Hi folks Is there any reason `launch`ing a new coroutine in

Question

Hi folks Is there any reason `launch`ing a new coroutine in a cancelled scope doesn t error at all and instead fails silently

streetsofboston · Answer

Maybe for the same reason what ever that reason is that Coroutines created but not yet started dispatched while inside an active CoroutineScope will silently go away when they finally are about to get started dispatched at the time the CoroutineScope is inactive

Joffrey · Answer

Launching a coroutine right before or right after the scope is cancelled should not behave differently IMO Throwing an exception when launching a coroutine in a cancelled scope would create an artificial difference that is not desirable Imagine if you had to deal with races between launches and scope cancellation to handle this difference

kevin.cianfarini · Answer

We re already having to deal with races when our scope is cancelled except now we re chasing a silent bug instead of one that crashes

Joffrey · Answer

gt We re already having to deal with races when our scope is cancelled In which way

kevin.cianfarini · Answer

We share viewmodels between iOS and Android Right now they have a lifecycle associated with them This is something I would like to remove in the future and make them stateless However we currently have to rectify lifecycle difference per platform with a common API Our iOS viewmodels are cancelling their scope at different times than our Android ones Therefore certain code paths try to `launch` something and it fails silently This is a problem only in native for us I realize this problem ultimately stems from our poor viewmodel arch However it made this bug difficult to track down because there was no indication that the coroutinescope was defunct Instead the `block` to launch would never get invoked

julian · Answer

`Job join ` on a job that s part of a structured hierarchy at the root of which is a cancelled scope is not silent It will throw an exception if you need that as a signal

Adam Powell · Answer

It does not <https pl kotl in K4vWokfsY> `join` returns when all children have joined it does not throw an exception if parts of the job hierarchy have failed

Adam Powell · Answer

cancellation means stop and clean up if a coroutine never started running before it was cancelled it has nothing to clean up e g `finally` blocks to run as the `CancellationException` tears it down

julian · Answer

< Adam Powell> It throws And from the on `join` gt This suspending function is cancellable and always checks for a cancellation of the invoking coroutine s Job If the Job of the invoking coroutine is cancelled or completed when this suspending function is invoked or while it is suspended this function throws CancellationException

Joffrey · Answer

I don t think cancellation exception counts in the eyes of the original question It s just how cancellation works

Adam Powell · Answer

yes if the caller of `join` is cancelled then `join` will throw the accompanying `CancellationException` that s standard cancellation on the caller s side It s not dependent on the job being joined though

Adam Powell · Answer

`launch`ing a new coroutine in a cancelled scope doesn t produce an error for the same reason that ```val job = launch job cancel ``` doesn t produce an error either