Hi guys There this problem with `viewModelScope` from Androi

Question

Hi guys There this problem with `viewModelScope` from Android Architecture Components `ViewModel` that i ve been thinking over lately I run a lot of background operations inside some `ViewModel` because I use its `ViewModelScope` These operations are `use cases` injected into constructor of this `ViewModel` For a better separation of concerns I would like to extract some of these operations in some other class that I would then inject in this `ViewModel` but in order to run these operations I need that `ViewModelScope` Is there a way to provide it to injected components without passing the scope every time in some function signature

Erik · Answer

Make the injected components functions `suspend fun` so the view model can call these from its `viewModelScope` Then there s no need to pass a `CoroutineScope` because these functions can only be called from a coroutine scope Does this fit your case

uli · Answer

A common pattern would be to pass the scope as receiver to every method that creates new coroutines i e make your launching methods extensions on `CoroutineScope`

ubu · Answer

< Erik> unfortunately in our current implementation in order to invoke a use case we need to pass a `CoroutineScope`

ubu · Answer

```abstract class BaseUseCase lt out Type in Params gt private val context CoroutineContext = <http Dispatchers IO|Dispatchers IO> where Type Any abstract suspend fun run params Params Either lt Throwable Type gt open operator fun invoke scope CoroutineScope params Params onResult Either lt Throwable Type gt gt Unit = val job = scope async context run params scope launch onResult job await object None ```

Erik · Answer

Or what Uli says depending on if your components can return quickly or return values

ubu · Answer

and an example of usage ```private fun proceedWithGettingAccount getCurrentAccount invoke viewModelScope BaseUseCase None result gt result either fnL = Timber e it Error while getting account fnR = account gt profile postValue ProfileView name = account name loadAvatarImage account ```

Erik · Answer

The `invoke` operator could be a `suspend fun` too and it can return any type It would then run asynchronously depending on the scope s context but you can switch context e g using `withContext context `

ubu · Answer

< uli> could you give an example

Erik · Answer

```fun CoroutineScope myBuilder = async ```

uli · Answer

```private fun CoroutineScope proceedWithGettingAccount getCurrentAccount invoke BaseUseCase None fun CoroutineScope getCurrentAccount ```

uli · Answer

```viewModelScope proceedWithGettingAccount ```

ubu · Answer

thanks as far as i understand this means that i have to put all these extensions inside `ViewModel` whereas I would like to abstract threading logic in my `Domain` module

Erik · Answer

The extensions can go in your domain

Erik · Answer

The coroutine scope is provided by the view model

ubu · Answer

in < uli> s example there is a use case instance already available because it s injected in vm i actually don t get where i would put this extension in use case implementation

ubu · Answer

``` Use case for starting downloading files see Params class DownloadFile private val downloader Downloader context CoroutineContext BaseUseCase lt Unit Params gt context override suspend fun run params Params = try downloader download url = params url name = params name let Either Right it catch t Throwable Either Left t Params for downloading file property name file name property url url of the file to download data class Params val name String val url Url ``` ```abstract class BaseUseCase lt out Type in Params gt private val context CoroutineContext = <http Dispatchers IO|Dispatchers IO> where Type Any abstract suspend fun run params Params Either lt Throwable Type gt open operator fun invoke scope CoroutineScope params Params onResult Either lt Throwable Type gt gt Unit = val job = scope async context run params scope launch onResult job await object None ``` invocation ```downloadFile invoke scope = viewModelScope params = DownloadFile Params url = urlBuilder file file hash name = file name orEmpty result gt result either fnL = Timber e it Error while trying to download file $file fnR = Timber d Started download file $file ``` as you can see execution context is injected but vm calls method from BaseUseCase class

uli · Answer

``` open operator fun invoke scope CoroutineScope params Params onResult Either lt Throwable Type gt gt Unit``` Would become ``` open operator fun CoroutineScope invoke params Params onResult Either lt Throwable Type gt gt Unit``` You can do that inside `BaseUseCase`

uli · Answer

btw just as a side note why do you use a callback in `invoke` Instead you could also make it suspend and just return `Either`

ubu · Answer

thanks guys

uli · Answer

If you don t want to make `invoke` suspend you can still simplify ``` scope launch val result = withContext context run params onResult result ```

uli · Answer

`async`is primarily for starting multiple jobs in paralel then waiting for all

ubu · Answer

< uli> I used callback because I don t know how to return type not a coroutine job

uli · Answer

```open suspend operator fun invoke params Params Either lt Throwable Type gt = withContext context run params ```

uli · Answer

Use like this ```viewModelScope launch var result = downloadFile invoke params = DownloadFile Params url = urlBuilder file file hash name = file name orEmpty result either fnL = Timber e it Error while trying to download file $file fnR = Timber d Started download file $file ```

uli · Answer

This way you also eliminate the original issue of needing the VM scope in BaseUsecase

uli · Answer

because you are now launching inside the VM

ubu · Answer

thanks a lot < uli> this seems to be what i ve been looking for great

ubu · Answer

this way I also no longer need `CoroutineScope` extensions