If I want to output a value from `Flow` if nothing else is output, and it completes, what’s the best...

# coroutines

[spierce7]

If I want to output a value from Flow if nothing else is output, and it completes, what’s the best way to do that?

[Zach Klippenstein (he/him) [MOD]]

https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines.flow/on-completion.html

[spierce7]

yeah - I know about onCompletion, but it doesn’t tell me if other items have been emitted.

[spierce7]

This is what I have, but it feels overly convoluted:

return flow {
    var isEmpty = true

    emitAll(
        baseFlow
            .onEach { isEmpty = false }
            .onCompletion {
                if (isEmpty) {
                    emit(defaultData)
                }
            }
    )
}

[araqnid]

I’d do basically that, although just wrapping a collect rather than using onEach and onCompletion .. just a stylistic difference, I think

[araqnid]

sth like

fun <T> Flow<T>.orElse(fn: () -> T): Flow<T> {
  return flow {
    var isEmpty = true
    collect { value ->
      isEmpty = false
      emit(value)
    }
    if (isEmpty) emit(fn())
  }
}

[Erik]

Since you must have collected the entire flow to know it's empty, why not collect it as a list first and then use a default value if the list is empty?

val emptyFlow = emptyFlow<Int>()
val nonEmptyFlow = flowOf(1, 2, 3)
val default = listOf(0)

println(emptyFlow.toList().ifEmpty { default }) // [0]
println(nonEmptyFlow.toList().ifEmpty { default }) // [1, 2, 3]

[elizarov]

What are you use-cases for this kind of operator? Can you, please, add more details on where you encounter this need here: https://github.com/Kotlin/kotlinx.coroutines/issues/1890