```override suspend fun getProducts()=flow<Stat...

# coroutines

[Noushad Chullian]

override suspend fun getProducts()=flow<State<List<ProductListFromEntity>>> {
            try {
                val it = productDao.getProducts()
                if (it.isEmpty()) {
                    loadProducts()
                } else {
                   emit(State.success(it))
                }
            }catch(e:Exception){
                emit(State.error(e.localizedMessage.toString()))
                e.printStackTrace()
            }
}



//Dao function
 @Query("SELECT product.productId, product.productCode, product.name FROM Product product")
    abstract suspend fun getProducts(): List<ProductListFromEntity>
    

//viewModel
suspend fun getProducts() {
        productRepository.getProducts().collectLatest {
            when (it) {
                is State.Error -> {
                    _productListStates.value =
                        ProductListStates.Error(it.message, UIComponentType.Dialog())
                }
                is State.Success -> {
                    parseGivenProductList(it.data)
                }
            }
        }
    }

i am calling this function from viewmodel (android), But it is always throwing error saying, Child of the scoped flow was cancelled without any stacktrace. Am i doing anything wrong here...

[gildor]

without any stacktrace

It’s strange, it should have at least some stacktrace, could you show full error message

[gildor]

I’m not quite sure how you use productDao.getProducts() as I understand from your example it should return Flow, but you use it a way that it returns List

[Noushad Chullian]

When i am copying here i was trying to change somthing else that is why it is returning a flow here, i will edit the question

[gildor]

When you run this code, could you show full error message

[gildor]

For example instead of doing State.error(e.localizedMessage.toString())) pass whole exception:

State.error(e)

[Noushad Chullian]

Okay

[gildor]

otherwise for now it looks that it can hide some error (for example just by hiding cause

[Noushad Chullian]

kotlinx.coroutines.flow.internal.ChildCancelledException: Child of the scoped flow was cancelled
this is the Exception returning, no stackTrace found

[gildor]

It doesn’t look as full exceptiuon

[gildor]

it looks as exception.toString()

[Noushad Chullian]

yes, But that is the only thing iam getting

[gildor]

well, exception.toString() is not a full exception, make sure that you print stacktrace too

[gildor]

actual exception has some stacktrace anyway

[gildor]

With code you provided there is no way to understand what is going on and which part of your code is actually throwing it and who is handling it

[Noushad Chullian]

message has been deleted

[gildor]

So which part of code catches this exception?

[gildor]

Open cause and check, maybe there is stacktrace

[Noushad Chullian]

i thought it is because of any error in Room library, but i tried with network call in the same and same error

[gildor]

Just checking, what loadProducts is doing, there is no related code in your example

[Noushad Chullian]

"So which part of code catches this exception?" when executing productDao.getProducts()

[gildor]

Ah, I probably got it, it just means that flow where you run this code is already cancelled

[Noushad Chullian]

loadProducts is a network call to fetch from server and it is working, because iam using that one to store into my sqlite

[gildor]

on practice it probably means that scope where you call viewModel.getProducts is already cacncelled

[Noushad Chullian]

"Ah, I probably got it, it just means that flow where you run this code is already cancelled"
can you please explain that

[gildor]

flow is active only while someone is collecting it

[gildor]

if coroutine which collects this flow is cancelled, you will get this exception every time when you call any suspend function

[gildor]

The right way would be instead of

}catch(e:Exception){

do

}catch(e: CancellationException){
  throw e
}catch(e:Exception){

[gildor]

so it will propagate cancellation

[Noushad Chullian]

Okay thanks, LEt me try

[Noushad Chullian]

so meaning this getProducts() from repository is running viewModelScope?

[gildor]

It’s opposite, viewModel.getProducts collecting your productRepository.getProducts, so it depends on where this viewModel.getProducts is called

[Noushad Chullian]

confused now, so which of my scope is cancelled?? the flow from repository?

