Hey guys, I have a list `returnedExpertiseFields` ...

# android

[Ofir Bar]

Hey guys, I have a list returnedExpertiseFields inside a ViewModel.
I want to initialize this list using a query to Room.
returnedExpertiseFields gets returned before it is initialized with values from Room, as in the code below:

fun getExpertiseFieldsById(expertiseFieldsIds: IntArray) : List<ExpertiseField>{
    var returnedExpertiseFields = listOf<ExpertiseField>()

    viewModelScope.launch {
        returnedExpertiseFields = mExpertiseFieldFieldsRepository.getExpertiseFieldsFromDatabaseById(expertiseFieldsIds)
    }

    return returnedExpertiseFields
}

returnedExpertiseFields will return an empty list of ExpertiseField .
What can I do to make sure I always wait to Room query to be returned? I thought calling launch is synchronous?
(If you need my repository code I will post it, I just thought it might not be necessary)

Also, If any of you have any tips for debugging when using coroutines I will be glad to hear. I initially tought my Room query is broken. After long debugging I realized I am jumping between breakpoints rather them skipping them.

[Oleg Siboglov]

If I understand your problem correctly then your method should look something like this.

fun getExpertiseFieldsById(expertiseFieldsIds: IntArray) : List<ExpertiseField> = withContext(<http://Dispatches.IO|Dispatches.IO>) {
mExpertiseFieldFieldsRepository.getExpertiseFieldsFromDatabaseById(expertiseFieldsIds)
}

Then you would call this method like so.

fun someMethod() {
viewModelScope.lauch {
val returnedExpertiseFields = getExpertiseFieldsById(ids)
// do something with the fields
}
}

[Brian]

what do you think about using LiveData in the view model and observing that? something like:

val expertiseFields = MutableLiveData<List<ExpertiseField>>()

fun getExpertiseFieldsById(expertiseFieldsIds: IntArray) {
    viewModelScope.launch {
        expertiseFields.postValue(mExpertiseFieldFieldsRepository.getExpertiseFieldsFromDatabaseById(expertiseFieldsIds))
    }
}

and then, from your activity or fragment:

// ... inside on onCreate or onViewCreated, for example
yourViewModel.expertiseFields.observe(this, { expertiseFields ->
    // expertiseFields is your list
})

[Ofir Bar]

Hey @Oleg Siboglov I tried using your code, however I can’t use withContext(IO) inside my function, do you have a suggestion how to restructure that?

Isn’t Room queries run on background threads anyway?

[Ofir Bar]

@Brian
I am thinking about using that, seems like a good Idea, as I just realized that Room queries should be asynchronous
https://stackoverflow.com/questions/47112114/android-room-synchronous-queries-return-null

[Brian]

yeah, LiveData and ViewModel are pretty well set up to handle situations like this so the fragment or activity can observe when the data is returned from whatever async process might happen in the viewModelScope

[Oleg Siboglov]

Room queries are run on whatever thread that started them. Why can’t you use <http://Dispatchers.IO|Dispatchers.IO>?

[Oleg Siboglov]

Meaning you have to start a query on a background thread and not the main thread.

[Ofir Bar]

Hmm I might did something wrong I am checking @Oleg Siboglov

[Ofir Bar]

I combined your two methods into one and I get an error:

private fun someMethod(expertiseFieldsIds: IntArray) : List<ExpertiseField>{
    viewModelScope.launch {
        withContext(<http://Dispatchers.IO|Dispatchers.IO>) {
            mExpertiseFieldFieldsRepository.getExpertiseFieldsFromDatabaseById(expertiseFieldsIds)
        }
    }
}

How can I return value from an IO context?

[Oleg Siboglov]

In regards to the code you posted

viewModelScope.launch {
        returnedExpertiseFields = mExpertiseFieldFieldsRepository.getExpertiseFieldsFromDatabaseById(expertiseFieldsIds)
    }

launch here is consuming an exception that Room would throw warning you about running a query on the main thread.

[Oleg Siboglov]

fun someMethod() {
viewModelScope.launch {
val ids = getIds()
val fields = withContext(<http://Dispatchers.IO|Dispatchers.IO>) {
mExpertiseFieldFieldsRepository.getExpertiseFieldsFromDatabaseById(ids)
}
//do something with fields
}
}

[Ofir Bar]

Got it, thank you Oleg @Oleg Siboglov
One more thing I didn’t mention.
The UI (Some Activity) needs to get these fields

Is is possible to pass the fields to the UI without making the UI observervable?

[Oleg Siboglov]

So I don’t use ViewModel and instead use MVP, so I don’t know all the capabilities of ViewModel. However heres an example where view is a reference to your UI.

fun loadFields() {
CoroutineScope(Dispatchers.Main).launch {
val ids = getIds()
val fields = withContext(<http://Dispatchers.IO|Dispatchers.IO>) {
mExpertiseFieldFieldsRepository.getExpertiseFieldsFromDatabaseById(ids)
}
// This coroutine is suspended until the room query completes. When it does complete, this coroutine is resumed and you can update the UI since you are back to using Dispatchers.Main

view.updateAdapterWithFields(fields)
}
}

[Oleg Siboglov]

You can try looking into this code lab to see how coroutines can be used with Room and ViewModel- https://codelabs.developers.google.com/codelabs/kotlin-coroutines/#2