Considering following code snippet ```viewLifecycleOwner.lifecycleScope.launch { val result = some...

# flow

[Marek Kubiczek]

Considering following code snippet

viewLifecycleOwner.lifecycleScope.launch {
  val result = someFlow.first()
  doSomethingInUI(result)
}

where doSomethingInUI is not a suspend function.
What will happen if scope is destroyed/cancelled while there is ongoing operation someFlow.first() (eg. database read). Is first() cancellable? Will it exit the launch block gracefully or do I need to always call ensureActive() before a call to non suspended function in such case?
The same stands for

someFlow.onEach {
  doSomnethinginUI(it)
}.launchIn(viewLifecycleOwner.lifecycleScope)

Is there any guarantee onEach won’t be called after scope is destroyed/cancelled?

[ephemient]

yes, .first() is cancelable. you're not catching CancellationException so it will leave scope naturally. .launchIn() is equivalent to launch { .collect() }, so it'll be sequenced with everything else on Main as long as you haven't switched to another dispatcher with withContext()

[Marek Kubiczek]

There is something I don’t understand there. I have crash on production where aprarently I land in doSomethingInUi despite the fact view is already destroyed.
I can reproduce it with the following code snippet

someFlow.onEach {
  nonCancellableDelay()
}.onEach {
  doSomnethinginUI(it)
}.launchIn(viewLifecycleOwner.lifecycleScope)

where the delay is non cancellable and defined as

suspend fun nonCancellableDelay() = suspendCoroutine<Unit> {
        Handler(Looper.getMainLooper()).postDelayed(Runnable {
            it.resume(Unit)
        }, 2000)
    }

I would expect the second onEach shouldn’t be called in case scope has been destroyed meanwhile. But it’s not the case. Making me think that I can only be safe by calling ensureActive first which is counterintuitive.

[ephemient]

well as you've worn it that ignores cancellation…

[ephemient]

suspendCancellableCoroutine would check for cancellation before resuming, but given the naming that seems intentional… why?

[Marek Kubiczek]

This is just some craft made code that I wrote to try to simulate long time needed to process the db query. So I made it noncancellable on purpose, just to ensure it is not cancelled while delayed but I was expected it to be cancelled before next emission from one onEach to the other one.

[ephemient]

onEach all run one after each other and there isn't an intervening suspension point that would throw if the scope gets cancelled

[ephemient]

another way to put it, what you wrote is equivalent to

launch(lifecycleScope) {
    flow {
        someFlow.collect {
            nonCancellableDelay()
            doSomnethinginUI(it)
            emit(it)
        }
    }.collect()
}

[ephemient]

if you take out all the flow stuff, you'll find that

launch(lifecycleScope) {
    nonCancellableDelay()
    doSomethingInUi(it)
}

has the same behavior: if there's an actual suspension it'll throw CancellationException, but otherwise it just keeps running

[ephemient]

normally this wouldn't arise, because either you've changed contexts away from Main and will check cancellation before resuming back on Main, or you're running blocking code on Main so the UI can't be destroyed asynchronously…

[Marek Kubiczek]

We still believe there is a bug. One of my colleagues investigated it more and reported a bug https://youtrack.jetbrains.com/issue/KT-48401