows emit at least one value before it starts emitting combined values If any of the flows don t emit a value the `combine` operator won t emit any value To solve this problem you can use the `onEach` operator to apply a side effect and ensure that each flow emits at least one value For example ```import kotlinx coroutines import kotlinx coroutines flow fun main = runBlocking lt Unit gt val flow1 = flowOf 1 2 3 val flow2 = flowOf A B C flow1 onEach Applying a side effect combine flow2 onEach Applying a side effect i s gt $i$s collect println it ```

kevin.cianfarini · Answer

Side effects have nothing to do with how flows work in combine Do not trust that ChatGPT is correct Consult the documentation

Leo Delon · Answer

I have onEach after combine

kevin.cianfarini · Answer

If either of your flows are not emitting an item you can wrap them with something like ` onStart emit item ` to ensure a flow has an item

Leo Delon · Answer

nothing comes through

Leo Delon · Answer

even though one of the flows emitted

kevin.cianfarini · Answer

Both of the flows need to emit for combine to emit

Leo Delon · Answer

the problem occurs on the second emission

kevin.cianfarini · Answer

if only one is emitting then there s your problem

Leo Delon · Answer

first time they both load and all works

Leo Delon · Answer

when one changes

Leo Delon · Answer

the combine doesn t work

kevin.cianfarini · Answer

You need to provide a better reproducer than what you ve given Give me something along the lines of the following ```val flow = flowOf 1 2 onEach delay 10 val flow2 = flowOf a b c onEach delay 15 flow combine flow2 i s gt i toString + s collect println it Will print 1a 2a 2b 2c ```

Leo Delon · Answer

ok

Leo Delon · Answer

```fun main val flow1 = MutableSharedFlow lt Int gt val flow2 = MutableSharedFlow lt String gt GlobalScope launch launch flow1 combine flow2 i Int s String gt i toString + s collect println it delay 100 flow1 emit 1 flow2 emit a delay 100 flow1 emit 2 delay 100 flow2 emit b ```

Leo Delon · Answer

this works point up

Leo Delon · Answer

```val collectionFlow = repository getCollection collectionId onEach Timber d loadData $ it name val itemsFlow = repository getCollectionItems collectionId onEach Timber d loadData $ it size collectionFlow combine itemsFlow collection items gt Timber d loadData $ items size collection to items ```

Leo Delon · Answer

this doesn t work after the first emission

Leo Delon · Answer

even though the individual flows emit

kevin.cianfarini · Answer

Do you collect this at all Right now it looks like it s not being `collected` ```val collectionFlow = repository getCollection collectionId onEach Timber d loadData $ it name val itemsFlow = repository getCollectionItems collectionId onEach Timber d loadData $ it size collectionFlow combine itemsFlow collection items gt Timber d loadData $ items size collection to items ```

Leo Delon · Answer

I do collect it at the end

Leo Delon · Answer

first collection works

kevin.cianfarini · Answer

What does your log look like

Leo Delon · Answer

```D loadData Newer Flow 1 Emission 1 D loadData 2 Flow 2 Emission 1 D loadData 2 Combine Emission 1 D loadData Newest Flow 1 Emission 2```

Leo Delon · Answer

I m collecting the flow inside of a function in my viewmodel

kevin.cianfarini · Answer

Can you change your debug logs just for the sake of clarity to be ```val collectionFlow = repository getCollection collectionId onEach Timber d getCollectionFlow $ it name val itemsFlow = repository getCollectionItems collectionId onEach Timber d getCollectionItems $ it size collectionFlow combine itemsFlow collection items gt Timber d combine $ collection name $ items size collection to items ```

kevin.cianfarini · Answer

and then post the logs

Leo Delon · Answer

I m trying returning my flow

Leo Delon · Answer

instead of collecting

Leo Delon · Answer

```D getCollectionFlow Newest D getCollectionItems 2 D combine Newest 2 D getCollectionFlow Newer```

Leo Delon · Answer

```kotlin=1 8 10 kotlinx coroutines=1 6 4```

kevin.cianfarini · Answer

Hmmmmm

kevin.cianfarini · Answer

Try and see if the flow is completing ```collectionFlow combine itemsFlow Pair onCompletion log completion here ```

kevin.cianfarini · Answer

I have one more idea after that but let s see that result first

kevin.cianfarini · Answer

That `onCompletion` function takes an optional throwable as the cause Would be good to log that

Leo Delon · Answer

tried

Leo Delon · Answer

not completing Meaning no log

Leo Delon · Answer

Trying coroutines 1 7 0 Beta with compatibility for kotlin 1 8 10

Leo Delon · Answer

also no change

kevin.cianfarini · Answer

can you ` take 2 toList ` to see what happens

kevin.cianfarini · Answer

Otherwise you should inline the body of `combine` to see where it s failing ```fun lt T E R gt Flow lt T gt myCombine other Flow lt E gt tansform T E gt R Flow lt R gt return combineTransform flow2 a b gt println Got values $a and $b val value = transform a b println emitting $value emit value println emitted $value ```

Leo Delon · Answer

how could I take after the first emission where the problem happens

kevin.cianfarini · Answer

Can you clarify

Leo Delon · Answer

I don t know how taking 2 would work to catch the issue

kevin.cianfarini · Answer

I was curious to see if it would suspend forever or throw some sort of exception

Leo Delon · Answer

```D getCollectionFlow yoyoyoyt D getCollectionItems 0 D Got values AssetCollection id=VX9MXud3QWUsAFwuAJlx name=yoyoyoyt itemCount=78 timestamp=Fri Mar 03 10 10 03 GMT+01 00 2023 and D combine yoyoyoyt 0 D emitting AssetCollection id=VX9MXud3QWUsAFwuAJlx name=yoyoyoyt itemCount=78 timestamp=Fri Mar 03 10 10 03 GMT+01 00 2023 D getCollectionFlow yoyoyoyx```

Leo Delon · Answer

the kotlin sample works so is definitely something with my flows

kevin.cianfarini · Answer

You should add `onCompletion` to each of your constituent flows to see if either of them are completing

Leo Delon · Answer

I did

Leo Delon · Answer

none is

Leo Delon · Answer

ok

Leo Delon · Answer

think I found it

kevin.cianfarini · Answer

What went wrong

Leo Delon · Answer

it s probably the flatmapConcat

Leo Delon · Answer

that other flow is also combining

Leo Delon · Answer

something wrong there could throw the whole thing off

Leo Delon · Answer

combining without it is collecting

kevin.cianfarini · Answer

Ah yep that ll do it

Leo Delon · Answer

```onCompletion kotlinx coroutines flow internal ChildCancelledException Child of the scoped flow was cancelled```