How do you guys handle coroutines in android which want to o

Question

How do you guys handle coroutines in android which want to outlive the viewmodel scope should repository or whatever does the couroutine have its scope and be transparent to the viewmodel or should that scope be passed into the viewmodel and viewmodel launches the coroutine into that scope

octylFractal · Answer

the scope should be tied to some lifecycle so probably the repository should have it if it is the one that should own the coroutines

ursus · Answer

okay but how can I make it composable in the viewmodel ``` fun Service doWhatever serviceScope launch fun ViewModel buttonClick viewModelScope launch service doWhatever ui displaySuccess ```

ursus · Answer

in this ssample ui displaySuccess wont wait for the coroutine

octylFractal · Answer

then it probably shouldn t be in its own scope that particular coroutine doesn t need to outlive the VM scope

ursus · Answer

why not say its posting a chat message you still want the post to finish without cancel when you switch screens no

octylFractal · Answer

sounds like you need an application level scope then that the original launch occurs on in this case the Service does not want to own the coroutine but the application as a whole

ursus · Answer

thats no difference in this case as Service is singleton anyways where would your app scope live or rather where its going to be used in service or vm

octylFractal · Answer

I m not sure I m not very familiar with android I would use it in the VM because that s the UI area

ursus · Answer

so youd pass the appScope via ctor to the vm

octylFractal · Answer

yea

ursus · Answer

okay but now viewmodel decides the scope is that smart client code basically dictating the lifecycle implicitly

octylFractal · Answer

I m not sure about the particular design pattern pros cons this appears to be reasonable to me because the action is supposed to outlive the VM in this case so you want whatever creates it to pass something in since it the creator typically has a longer lifecycle than the VM

ursus · Answer

yea but why not hide it inside the Service or whatever holds the coroutine I mean for viewmodel viewmodelScope is also private in the VM view which has even shorter lifecycle doesnt see it

octylFractal · Answer

it doesn t make sense to me because then the service would be calling out to the UI

ursus · Answer

what do you mean viewmodel scope service foo ui display

octylFractal · Answer

then that s still on the VM scope right

ursus · Answer

well that was my argument that each component that has a lifecycle should have its own scope

ursus · Answer

service gt viewmodel gt view

ursus · Answer

each have their lifecycle narrowing to the right

octylFractal · Answer

well it doesn t make sense because the outer coroutine is what needs to be launched on it

octylFractal · Answer

so you still need to pull it out of the service

ursus · Answer

well why then the view doesnt own the scope if its the actual callsite

octylFractal · Answer

because then it should be cancelled when the view is closed right

ursus · Answer

also ``` fun Viewmodel buttonClick appScope launch service foo ui display ``` doesnt this leak Ui

octylFractal · Answer

yes it does leak it until the coroutine finishes

octylFractal · Answer

there isn t much you can do to avoid that it would leak regardless

ursus · Answer

well yes and you dont want that cancelation hence you move the scope ownership upstream

octylFractal · Answer

that s not a thing in coroutines afaik

octylFractal · Answer

where you launch it is what scope it is tied to

ursus · Answer

so how would I jump that

ursus · Answer

I dont feel like UI should decide lifecycle of some mutating business logic

octylFractal · Answer

I suppose you could pass some event back and forth in this case

octylFractal · Answer

the trick is that imo the service shouldn t directly call the UI either

ursus · Answer

yea sure service has no knowledge about ui

ursus · Answer

okay so if that function will be nonsuspending and launching a coroutine inside its service scope how can I let the result go outside channel

ursus · Answer

but that breaks composability doesnt it

octylFractal · Answer

really I haven t run in to this case myself so I m not sure what is the best better design option I think what I originally proposed is the best fit for this scenario UI should dictate service lifecycle because it needs to run something after it

ursus · Answer

how about that leak though thats not cool

octylFractal · Answer

do you expect it to not leak

octylFractal · Answer

how else will the UI object be around for when it finishes

octylFractal · Answer

I would not consider it a leak because you really do want it to stay around for later

ursus · Answer

well youre keeping the ui in memory after it should be destroyed gced

octylFractal · Answer

no because you wanted to call it

octylFractal · Answer

did you not want that at the end of the coroutine

ursus · Answer

I did but when ui is gone its no longer necessary Why im beating at this is that with rxjava you can do this via autoConnect downstream just unsubscribes upstream keeps chugging

octylFractal · Answer

okay but when that `launch` block finishes UI is GC d

octylFractal · Answer

it doesn t stick around after the coroutine is done

kevindmoore · Answer

```suspend fun Service doWhatever fun ViewModel buttonClick viewModelScope launch service doWhatever ui displaySuccess ```

kevindmoore · Answer

Couldn t you do something like this

octylFractal · Answer

the issue is that the coroutine should stick beyond VM whereas this will be cancelled when the VM is closed

kevindmoore · Answer

I see Seems like you would start the service on it s own and then send an event back to a listener in this case the ViewModel that the data is ready If the ViewModel s lifecycle is done it would unsubscribe and not be notified

ursus · Answer

Well yes but youre comunicating via listener channel rx observsble i e composability of suspend functions is gone right also the listener needs to be keyed to identify that call if need be etc im not super happy about that

gildor · Answer

If you want to avoid cancellation of service operation it should be one of two 1 implementation details of service which never cancel operation so just run it on own scope for example by launching async and awaits it already on caller s scope just suspend function 2 call site runs operation with NonCancellable context but careful with second approach it s relatively easy to leak coroutine

ursus · Answer

1 how would that look like ``` suspend fun foo val deffered = scope async foo stuff return deffered await ``` this

gildor · Answer

yes like this

gildor · Answer

Check this example <https pl kotl in peLtoh5bh>

ursus · Answer

sorry that opens my previous snippet and a helloworld in incognito mode

gildor · Answer

Yeah looks like some bug of playground Try this one <https pl kotl in wdzhx7 IY>

ursus · Answer

Yea that I had in mind however have you seen this ``` If you need to launch a coroutine that keeps running after your function returns then make your function an extension of CoroutineScope or pass scope CoroutineScope as parameter to make your intent clear in your function signature Do not make these functions suspending ``` <https medium com elizarov coroutine context and scope c8b255d59055>

gildor · Answer

It s an explicit way to do this as I said it depends on what is your goal If you just do this on call site of course use different more global scope how was already suggested in this thread if it actually services logic do not lose requests even if client is cancelled and you do not want to add this logic on every client then do this on level of service it doesn t violate structured concurrency

gildor · Answer

And what you cited is not really your case because you want to wait for result