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How to do parallel network requests in the repository ? MVVM

I am working on an Android project and at the moment we are doing multiple network calls in a single repository, for example in the PostsRepository class there are multiple endpoints that needs to be called e.g. (/getNewspost /getPostPrice and maybe /get) then it returns a large Post data class back to the ViewModel.
Although it seems fine, the downside of this structure is being unable to do parallel network calls in the repository like the features of launch, or async/await which only exists in the ViewModel.
So question is can this logic be moved to the ViewModel so then i can do multiple network calls ? Or if this logic should stay in the repository how can we do parallel calls in the repo?

You can create coroutine in Repository class also.
Class PostsRepository{
suspend fun callAPIs() : String{
return withContext(Dispatchers.IO) {
val a = async { getPost() }
val b = async { getNews() }
return#withContext a.await() + b.await()
}
}
}

With Clean architecture , you can create a UseCase to handle this behavior
1.first way
Class GetPostsUseCase(private val postRepository : PostRepository){
suspend operator fun invoke():List<Post>{
// we assume that getPosts()
// and getPostsPricies() are also suspend functions
val posts = postRepository.getPosts()
val prices = postRepository.getPostPricies()
return build(posts , prices)
}
private fun build(posts,prices) :List<Post>{
// build your data object here
}
}
/////// OR ////////
Class GetPostsUseCase(private val postRepository : PostRepository){
suspend operator fun invoke():List<Post> = withContext(Dispatchers.IO){
val posts = async{postRepository.getPosts()}
val prices = async { postRepository.getPostPricies() }
posts.await()
prices.await()
return build(posts, prices)
}
private fun build(posts,prices) :List<Post>{
// build your data object here
}
}

You can achieve this by using suspend and withContext
class PostsRepository {
suspend fun fetchPostData(): Post {
return withContext(Dispatchers.IO) {
val fetchA = async { getA() }
val fetchB = async { getB() }
val fetchC = async { getC() }
//More if needed ...
//Then execute waitAll() to get them all as parallel
val (AResult, BResult, CResult) = awaitAll(fetchA, fetchB, fetchC)
//Finally use the result of these fetch when all of them is completed
return#withContext Post(AResult, BResult, CResult)
}
}
}

Kotlin KMM stop coroutine flow with infinite loop properly

I'm building a KMM app for retrieving news.
My app fetches news every 30 seconds and save it in a local database. User must be logged for use it. When user want to logout i need to stop refreshing news and delete the local database.
How do i stop a flow with an infinite loop properly without use static variabile?
I designed the app like follows:
ViewModel (separate for Android and iOS)
UseCase (shared)
Repository (shared)
Data source (shared)
Android Jetpack compose single activity
iOS SwiftUI
Android ViewModel:(iOS use ObservableObject, but logic is the same)
#HiltViewModel
class NewsViewModel #Inject constructor(
private val startFetchingNews: GetNewsUseCase,
private val stopFetchingNews: StopGettingNewsUseCase,
) : ViewModel() {
private val _mutableNewsUiState = MutableStateFlow(NewsState())
val newsUiState: StateFlow<NewsState> get() = _mutableNewsUiState.asStateFlow()
fun onTriggerEvent(action: MapEvents) {
when (action) {
is NewsEvent.GetNews -> {
getNews()
}
is MapEvents.StopNews -> {
//????
}
else -> {
}
}
}
private fun getNews()() {
startFetchingNews().collectCommon(viewModelScope) { result ->
when {
result.error -> {
//update state
}
result.succeeded -> {
//update state
}
}
}
}
}
UseCase:
class GetNewsUseCase(
private val newsRepo: NewsRepoInterface) {
companion object {
private val UPDATE_INTERVAL = 30.seconds
}
operator fun invoke(): CommonFlow<Result<List<News>>> = flow {
while (true) {
emit(Result.loading())
val result = newsRepo.getNews()
if (result.succeeded) {
// emit result
} else {
//emit error
}
delay(UPDATE_INTERVAL)
}
}.asCommonFlow()
}
Repository:
class NewsRepository(
private val sourceNews: SourceNews,
private val cacheNews: CacheNews) : NewsRepoInterface {
override suspend fun getNews(): Result<List<News>> {
val news = sourceNews.fetchNews()
//.....
cacheNews.insert(news) //could be a lot of news
return Result.data(cacheNews.selectAll())
}
}
Flow extension functions:
fun <T> Flow<T>.asCommonFlow(): CommonFlow<T> = CommonFlow(this)
class CommonFlow<T>(private val origin: Flow<T>) : Flow<T> by origin {
fun collectCommon(
coroutineScope: CoroutineScope? = null, // 'viewModelScope' on Android and 'nil' on iOS
callback: (T) -> Unit, // callback on each emission
) {
onEach {
callback(it)
}.launchIn(coroutineScope ?: CoroutineScope(Dispatchers.Main))
}
}
I tried to move the while loop inside repository, so maybe i can break the loop with a singleton repository, but then i must change the getNews method to flow and collect inside GetNewsUseCase (so a flow inside another flow).
Thanks for helping!

