Hello Kotlin experts, I'm a Kotlin/Flow beginner a...

# flow

[Guillaume Fleury]

Hello Kotlin experts,
I'm a Kotlin/Flow beginner and I'm trying to create a flow and then collect it but I am not getting the results I am expecting.

I'm creating a flow where errors(exceptions) can be thrown.

suspend fun getFlow(): Flow<Int> {
    return flow {
        emit(5)
        throw Exception("some exception")
    }
}

I am collecting the flow somewhere else.,,

launch {
    getFlow()
        .catch { throwable -> 
            // handle exception
        }
        .collect {
            // do someting
        }
}

The problem is that when a value is emitted and an exception thrown immediately after, the value is lost/not collected. It is possible to have the exception caught/flow cancelled only after all emitted values have been collected?

Thanks.

[CLOVIS]

First, note that a function returning a flow should very rarely be suspend . The flow encapsulates suspend between elements already, it is redundant to also allow suspend at the function level:

fun getFlow() = flow {
    emit(5)
    throw Exception("some exception")
}

[CLOVIS]

What kind of flow are you using? Literally like this example? Operators like sharedIn , stateIn , buffer, implémentations such as SharedFlow or StateFlow , Compose's collectAsState , will cause the behavior you're seeing (some values are skipped)

[CLOVIS]

Your example, as you wrote it, does print the value before the exception, so I think there's something else in your code that causes what you're seeing.

https://play.kotlinlang.org/#eyJ2ZXJzaW9uIjoiMS44LjEwIiwicGxhdGZvcm0iOiJqYXZhIiwiYXJncy[…]sgcHJpbnRsbihcIlZhbHVlIGNvbGxlY3RlZDogJGl0XCIpIH1cbn0ifQ==

Value collected: 5
Exception caught: java.lang.Exception: some error

[CLOVIS]

^ if you can manage to recreate your situation by editing the above link (in the playground), it will be much easier to explain what the problem is

[Guillaume Fleury]

@CLOVIS
Thank you very much for the reply and sorry for the lack of details. I have made some modifications and I think I've located the source of the problem. I cannot share the complete code but let me share some pseudo-code.

fun getFlow(): Flow<Data> = flow {

// fetch data from Room DB

emit(roomData)

// fetch data from web/remote

// networking error throws an exception

emit(remoteData)

}

```// cancel previous jobs

cancellableScope.coroutineContext.cancelChildren()

cancellableScope.launch {

// this emits once(remote), maybe twice(local + remote)

getFlow()

.flowOn(Dispatchers.IO) // when I comment this, value is collected

.catch { throwable ->

// handle

}

.collect {

// do something with value

}

}```

It seems that flowOn is the cause of the problem. When I comment it, the roomData is collected and then the exception is caught. However, when I update the playground above to run on Dispatchers.IO, it still works(value is collected THEN exception is caught).
Is it possible that emitting values in a different dispatcher causes values to be lost?

Thanks.

[Guillaume Fleury]

@CLOVIS
This seems to work, does that make sense?

```// scope must be Dispatcher.IO

private val cancellableScope = CoroutineScope(Dispatchers.IO + SupervisorJob())

fun getData() {

// cancel previous jobs

cancellableScope.coroutineContext.cancelChildren()

cancellableScope.launch {

getFlow()

.catch { throwable ->

viewModelScope.launch {

// handle

}

}

.collect {

viewModelScope.launch {

// do something with value

}

}

}```