Using livedata coroutine doesn't gets executed

Question

I am using the liveData coroutine as follows. My function takes 3 params - accessing database, make a API call and return the API result

fun <T, A> performGetOperation(
databaseQuery: () -> LiveData<T>,
networkCall: suspend () -> Resource<A>,
saveCallResult: suspend (A) -> Unit
): LiveData<Resource<T>> =
liveData(Dispatchers.IO) {
    emit(Resource.loading())
    val source = databaseQuery.invoke().map { Resource.success(it) }
    emitSource(source)

    val responseStatus = networkCall.invoke()
    if (responseStatus.status == SUCCESS) {
        saveCallResult(responseStatus.data!!)
    } else if (responseStatus.status == ERROR) {
        emit(Resource.error(responseStatus.message!!))
        emitSource(source)
    }
}

I am calling the function as

fun getImages(term: String) = performGetOperation(
    databaseQuery = {
        localDataSource.getAllImages(term) },
    networkCall = {
        remoteDataSource.getImages(term) },
    saveCallResult = {
        val searchedImages = mutableListOf<Images>()
        it.query.pages.values.filter {
            it.thumbnail != null
        }.map {
            searchedImages.add(Images(it.pageid, it.thumbnail!!.source, term))
        }
        localDataSource.insertAll(searchedImages)
    }
)

This is my viewmodel class

class ImagesViewModel @Inject constructor(
private val repository: WikiImageRepository
 ) : ViewModel() {

var images: LiveData<Resource<List<Images>>> = MutableLiveData()

fun fetchImages(search: String) {
    images = repository.getImages(search)
}
}

From my fragment I am observing the variable

viewModel.images?.observe(viewLifecycleOwner, Observer {
        when (it.status) {
            Resource.Status.SUCCESS -> {
                println(it)
            }
            Resource.Status.ERROR ->
                Toast.makeText(requireContext(), it.message, Toast.LENGTH_SHORT).show()

            Resource.Status.LOADING ->
                println("loading")

        }
    })

I have to fetch new data on click of button viewModel.fetchImages(binding.searchEt.text.toString())

Function doesn't gets executed. Is there something I have missed out?

I'm trying to move this file to the data layer but it's impossible, because it's pure java, and LiveData can't be imported. Any solution? — Patrick
– Patrick, Commented Aug 2, 2022 at 9:20
performGetOperation function. I have a module Data, which is a java-library, and I cannot not import this function to this layer for the operations — Patrick
– Patrick, Commented Aug 7, 2022 at 7:48

Santanu Sur · Accepted Answer · 2021-05-12 09:58:26Z

1

The liveData {} extension function returns an instance of MediatorLiveData

liveData { .. emit(T) } // is a MediatorLiveData which needs a observer to execute

Why is the MediatorLiveData addSource block not executed ?

We need to always observe a MediatorLiveData using a liveData observer else the source block is never executed

So to make the liveData block execute just observe the liveData,

performGetOperation(
    databaseQuery = {
        localDataSource.getAllImages(term) },
    networkCall = {
        remoteDataSource.getImages(term) },
    saveCallResult = {
        localDataSource.insertAll(it)
    }
).observe(lifecyleOwner) { // observing the MediatorLiveData is necessary
}

In your case every time you call

images = repository.getImages(search)

a new instance of mediator liveData is created which does not have any observer. The old instance which is observed is ovewritten. You need to observe the new instance of getImages(...) again on button click.

images.observe(lifecycleOwner) { // on button click we observe again.
     // your observer code goes here
}

See MediatorLiveData and this

edited May 12, 2021 at 9:58

answered May 12, 2021 at 9:02

Santanu Sur

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4 Comments

WISHY Over a year ago

I am observing it in my fragment and I have to fetch new data on button click. Still the function doesn't gets executed, I have updated the question. Can you please take a look.

Santanu Sur Over a year ago

but u are never observing perforGetOperation(...) which needs an observer. We need to observe perforGetOperation(...) atleast with a blank observer to make it execute as i have shown in the answer @WISHY

WISHY Over a year ago

I am observing that from getImages fun

Santanu Sur Over a year ago

on button click u need to observe again , its a separate instance now @WISHY it creates a new instance every time

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