[gildor]

no, scope on which viewModel.getProducts is called

[gildor]

So, in general there is no issue in code which you show,except that you do not propagate cancellation, and end up with State.Error with cancellation error, which you usually don’t want to have, you want instead propagate cancellation

But it doesn’t solve your issue, it looks that your problem not with this code, but with the fact that something cancelling your viewModel.getProducts()

[gildor]

loadProducts is a network call to fetch from server and it is working, because iam using that one to store into my sqlite

But as I see it doesn’t emit anything, looks that you return nothing there

[Noushad Chullian]

yes the dao call must be flowable, that is how it was, and i was trying to find this thing, so for now i changed not to flow

[gildor]

Well, if dao is flowable, it will be very different code

[gildor]

and I just don’t understand how it supose to work

[Noushad Chullian]

yes

[gildor]

probably you need something like thius:

if (it.isEmpty()) {
                    emit(State.success(loadProducts()))
                } else {
                   emit(State.success(it))
                }

[Noushad Chullian]

// this is how i am calling viewmodel to load

lifecycleScope.launchWhenStarted {
            vm.action.emit(ProductListActions.FetchProductList)
        }

//the init of viewmodel
init {
        viewModelScope.launch { handleIntent() }
    }

private suspend fun handleIntent() {
        action.collectLatest {
		when (it) {
                ProductListActions.FetchProductList -> {
                    getProducts()
                }
			}
}

[Noushad Chullian]

is there anything wrong here?

[Noushad Chullian]

because i am using the same way in another place, and it is working like expected

[gildor]

It can be related on the fact that you use collectLatest everywhere, which cancells coroutine, so every new action will cancell currently running getPRoducts

[gildor]

it’s not an issue if you propagate cancellation correctly, as I suggested above

[Noushad Chullian]

THanaks Brother

[gildor]

Hope it will help

[gildor]

I would also suggest you instead of flow<State<List<ProductListFromEntity>>> { to use simple map/catch on flow, it would be more natural code and more flexible and will easily work with flow

[Noushad Chullian]

like how??

[gildor]

I mean implement this function as mapping of values

[Noushad Chullian]

THanks, I will try

[gildor]

depend of course on how productDao.getProducts() is implemented, now there is no reason to use Flow there, it always return one value

[Noushad Chullian]

yea, but i will make the query back to flow

[gildor]

even with current suspend function it can be implemented like:

productDao::getProducts.toFlow() // convert reference on suspend function to flow, later you can just use any other flow
  .map { if (it.isEmpty()) {
                    loadProducts() // Suppose it returns State
                } else {
                   State.success(it)
                }
  }
  .catch {
        emit(State.error(..))
  }

[Noushad Chullian]

THanks, Can i ask another question

[Noushad Chullian]

since i am struggling with this flow/collect thing

[Noushad Chullian]

private suspend fun handleIntent() {
action.collectLatest {
when (it) {
ProductListActions.FetchProductList -> {
getProducts()
}
}
}
this function has another events, but when i am using just collect, it is not collecting from second time, it is working for only first time? any idea about that

[gildor]

Is it with flow or suspend function on DAO

[gildor]

because when youy change dao to flow it never complete, it will be subscribed forever,. as result getProducts() function never return and can be cancelled

[Noushad Chullian]

no. from fragment/activity to viewmodel

[gildor]

not sure what you mean

[gildor]

I just said that your current implementation will work very different with suspend or with dao

[gildor]

it shouldn’t be an issue about collect if you use suspend function

[gildor]

but it will be with Flow

[Noushad Chullian]

i am passing action via sharedFlow from activity to viewModel, and i am collecting it here
private suspend fun handleIntent() {
action.collect {
when (it) {
ProductListActions.FetchProductList -> {
getProducts()
}
}
}
But for the first time the sharedFlow is collected, and not collecting afterwards

[gildor]

Sorry, it hard to understand what is going on which this example

[Noushad Chullian]

it is not related to my first question

[Noushad Chullian]

this is a different one

[Noushad Chullian]

i am passing action via sharedFlow from activity to viewModel, and i am collecting it here
private suspend fun handleIntent() {
action.collect {
when (it) {
ProductListActions.FetchProductList -> {
getProducts()
}
}
}
But for the first time the sharedFlow is collected, and not collecting afterwards

[Noushad Chullian]

this handleIntent() is called from the init of the viewModel, So I am thinking the collecting of action inside ths function will persist till the viewModelScope is alive.

[gildor]

it is not related to my first question

I understand, and I have even less context, so don’t understand what is going on there

[Noushad Chullian]

Emiting action from activity/fragment works for the first time like getProgucts(), but second time when clicked on the product from the list, i am passing another action to viewmodel about this click event, but it is not collecting

[gildor]

I already told you, it depends on how getProducts implemented

[Noushad Chullian]

THanks, Now I understtod

[Noushad Chullian]

THank you for clearing my doubts....

[Nick Allen]

FYI, you should pretty much never call emit from try/catch. See https://kotlinlang.org/docs/flow.html#exception-transparency

[Noushad Chullian]

THanks