anyone know how to get coroutine context from a suspend func

Question

anyone know how to get coroutine context from a suspend function in order to launch a job within a suspend function we need to switch context with `withContext` to get `this` as a coroutine context

tseisel · Answer

To launch a new job you need a `CoroutineScope` not a `CoroutineContext` The best way to obtain the CoroutineScope of a coroutine is to pass it as argument to the function that needs it

tseisel · Answer

Also There is a convention for defining functions that starts coroutines `fun CoroutineScope launchMyJob ` Functions defined like this does not suspend until the job is completed as opposed to `suspend fun`

simon.vergauwen · Answer

There is `coroutineContext` as a property available within any `suspend` function or lambda

Tuan Kiet · Answer

< simon vergauwen> that is a function

Tuan Kiet · Answer

and not sure which is less overhead `suspendCoroutineUninterceptedOrReturn` or `withContext`

simon.vergauwen · Answer

I don t think `suspendCoroutineOrReturn` is what you re looking for It s rather low level

simon.vergauwen · Answer

Do you just want to launch and forget Or do you depend on the result

Tuan Kiet · Answer

that is coroutineContext implemetation

Tuan Kiet · Answer

again which is less expensive coroutineContext or withContext

gildor · Answer

coroutineContext is not a function this is a property

gildor · Answer

What is your use case From your first message is completely not clear what you want to achieve if you just want to use launch async from suspend function use coroutineScope or supervisorScope functions which create scopes and suspend until child coroutineScope active but you asking about coroutine context which cannot be used to start launch async coroutines

gildor · Answer

withContext also works for this but it make sense to use only if you really want to change context otherwise just use coroutineScope supervisorScope

Tuan Kiet · Answer

wrong typing though I mean `coroutineScope` pretty sure within a suspend function coroutineScope is a function

Tuan Kiet · Answer

`public suspend fun lt R gt coroutineScope block suspend CoroutineScope gt R R`

gildor · Answer

coroutineScope is a function of course coroutineContext is a property and actually intrinsic

Tuan Kiet · Answer

oh

Tuan Kiet · Answer

so same question again not what I should use which is less overhead `coroutineScope ` or `withContext `

gildor · Answer

they are very similar but if you don t want to switch context only create a new scope just use coroutineScope not because of performance bit just semantically

Tuan Kiet · Answer

so the suspend function does not have scope

gildor · Answer

Probably coroutineScope a bit more performant just because less context objects will be created but actually not sure because coroutineScope also create own context so I think it s the same but use withContext just to create scope is just strange and confusing

gildor · Answer

Sure there is no scope in suspend function and it s not needed for most of cases

Tuan Kiet · Answer

apparently I need a function which will return a Job which lead me to this not sure it s the best way ``` suspend fun sdfsdf Job return coroutineScope launch ```

gildor · Answer

Oh no it doesn t make sense and will not work

Tuan Kiet · Answer

why

gildor · Answer

Job will be always closed in this case because coroutineScope will not return until launch is finished this is whole point of this function

Tuan Kiet · Answer

so what should I do

gildor · Answer

Could you please give your use case Suspend function that returns Job is not a use case

Tuan Kiet · Answer

my previous function does return a Job but that Job is already finish am I correct

gildor · Answer

Yes

Tuan Kiet · Answer

lol

gildor · Answer

Do you want to use this job to cancel invocation of suspend function body

Tuan Kiet · Answer

``` override suspend fun doWork Result = coroutineScope workJob = launch doActualWork workJob invokeOnCompletion if it = null showStoppedNotification Result success private suspend fun doActualWork stuff ```

Tuan Kiet · Answer

doWork is a framework method

gildor · Answer

If my assumption is correct than no need to return Job from suspend function for this just wrap to launch or any other builder on call site to control lifecycle it s common idiom in coroutines instead of putting launch inside wrap it on call site it make it universal call site controls lifecycle and safe to use no way to leak it ``` suspend fun sdfsdf On call site val job = launch sdfsdf Launch invocation in background and also may be cancelled job cancel ```

Tuan Kiet · Answer

and this can be cancel any time by the framework so I need a job instance to clean up stuff after it got canceled

Tuan Kiet · Answer

and I don t want to put everything I launch so I made a function for it

gildor · Answer

To handle cancellation just wrap suspend function to try catch finally

Tuan Kiet · Answer

doWork is called by the framework

gildor · Answer

In finally you can clean resources

gildor · Answer

So

gildor · Answer

This is just suspend function use try catch to handle cancellation

Tuan Kiet · Answer

do you mean I need to try catch in doActualWork

Tuan Kiet · Answer

or in doWork

gildor · Answer

anywhere depends on where resource is acquired

gildor · Answer

If you want to keep current semantics than in doWork

Tuan Kiet · Answer

I thought when suspend function got canceled it will stop immediately no change to reach catch or final

gildor · Answer

``` override suspend fun doWork Result try doActualWork catch e CancellationException You can handle cancellation separately if you want catch e Exception showStoppedNotification Should we return Result fail return Result success private suspend fun doActualWork stuff ```

gildor · Answer

when coroutine where suspend function should be invoked is cancelled suspend function will throw cancellation exception and this exception propagated to parent suspend functions

Tuan Kiet · Answer

note that `doWork` is called by that framework and canceled by the framework too do we need doActualWork at all

gildor · Answer

but you should put ` stuff` somewhere isn t it I mean nothing prevents you from inline it in doWork like with any other function I m not sure what is your concern in this case

Tuan Kiet · Answer

assume that I could do it in try doWork got canceled does the catch and final statement get called

gildor · Answer

Of course otherwise it would break error handling and resource handling your case suspend function cancellation based on throwing CancellationException that then handled differently by coroutines runtime but try catch finally contract is still work for suspend functions

Tuan Kiet · Answer

thank for your support

Tuan Kiet · Answer

I appreciate it

gildor · Answer

You are welcome

Dico · Answer

Yes they get called keep in mind that errors other than cancellation might also cancel parent coroutines regardless of catching the error in code

Dico · Answer

Am I right Andrey

gildor · Answer

No suspend function exception do not scope by itself I mean handled exception unhandled will be propagated and may cause scope cancellation or crash the same as any other exception if suspend function throws it will not cancel scope automatically so it s fine to try catch it with no scope cancellation

gildor · Answer

Behavior with scope cancellation if one of child background coroutines throws designed to avoid coroutine resources leaking Without this if you would run 2 parallel jobs in one scope and one of them failed it wouldn t cause cancellation of second one so it will continue invocation but usually result is useless because another part of result is failed