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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 and Repository Cache Doesn't Work

According to this article Kotlin Flows. What I understand is DataSource emits latestnews list to repository and in repository it's mapping. Then inside of onEach{} method
the mapped list is using for cache function.
Question is I'm trying this method but onEach method isn't sending data to my cache function.
I tried lot's of way but I didn't make it possible. Why this pattern is not working or what is my misstake.
DATASOURCE
class NewsRemoteDataSource(
private val newsApi: NewsApi,
private val refreshIntervalMs: Long = 5000
) {
val latestNews: Flow<List<ArticleHeadline>> = flow {
while(true) {
val latestNews = newsApi.fetchLatestNews()
emit(latestNews) // Emits the result of the request to the flow
delay(refreshIntervalMs) // Suspends the coroutine for some time
}
}
}
// Interface that provides a way to make network requests with suspend functions
interface NewsApi {
suspend fun fetchLatestNews(): List<ArticleHeadline>
}
REPOSITORY
class NewsRepository(
private val newsRemoteDataSource: NewsRemoteDataSource,
private val userData: UserData
) {
val favoriteLatestNews: Flow<List<ArticleHeadline>> =
newsRemoteDataSource.latestNews
.map { news -> news.filter { userData.isFavoriteTopic(it) } }
.onEach { news -> saveInCache(news) }
}
These are what I tried
ExampleDataSource
class DataSource {
val dummyList:List<User> = listOf(User(
firstName = "example", lastName = "example"
),User(firstName = "example1", lastName = "example2"))
fun getData():Flow<List<User>> = flow {
emit(dummyList)
}
}
ExampleRepository
class Repository(val datasource:DataSource,val dao:UserDao) {
suspend fun getdataRepo():Flow<List<User>>{
return datasource.getData().onEach {
addDatabase(it)
}
}
suspend fun addDatabase(list:List<User>){
dao.insertAll(list)
}
}
Example MainActivity
//This is just for testing...
GlobalScope.launch {
Repository(dao = db.userDao(), datasource = DataSource()).getdataRepo()
}
Why addDatabase Function is not working. According to the article it should be working. How can I solve this.
UPDATE
The answer is Flow should be collected somewhere to make all intermediate operations work because they are lazy. (Answered by "bylazy")

My MutableStateFlow doesnt emit when called from suspend function in test

I am trying to write tests for my Repository which provides access to my Room database.
For this, I wrote a Mock Database and a mock DAO:
My Database:
abstract class JoozdlogDatabase protected constructor(): RoomDatabase() {
abstract fun aircraftTypeDao(): AircraftTypeDao
// (...)
}
class MockDatabase: JoozdlogDatabase() {
override fun aircraftTypeDao(): AircraftTypeDao = MockAircraftTypeDao()
}
My DAO:
interface AircraftTypeDao {
#Query("SELECT * FROM AircraftTypeData")
suspend fun requestAllAircraftTypes(): List<AircraftTypeData>
#Query("SELECT * FROM AircraftTypeData")
fun aircraftTypesFlow(): Flow<List<AircraftTypeData>>
// etc etc //
}
class MockAircraftTypeDao: AircraftTypeDao {
private val simulatedDatabase = ArrayList<AircraftTypeData>()
private val simulatedFlow = MutableStateFlow<List<AircraftTypeData>>(listOf(AircraftTypeData("aap", "noot", false, multiEngine = false)))
override suspend fun requestAllAircraftTypes(): List<AircraftTypeData> = simulatedDatabase
override fun aircraftTypesFlow(): Flow<List<AircraftTypeData>> = simulatedFlow
override suspend fun save(vararg aircraftTypeData: AircraftTypeData) {
//println("${this::class.simpleName} Saving ${aircraftTypeData.size} type data")
simulatedDatabase.addAll(aircraftTypeData)
emit()
}
override suspend fun clearDb() {
println("${this::class.simpleName} Clear DB")
simulatedDatabase.clear()
emit()
}
private fun emit(){
println("emit() should emit ${simulatedDatabase.size} items")
simulatedFlow.update { simulatedDatabase.toList() } // also tried: simulatedFlow.value = simulatedDatabase.toList()
println("simulatedFlow.value is now ${simulatedFlow.value.size}")
}
My Test data:
object TestData {
val aircraftTypes = listOf(
AircraftType("Test Aircraft 1 (MP/ME)", "TAC1", multiPilot = true, multiEngine = true),
AircraftType("Test Aircraft 2 (SP/SE)", "TAC2", multiPilot = false, multiEngine = false)
)
}
and my test:
#Test
fun test() {
runTest {
var currentTypesList: List<AircraftType> = emptyList()
val aircraftRepository = AircraftRepository.mock(MockDatabase())
// DispatcherProvider.default() provides UnconfinedTestDispatcher(TestCoroutineScheduler()) for my test.
launch(DispatcherProvider.default()) {
aircraftRepository.aircraftTypesFlow.collect {
println("emitted ${it.size} flights: $it")
currentTypesList = it
}
}
aircraftRepository.replaceAllTypesWith(TestData.aircraftTypes)
delay(500)
println("Done waiting")
assertEquals (2, currentTypesList.size)
}
}
Expected result: Test passed.
received result: java.lang.AssertionError: expected:<2> but was:<1> for the single assert
received output:
emitted 1 flights: [AircraftType(name=aap, shortName=noot, multiPilot=false, multiEngine=false)]
MockAircraftTypeDao Clear DB
emit() should emit 0 items
simulatedFlow.value is now 0
emit() should emit 2 items
simulatedFlow.value is now 2
Done waiting
Now, I have been at this all morning and I just don't get why it won't collect anything but the first value.
Things I tried:
Making a flow object to test my collector -> collector is OK
Accessing the flow item in DAO directly -> Does not work
Setting value of MutableStateFlow with update and with value = -> neither works.
Making a different flow object the is exactly the same but not called from a suspend function: Works.
So, I guess something about the calling suspend function is doing something wrong, but the Flow object is being updated before the delay is over, and it just won't collect.
If anybody is much smarter than me and can explain what I am doing wrong, I would very much appreciate it.

I fixed this by using the suggestion posted here and switching to turbine for all my flow testing needs.

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()
}
}