can someone lead me to a way to collect a flow until some other flow emits something as a condition?...

# flow

[Alexander Black]

can someone lead me to a way to collect a flow until some other flow emits something as a condition??

example:

someFlow.onEach { /*Do something*/}.takeUntil(someOtherFlowAsPredicate).collect()

[baxter]

That sounds like `takeWhile`: https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines.flow/take-while.html

[Alexander Black]

So exactly this, but if the predicate was another flow not a boolean.

[gildor]

https://github.com/Kotlin/kotlinx.coroutines/issues/1850

[Alexander Black]

@gildor perfect! Thank you so much

[baxter]

I mean... the down and dirty way of achieving what you are looking to do would be:

fun <T> Flow<T>.takeUntil(otherFlow: Flow<*>): Flow<T> {
    return channelFlow {
        val job = launch {
            collect { send(it) }
        }

        otherFlow.first()
        job.cancel()
    }
}

But I literally just wrote this in about 5 min.

The idea here is that both flows start to collect, but only the upstream emits. The moment that otherFlow emits a single item, the job collecting the upstream is cancelled, and this flow ends.

[baxter]

You'd be able to customize this too by adding in some sort of terminal item to send downstream at the end of the channelFlow.

[Nick Allen]

Generally, when I want to cancel behavior based on a Flow, I look to the *Latest methods. For example:

flow{
   emit(someFlow)
   otherFlow.first()
   emit(emptyFlow())
}.flatMapLatest {it}

Not any better than what’s above (and untested, on phone) but just some advice for getting to a solution when you end up in a similar situation.

[Alexander Black]

@baxter @Nick Allen Thank you both very much for the code/advice. I very much appreciate it.

[Alexander Black]

I've never tried the flatMapLatest function... that's really cool! going to give that a shot.