How can I propagate an upstream exception in a Sha...

# flow

[Jason Ankers]

How can I propagate an upstream exception in a SharedFlow? For example, I have an Android screen which combines multiple data sources: val uiState = combine(sharedFlow1, sharedFlow2, sharedFlow2) { … } and I would like to catch anything which goes wrong in any of the flows. How could I achieve that? .catch doesnt work because shared flows never complete

[Jason Ankers]

I’ve been considering creating a Result type which gets emitted from my shared flows, and create a flow extension fun <T> Flow<Result<T>>.resultOrError() which maps the errors to exceptions

[wasyl]

A wrapper type is the proper solution. The docs for SharedFlow state

All errors and completion signals should be explicitly materialized if needed

[Jason Ankers]

@wasyl Would something like this work?

fun <T, U> Flow<Result<T, U>>.resultOrError(): Flow<T> { result ->
    return map { result ->
        when(result) {
            is Result.Success -> result.body
            is Result.Error -> throw Exception(result.error)
        }
    }
}

val flow1 = flow { 
    // catch errors and convert to Result.Error
 }.shareIn(..).resultOrError()
val flow2 = flow { .. }.shareIn(..).resultOrError()
val combined = flow1.combine(flow2).catch(// handle errors)

[wasyl]

I think so, yes. I’m not 100% sure but I think the shared flow will stay active though, because well it’s not completing. Overall if you’re using shared flow consider not converting to errors at all and only operating on the result class, but at first glance your snippet would work as you expect

[Jason Ankers]

Yeah, the problem I found with working with a Result type is it’s not easy to quickly determine if anything failed during a large combine. I.e. if I’m combining 4 different data sources, I would need to individually check if each source is of type Result.Error. catch is a nice shortcut to grab any error