When you call launchIn on a Flow, it returns a Job. Hang on to a reference to this Job in a property, and you can call cancel() on it when you want to stop collecting it.
I don't see the point of the CommonFlow class. You could simply write collectCommon as an extension function of Flow directly.
fun <T> Flow<T>.collectCommon(
coroutineScope: CoroutineScope? = null, // 'viewModelScope' on Android and 'nil' on iOS
callback: (T) -> Unit, // callback on each emission
): Job {
return onEach {
callback(it)
}.launchIn(coroutineScope ?: CoroutineScope(Dispatchers.Main))
}
// ...
private var fetchNewsJob: Job? = null
private fun getNews()() {
fetchNewsJob = startFetchingNews().collectCommon(viewModelScope) { result ->
when {
result.error -> {
//update state
}
result.succeeded -> {
//update state
}
}
}
}
In my opinion, collectCommon should be eliminated entirely because all it does is obfuscate your code a little bit. It saves only one line of code at the expense of clarity. It's kind of an antipattern to create a CoroutineScope whose reference you do not keep so you can manage the coroutines running in it--might as well use GlobalScope instead to be clear you don't intend to manage the scope lifecycle so it becomes clear you must manually cancel the Job, not just in the case of the news source change, but also when the UI it's associated with goes out of scope.

How can I update the flow source data in android coroutine?

I am developing an android app using Coroutine.
Because I have an experience with the Rx, I can understand Coroutine Flow's concept.
But I don't know about emitting new data to the existing flow.
In Rx, I can use BehaviorSubject and it's onNext() function.
Producer:
class ProfileRepository(
private val profileDao: ProfileDao
) {
init {
val profile = profileDao.getMyProfile()
myProfile.onNext(profile)
}
val myProfile: PublishSubject<Profile> = BehaviorSubject.create()
fun updateMyProfile(profile: Profile) {
myProfile.onNext(profile)
}
}
Subscriber:
class ProfileViewModel(
private val profileRepo: ProfileRepository
) {
val myProfile = MutableLiveData<Profile>()
fun loadMyProfile() {
profileRepo.subscribe {
myProfile.postValue(it)
}
}
}
And other UI (like Dialog) can update the source data like:
class SignInDialog(
private val profileRepo: ProfileRepository
) {
fun onSignInSuccess() {
profileRepo.updateMyProfile(profile)
}
}
I want to do this using Coroutine Flow, Is this possible on Flow?

You are using BehaviorSubject, which emits the most recently item at the time of subscription and then continues the sequence until complete. So basically it is a hot stream of data.
The similar behavior has SharedFlow:
A hotFlow that shares emitted values among all its collectors in a broadcast fashion, so that all collectors get all emitted values. A shared flow is called hot because its active instance exists independently of the presence of collectors.
For your case it will look something like the following:
class ProfileRepository(
private val profileDao: ProfileDao
) {
init {
val profile = profileDao.getMyProfile()
_myProfile.tryEmit(profile)
}
private val _myProfile = MutableSharedFlow<String>(replay = 1, extraBufferCapacity = 64)
val myProfile: SharedFlow<Profile> = _myProfile
fun updateMyProfile(profile: Profile) {
_myProfile.tryEmit(profile)
}
}
class ProfileViewModel(
private val profileRepo: ProfileRepository
) {
val myProfile = MutableLiveData<Profile>()
fun loadMyProfile() = viewModelScope.launch {
profileRepo.myProfile.collect {
myProfile.postValue(it)
}
}
}
viewModelScope - Any coroutine launched in this scope is automatically canceled if the ViewModel is cleared. For ViewModelScope, use
androidx.lifecycle:lifecycle-viewmodel-ktx:2.4.0 or higher.

