Hey guys, With the new viewModelScope, when it gets cancelled, why doesn’t future launch call do an...

# android

[Gautam Lad]

Hey guys,

With the new viewModelScope, when it gets cancelled, why doesn’t future launch call do anything?

viewModelScope.launch(<http://Dispatchers.IO|Dispatchers.IO>) {
            contentLiveData.postValue(getDiscoverContentUseCase.execute())
        }

If I put a breakpoint on launch line I see it’s cancelled.

viewModelScope = {CloseableCoroutineScope@5985} 
 coroutineContext = {CombinedContext@5986} "[SupervisorJobImpl{Cancelled}@7ed8dea, Main]"

[itnoles]

"A failure of a child job that was created using launch can be handled via CoroutineExceptionHandler in the context."

[Gautam Lad]

Fairly new to this so not sure what that means

[Gautam Lad]

i dont see any exceptions or logs indicating something had failed

[itnoles]

I think you are not really catching it.

[itnoles]

https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines/-supervisor-job.html

[Gautam Lad]

Are you saying something inside the launch { } is failing?

[itnoles]

yeah

[Gautam Lad]

I did something like:

viewModelScope.launch(<http://Dispatchers.IO|Dispatchers.IO>) {
            Log.i(TAG, "Inside launch")
         }

And it’s not getting to the Log statement

[itnoles]

try without (Dispatchers.IO)

[Gautam Lad]

I am switching between fragments (where one of hte fragment has the viewmodel) and if the fragment is created/destroyed fast the viewmodel enters a cancelled state

[Gautam Lad]

ok will try

[Gautam Lad]

Same thing:

viewModelScope = {CloseableCoroutineScope@5902} 
 coroutineContext = {CombinedContext@5929} "[SupervisorJobImpl{Cancelled}@e3d3400, Main]"

[Gautam Lad]

Also tried:

val handler = CoroutineExceptionHandler { _, exception ->
            Log.e(TAG, "Error $exception", exception)
        }
        viewModelScope.launch(handler) {

And not getting the log

[liemvo]

viewModelScope is belonged to the ViewModel so it will cancel the future launch when you switch fragments quickly

[Gautam Lad]

so once a scope is cancelled there’s no way to restart it?

[Gautam Lad]

https://developer.android.com/topic/libraries/architecture/coroutines#viewmodelscope

[liemvo]

It depends on your ViewModel

[Gautam Lad]

it’s the same viewmodel instance

[Gautam Lad]

the fragment is destroyed and re-created

[liemvo]

Your ViewModel should belong to the activity???

[Gautam Lad]

it does

[Gautam Lad]

in the fragment it’s

viewModel = ViewModelProviders.of(requireActivity(), viewModelFactory).get(MyViewModel::class.java)

[Melih Aksoy]

Where do you call launch ? You sure you’re calling that function to start it ?

[Gautam Lad]

The call is made in onviewcreated and it's called since i can reach breakpoint on launch line.

[Gautam Lad]

It just never enters the inside of lsuch block since the scope is cancelled as I posted

[Gautam Lad]

Isuch =launch

[Melih Aksoy]

hmm, I also use viewModelScope but didn’t encounter this issue so far. Sounds more like a lifecycle issue. ViewModel should be cancelling this scope in onCleared method. Overriding and debugging it might reveal the reason

[Gautam Lad]

https://discuss.kotlinlang.org/t/coroutinescope-job-cancel-issue/11642

[Gautam Lad]

That might be a clue

[Melih Aksoy]

Yes indeed. cancel is called in ViewModel.kt for viewModelScope. Did you hit onCleared in debugging ?

[Gautam Lad]

Yup I did

[Melih Aksoy]

Based on your previous snippets, you’re sharing this viewmodel by using activity lifecycle right ? Didn’t really debug similar situation, but it seems viewModel gets cleared, but not destroyed, and doesn’t re-create the coroutine scope 🤔

[Gautam Lad]

Yeah the activity hasn't changed

[Gautam Lad]

I'm actually making a leanback TV app

[Gautam Lad]

And I have one activity and one fragment.. Inside the fragment I have a tab system that changed pages (child fragment manager transactions)

[Gautam Lad]

When I change tabs I replace the (child) fragment

[Gautam Lad]

So the child fragment rely on view model that gets a cleared event and subsequently the scope is set to cancelled

[Gautam Lad]

I might just have to use my own scope and job cancelations on clear or on repetitive load calls

[Melih Aksoy]

Seems like a solution, cancelChildren instead of cancel in onCleared. Wonder what causes VM to clear and cancel it’s scope. Would be nice to update here if you find out !