Hey guys, I have 3 methods inside a `ViewModel` in the image below, calling methods on some reposito...

# coroutines

[Ofir Bar]

Hey guys, I have 3 methods inside a ViewModel in the image below, calling methods on some repository.
All methods attempt to do the same thing:

Fetch a value from the local DB (a Room DB), suspend the method execution until that value is fetched so the method is guaranteed to return a value.

In each method I added a comment that specifies what issue I have with this method.

What is the safe and correct way to accomplish what I am trying to do?
Slack Conversation

[Evan R.]

launch is a fire-and-forget construct which exits immediately after you invoke it, so it’s not guaranteed that your viewModelScope.launch will run or complete before your function returns in getCityByID. What you probably want to do there is turn getCityByID into a suspend fun and just do something to the effect of:

val city = withContext(viewModelScope) {
    locationRepository.getSingleCityFromDatabaseByID(selectedCityID)
}
return city

if you need to run that inside the viewModelScope and not the caller’s scope. If you’d rather run in the caller’s scope you can simply do:

return locationRepository.getSingleCityFromDatabaseByID(selectedCityID)

as long as you make it a suspend fun.

Same concept applies across your other two functions as well.

[Evan R.]

If you’re okay with blocking a thread you can also use runBlocking(viewModelScope) { …

[Evan R.]

That way you wouldn’t need to make it a suspend fun

[Evan R.]

I’d recommend making it a suspend fun though

[Ofir Bar]

Thanks @Evan R.! your solution is way much cleaner than what I used by far (I used LiveData and observables)

[Evan R.]

Yup, coroutines should just look like blocking code 🙂

[Ofir Bar]

@Evan R.

I can’t call the above code:

suspend fun test(selectedCityId: Int, selectedLanguages: List<Int>, selectedExpertiseFieldId: Int){
    val city = withContext(viewModelScope){
        locationRepository.getSingleCityFromDatabaseByID(selectedCityID)
    }
    return city
}

[Ofir Bar]

message has been deleted

[Evan R.]

Try viewModelScope.context

[dfriehs]

how about just using async instead of launch:

suspend fun getCityById(selectedCityId: Int): City =
    viewModelScope.async {
        locationRepository.getSingleCityFromDatabaseByID(selectedCityID)
    }.await()

[Ofir Bar]

@dfriehs
Seems cleaner, but where did I called launch?
I assume I implicitly did that when calling withContext am I correct?

[dfriehs]

You used launch in your initial example; I am not exactly sure about implementation details of withContext , sadly.

[Ofir Bar]

Sure thank you @dfriehs