Kotlin Flow: emitAll is never collected

I am trying to write a UnitTest for the kotlin-version of networkBoundResource that can be found on serveral sources with several features
Here is my version of it with marker-comments for the following question.
inline fun <ResultType, RequestType> networkBoundResource(
...
coroutineDispatcher: CoroutineDispatcher
) = flow {
emit(Resource.loading(null)) // emit works!
val data = queryDatabase().firstOrNull()
val flow = if (shouldFetch(data)) {
emit(Resource.loading(data)) // emit works!
try {
saveFetchResult(fetch())
query().map { Resource.success(it) }
} catch (throwable: Throwable) {
onFetchFailed(throwable)
query().map { Resource.error(throwable.toString(), it) }
}
} else {
query().map { Resource.success(it) }
}
emitAll(flow) // emitAll does not work!
}.catch { exception ->
emit(Resource.error("An error occurred while fetching data! $exception", null))
}.flowOn(coroutineDispatcher)
This is one of my UnitTests for this code. The code is edited a bit to focus on my question:
#get:Rule
val testCoroutineRule = TestCoroutineRule()
private val coroutineDispatcher = TestCoroutineDispatcher()
#Test
fun networkBoundResource_noCachedData_shouldMakeNetworkCallAndStoreUserInDatabase() = testCoroutineRule.runBlockingTest {
...
// When getAuthToken is called
val result = networkBoundResource(..., coroutineDispatcher).toList()
result.forEach {
println(it)
}
}
The problem is that println(it) is only printing the Resource.loading(null) emissions. But if you have a look at the last line of the flow {} block, you will see that there should be another emission of the val flow. But this emission never arrives in my UnitTest. Why?

I'm not too sure of the complete behaviour, but essentially you want to get a resource, and current flow is all lumped into the FlowCollector<T> which makes it harder to reason and test.
I have never used or seen the Google code before and if I'm honest only glanced at it. My main take away was it had poor encapsulation and seems to break separations of concern - it manages the resource state, and handles all io work one one class. I'd prefer to have 2 different classes to separate that logic and allows for easier testing.
As simple pseudo code I would do something like this :
class ResourceRepository {
suspend fun get(r : Request) : Resource {
// abstract implementation details network request and io
// - this function should only fulfill the request
// can now be mocked for testing
delay(3_000)
return Resource.success(Any())
}
}
data class Request(val a : String)
sealed class Resource {
companion object {
val loading : Resource get() = Loading
fun success(a : Any) : Resource = Success(a)
fun error(t: Throwable) : Resource = Error(t)
}
object Loading : Resource()
data class Success(val a : Any) : Resource()
data class Error(val t : Throwable) : Resource()
}
fun resourceFromRequest(r : Request) : Flow<Resource> =
flow { emit(resourceRepository.get(r)) }
.onStart { emit(Resource.loading) }
.catch { emit(Resource.error(it)) }
This allows you to massively simplify the actual testing of the resourceFromRequest() function as you only have to mock the repository and one method. This allows you to abstract and deal with the networking and io work elsewhere, independently which again can be tested in isolation.

As #MarkKeen suggested, I now created my own implementation and it works quite well. Compared to the code that is going around on SO, this version now injects the coroutineDispatcher for easier testing, it lets flow take care of error handling, it does not contain nested flows and is imho easier to read and understand, too. There is still the side-effect of storing updated data to the database, but I am too tired now to tackle this.
import kotlinx.coroutines.CoroutineDispatcher
import kotlinx.coroutines.flow.*
inline fun <ResultType, RequestType> networkBoundResource(
crossinline query: () -> Flow<ResultType?>,
crossinline fetch: suspend () -> RequestType,
crossinline saveFetchResult: suspend (RequestType) -> Unit,
crossinline shouldFetch: (ResultType?) -> Boolean = { true },
coroutineDispatcher: CoroutineDispatcher
) = flow<Resource<ResultType>> {
// check for data in database
val data = query().firstOrNull()
if (data != null) {
// data is not null -> update loading status
emit(Resource.loading(data))
}
if (shouldFetch(data)) {
// Need to fetch data -> call backend
val fetchResult = fetch()
// got data from backend, store it in database
saveFetchResult(fetchResult)
}
// load updated data from database (must not return null anymore)
val updatedData = query().first()
// emit updated data
emit(Resource.success(updatedData))
}.onStart {
emit(Resource.loading(null))
}.catch { exception ->
emit(Resource.error("An error occurred while fetching data! $exception", null))
}.flowOn(coroutineDispatcher)
One possible UnitTest for this inline fun, which is used in an AuthRepsitory:
#ExperimentalCoroutinesApi
class AuthRepositoryTest {
companion object {
const val FAKE_ID_TOKEN = "FAkE_ID_TOKEN"
}
#get:Rule
val testCoroutineRule = TestCoroutineRule()
private val coroutineDispatcher = TestCoroutineDispatcher()
private val userDaoFake = spyk<UserDaoFake>()
private val mockApiService = mockk<MyApi>()
private val sut = AuthRepository(
userDaoFake, mockApiService, coroutineDispatcher
)
#Before
fun beforeEachTest() {
userDaoFake.clear()
}
#Test
fun getAuthToken_noCachedData_shouldMakeNetworkCallAndStoreUserInDatabase() = testCoroutineRule.runBlockingTest {
// Given an empty database
coEvery { mockApiService.getUser(any()) } returns NetworkResponse.Success(UserFakes.getNetworkUser(), null, HttpURLConnection.HTTP_OK)
// When getAuthToken is called
val result = sut.getAuthToken(FAKE_ID_TOKEN).toList()
coVerifyOrder {
// Then first try to fetch data from the DB
userDaoFake.get()
// Then fetch the User from the API
mockApiService.getUser(FAKE_ID_TOKEN)
// Then insert the user into the DB
userDaoFake.insert(any())
// Finally return the inserted user from the DB
userDaoFake.get()
}
assertThat(result).containsExactly(
Resource.loading(null),
Resource.success(UserFakes.getAppUser())
).inOrder()
}
}

