After cancelling scope, should I throw away the object? For example, in android activity lifecycle, ...

# coroutines

[Maciek]

After cancelling scope, should I throw away the object? For example, in android activity lifecycle, I'd want to have coroutine started on resume and cancelled on paused, but for every lifecycle pass, I'd need a new scope object, correct?

[Zach Klippenstein (he/him) [MOD]]

Technically you just need a new CoroutineContext that contains a new Job, but if you're following best practice and using the CoroutineScope factory function then yes, you'd need a new scope.

[Maciek]

oh, thanks! That makes sense. Docs could be clearer on that part in CoroutineScope section 😄 For factory function are you referring functions like MainScope()? I'm using them but still keeping the object in the field for reuse, is this something that should be avoided?

[Zach Klippenstein (he/him) [MOD]]

I mean the val scope: CoroutineScope = CoroutineScope(yourContext) factory function.

[Zach Klippenstein (he/him) [MOD]]

Which MainScope() is closely related to. Keeping scopes in a field like that is fine, even encouraged.

[Maciek]

right, fyi that is what caused problems for me in the first place. Putting val scope in activity or fragment, at least when you want to cancel coroutines on pause or on stop. If the system will use same activity object to go back to on start and on pause then your coroutines will just silently not run 🙂

[Zach Klippenstein (he/him) [MOD]]

Doesn’t KTX solve this problem for you? https://developer.android.com/topic/libraries/architecture/coroutines

[Marcelo Hernandez]

If you wish to reuse the scope in onResume(), then you can instead do scope.coroutineContext.cancelChildren() in onPause(). This keeps the scope active for reuse but still makes sure to cancel any launched coroutines.

[Marcelo Hernandez]

If you're familiar with RxJava and its `CompositeDisposable`:
scope.cancel() <-> compositeDisposable.dispose()

scope.coroutineContext.cancelChildren() <-> compositeDisposable.clear()

[Maciek]

KTX solves it only partially, they're providing scope that is alive until onDestroy.
Thanks Marcelo! That's exactly what I needed

[aipok]

Recent activity and fragement KTX provide solution for this problem

viewLifecycleOwner.lifecycleScope.launchWhenResumed {
someWorkThatOnlyPerformedWHileResumed
}

That method will stop coroutine in case app is on pause and will cancel it properly in case activity is destroyed.

[dekans]

it will suspend coroutines when activity is paused

[aipok]

yes, this is what I meant 😄

[dekans]

also beware of, scope.cancelChildren()
It's dangerous if you use channels/actors they will be closed, you'd have to reinstanciate them