Cancel kotlin flow collection on signal

Question

I'm struggling to create a 'takeUntilSignal' operator for a Flow - an extension method that will cancel a flow when another flow generates an output.

fun <T> Flow<T>.takeUntilSignal(signal: Flow<Unit>): Flow<T>

My initial effort was to try to launch collection of the signal flow in the same coroutine scope as the primary flow collection, and cancel the coroutine scope:

fun <T> Flow<T>.takeUntilSignal(signal: Flow<Unit>): Flow<T> = flow {
    kotlinx.coroutines.withContext(coroutineContext) {
        launch {
            signal.take(1).collect()
            println("signalled")
            cancel()
        }
        collect {
            emit(it)
        }
    }
}

But this isn't working (and uses the forbidden "withContext" method that is expressly stubbed out by Flow to prevent usage).

edit I've kludged together the following abomination, which doesn't quite fit the definition (resulting flow will only cancel after first emission from primary flow), and I get the feeling there's a far better way out there:

fun <T> Flow<T>.takeUntilSignal(signal: Flow<Unit>): Flow<T> =
    combine(
        signal.map { it as Any? }.onStart { emit(null) }
    ) { x, y -> x to y }
        .takeWhile { it.second == null }
        .map { it.first }

edit2 another try, using channelFlow:

fun <T> Flow<T>.takeUntilSignal(signal: Flow<Unit>): Flow<T> =
    channelFlow {
        launch {
            signal.take(1).collect()
            println("hello!")
            close()
        }
        collect { send(it) }
        close()
    }

Answer 1

Use couroutineScope and start the new coroutine inside:

fun <T> Flow<T>.takeUntilSignal(signal: Flow<Unit>): Flow<T> = flow {
    try {
        coroutineScope {
            launch {
                signal.take(1).collect()
                println("signalled")
                [email protected]()
            }

            collect {
                emit(it)
            }
        }

    } catch (e: CancellationException) {
        //ignore
    }
}