How to use Fuel with coroutines in Kotlin?

I want to get an API request and save request's data to a DB. Also want to return the data (that is written to DB). I know, this is possible in RxJava, but now I write in Kotlin coroutines, currently use Fuel instead of Retrofit (but a difference is not so large). I read How to use Fuel with a Kotlin coroutine, but don't understand it.
How to write a coroutine and methods?
UPDATE
Say, we have a Java and Retrofit, RxJava. Then we can write a code.
RegionResponse:
#AutoValue
public abstract class RegionResponse {
#SerializedName("id")
public abstract Integer id;
#SerializedName("name")
public abstract String name;
#SerializedName("countryId")
public abstract Integer countryId();
public static RegionResponse create(int id, String name, int countryId) {
....
}
...
}
Region:
data class Region(
val id: Int,
val name: String,
val countryId: Int)
Network:
public Single<List<RegionResponse>> getRegions() {
return api.getRegions();
// #GET("/regions")
// Single<List<RegionResponse>> getRegions();
}
RegionRepository:
fun getRegion(countryId: Int): Single<Region> {
val dbSource = db.getRegion(countryId)
val lazyApiSource = Single.defer { api.regions }
.flattenAsFlowable { it }
.map { apiMapper.map(it) }
.toList()
.doOnSuccess { db.updateRegions(it) }
.flattenAsFlowable { it }
.filter({ it.countryId == countryId })
.singleOrError()
return dbSource
.map { dbMapper.map(it) }
.switchIfEmpty(lazyApiSource)
}
RegionInteractor:
class RegionInteractor(
private val repo: RegionRepository,
private val prefsRepository: PrefsRepository) {
fun getRegion(): Single<Region> {
return Single.fromCallable { prefsRepository.countryId }
.flatMap { repo.getRegion(it) }
.subscribeOn(Schedulers.io())
}
}

Let's look at it layer by layer.
First, your RegionResponse and Region are totally fine for this use case, as far as I can see, so we won't touch them at all.
Your network layer is written in Java, so we'll assume it always expects synchronous behavior, and won't touch it either.
So, we start with the repo:
fun getRegion(countryId: Int) = async {
val regionFromDb = db.getRegion(countryId)
if (regionFromDb == null) {
return apiMapper.map(api.regions).
filter({ it.countryId == countryId }).
first().
also {
db.updateRegions(it)
}
}
return dbMapper.map(regionFromDb)
}
Remember that I don't have your code, so maybe the details will differ a bit. But the general idea with coroutines, is that you launch them with async() in case they need to return the result, and then write your code as if you were in the perfect world where you don't need to concern yourself with concurrency.
Now to the interactor:
class RegionInteractor(
private val repo: RegionRepository,
private val prefsRepository: PrefsRepository) {
fun getRegion() = withContext(Schedulers.io().asCoroutineDispatcher()) {
val countryId = prefsRepository.countryId
return repo.getRegion(countryId).await()
}
}
You need something to convert from asynchronous code back to synchronous one. And for that you need some kind of thread pool to execute on. Here we use thread pool from Rx, but if you want to use some other pool, so do.

