How can you emit (infinite) values before the real...

# coroutines

[hfhbd]

How can you emit (infinite) values before the real flow emits? I want to combine two flows: flow b into flow a. Flow a should not wait until the first emission of flow b, but should emit null until b emits values. After b emits some values, the last value of b should be used (combine).

[hfhbd]

val a = flow {
    var counter = 0
    while (true) {
        emit(counter)
        counter++
        delay(1.seconds)
    }
}

val b = flow {
    delay(2.seconds)
    emit("a")
    delay(2.seconds)
    emit("b")
    delay(2.seconds)
}

val expected = listOf(1 to null, 2 to null, 3 to null, 4 to "a", 5 to "a", 6 to "b", 7 to "b")

[Joffrey]

First thing that comes to mind is to use .onStart { emit(null) } on b to ensure you get the first (null) value of b to combine with all first values of a - didn't think it through but I think it should work

[ephemient]

.onStart { emit(null) } should work with `.combine()`; not .zip() of course

[Joffrey]

oops, forgot emit

[hfhbd]

Ohh, nice. I thought onStart will only be called once.

[ephemient]

it is called only once (per subscribe)

[ephemient]

it's combine that's re-using values when the two flows are producing at different times

[hfhbd]

Thanks!

[Joffrey]

Note that you'll probably need to make b a Flow<String?> up front for onStart to be able to emit null

[hfhbd]

Yeah, thats true, I use a simple map