After researching How to use Fuel with a Kotlin coroutine, Fuel coroutines and https://github.com/kittinunf/Fuel/ (looked for awaitStringResponse), I made another solution. Assume that you have Kotlin 1.3 with coroutines 1.0.0 and Fuel 1.16.0.
We have to avoid asynhronous requests with callbacks and make synchronous (every request in it's coroutine). Say, we want to show a country name by it's code.
// POST-request to a server with country id.
fun getCountry(countryId: Int): Request =
"map/country/"
.httpPost(listOf("country_id" to countryId))
.addJsonHeader()
// Adding headers to the request, if needed.
private fun Request.addJsonHeader(): Request =
header("Content-Type" to "application/json",
"Accept" to "application/json")
It gives a JSON:
{
"country": {
"name": "France"
}
}
To decode the JSON response we have to write a model class:
data class CountryResponse(
val country: Country,
val errors: ErrorsResponse?
) {
data class Country(
val name: String
)
// If the server prints errors.
data class ErrorsResponse(val message: String?)
// Needed for awaitObjectResponse, awaitObject, etc.
class Deserializer : ResponseDeserializable<CountryResponse> {
override fun deserialize(content: String) =
Gson().fromJson(content, CountryResponse::class.java)
}
}
Then we should create a UseCase or Interactor to receive a result synchronously:
suspend fun getCountry(countryId: Int): Result<CountryResponse, FuelError> =
api.getCountry(countryId).awaitObjectResponse(CountryResponse.Deserializer()).third
I use third to access response data. But if you wish to check for a HTTP error code != 200, remove third and later get all three variables (as Triple variable).
Now you can write a method to print the country name.
private fun showLocation(
useCase: UseCaseImpl,
countryId: Int,
regionId: Int,
cityId: Int
) {
GlobalScope.launch(Dispatchers.IO) {
// Titles of country, region, city.
var country: String? = null
var region: String? = null
var city: String? = null
val countryTask = GlobalScope.async {
val result = useCase.getCountry(countryId)
// Receive a name of the country if it exists.
result.fold({ response -> country = response.country.name }
, { fuelError -> fuelError.message })
}
}
val regionTask = GlobalScope.async {
val result = useCase.getRegion(regionId)
result.fold({ response -> region = response.region?.name }
, { fuelError -> fuelError.message })
}
val cityTask = GlobalScope.async {
val result = useCase.getCity(cityId)
result.fold({ response -> city = response.city?.name }
, { fuelError -> fuelError.message })
}
// Wait for three requests to execute.
countryTask.await()
regionTask.await()
cityTask.await()
// Now update UI.
GlobalScope.launch(Dispatchers.Main) {
updateLocation(country, region, city)
}
}
}
In build.gradle:
ext {
fuelVersion = "1.16.0"
}
dependencies {
...
implementation 'org.jetbrains.kotlinx:kotlinx-coroutines-android:1.0.0'
// Fuel.
//for JVM
implementation "com.github.kittinunf.fuel:fuel:${fuelVersion}"
//for Android
implementation "com.github.kittinunf.fuel:fuel-android:${fuelVersion}"
//for Gson support
implementation "com.github.kittinunf.fuel:fuel-gson:${fuelVersion}"
//for Coroutines
implementation "com.github.kittinunf.fuel:fuel-coroutines:${fuelVersion}"
// Gson.
implementation 'com.google.code.gson:gson:2.8.5'
}
If you want to work with coroutines and Retrofit, please, read https://medium.com/exploring-android/android-networking-with-coroutines-and-retrofit-a2f20dd40a83 (or https://habr.com/post/428994/ in Russian).

You should be able to significantly simplify your code. Declare your use case similar to the following:
class UseCaseImpl {
suspend fun getCountry(countryId: Int): Country =
api.getCountry(countryId).awaitObject(CountryResponse.Deserializer()).country
suspend fun getRegion(regionId: Int): Region =
api.getRegion(regionId).awaitObject(RegionResponse.Deserializer()).region
suspend fun getCity(countryId: Int): City=
api.getCity(countryId).awaitObject(CityResponse.Deserializer()).city
}
Now you can write your showLocation function like this:
private fun showLocation(
useCase: UseCaseImpl,
countryId: Int,
regionId: Int,
cityId: Int
) {
GlobalScope.launch(Dispatchers.Main) {
val countryTask = async { useCase.getCountry(countryId) }
val regionTask = async { useCase.getRegion(regionId) }
val cityTask = async { useCase.getCity(cityId) }
updateLocation(countryTask.await(), regionTask.await(), cityTask.await())
}
}
You have no need to launch in the IO dispatcher because your network requests are non-blocking.
I must also note that you shouldn't launch in the GlobalScope. Define a proper coroutine scope that aligns its lifetime with the lifetime of the Android activity or whatever else its parent is.