Solution: NewsViewModel
class NewsViewModel(
newsRepository: NewsRepository,
) : BaseViewModel() {
private val _progressVisible = MutableStateFlow(false)
val progressVisible = _progressVisible.asStateFlow()
private val _newsToShow = MutableStateFlow(emptyList<News>())
val newsToShow = _newsToShow.asStateFlow()
private val _errors = Channel<Throwable>(Channel.UNLIMITED)
val errors = _errors.receiveAsFlow()
init {
newsRepository.fetchNews()
.retry { error -> error is ApiException }
.catch { error -> _errors.send(error) }
.onStart { _progressVisible.value = true }
.onCompletion { _progressVisible.value = false }
.onEach { news -> _newsToShow.update { it + news } }
.launchIn(viewModelScope)
}
}
Example solution in playground
import junit.framework.TestCase.assertEquals
import kotlinx.coroutines.CoroutineScope
import kotlinx.coroutines.Dispatchers
import kotlinx.coroutines.SupervisorJob
import kotlinx.coroutines.channels.Channel
import kotlinx.coroutines.delay
import kotlinx.coroutines.flow.*
import kotlinx.coroutines.test.StandardTestDispatcher
import kotlinx.coroutines.test.TestDispatcher
import kotlinx.coroutines.test.resetMain
import kotlinx.coroutines.test.setMain
import org.junit.After
import org.junit.Before
import org.junit.Test
class NewsViewModel(
newsRepository: NewsRepository,
) : BaseViewModel() {
private val _progressVisible = MutableStateFlow(false)
val progressVisible = _progressVisible.asStateFlow()
private val _newsToShow = MutableStateFlow(emptyList<News>())
val newsToShow = _newsToShow.asStateFlow()
private val _errors = Channel<Throwable>(Channel.UNLIMITED)
val errors = _errors.receiveAsFlow()
init {
newsRepository.fetchNews()
.retry { error -> error is ApiException }
.catch { error -> _errors.send(error) }
.onStart { _progressVisible.value = true }
.onCompletion { _progressVisible.value = false }
.onEach { news -> _newsToShow.update { it + news } }
.launchIn(viewModelScope)
}
}
class ApiException : Exception()
interface NewsRepository {
fun fetchNews(): Flow<News>
}
abstract class BaseViewModel {
protected val viewModelScope = CoroutineScope(
Dispatchers.Main.immediate + SupervisorJob()
)
}
data class News(
val title: String,
val description: String,
val imageUrl: String,
val url: String,
)
class FakeNewsRepository : NewsRepository {
val newsList = List(100) { News("Title $it", "Description $it", "ImageUrl $it", "Url $it") }
var fetchNewsStartDelay = 0L
var fetchNewsDelay = 0L
val failWith = mutableListOf<Throwable>()
override fun fetchNews(): Flow<News> = flow {
delay(fetchNewsStartDelay)
failWith.removeFirstOrNull()?.let { throw it }
newsList.forEach {
delay(fetchNewsDelay)
emit(it)
}
}
}
class NewsViewModelTest {
lateinit var dispatcher: TestDispatcher
lateinit var newsRepository: FakeNewsRepository
@Before
fun setUp() {
newsRepository = FakeNewsRepository()
dispatcher = StandardTestDispatcher()
Dispatchers.setMain(dispatcher)
}
@After
fun tearDown() {
Dispatchers.resetMain()
}
@Test
fun `should show all news`() {
val viewModel = NewsViewModel(newsRepository)
newsRepository.fetchNewsDelay = 1000
dispatcher.scheduler.advanceUntilIdle()
assertEquals(newsRepository.newsList, viewModel.newsToShow.value)
assertEquals(newsRepository.newsList.size * newsRepository.fetchNewsDelay, dispatcher.scheduler.currentTime)
}
@Test
fun `should show progress bar when loading news`() {
val viewModel = NewsViewModel(newsRepository)
newsRepository.fetchNewsDelay = 1000
var progressChanges = listOf<Pair<Long, Boolean>>()
viewModel.progressVisible.onEach {
progressChanges += dispatcher.scheduler.currentTime to it
}.launchIn(CoroutineScope(dispatcher))
dispatcher.scheduler.advanceUntilIdle()
assertEquals(
listOf(0L to true, newsRepository.newsList.size * newsRepository.fetchNewsDelay to false),
progressChanges
)
}
@Test
fun `should retry API exceptions`() {
val exceptionsNum = 10
newsRepository.failWith.addAll(List(exceptionsNum) { ApiException() })
newsRepository.fetchNewsStartDelay = 1000
val viewModel = NewsViewModel(newsRepository)
var errors = listOf<Throwable>()
viewModel.errors.onEach {
errors += it
}.launchIn(CoroutineScope(dispatcher))
var newsShown = listOf<News>()
viewModel.newsToShow.onEach {
newsShown += it
}.launchIn(CoroutineScope(dispatcher))
dispatcher.scheduler.advanceUntilIdle()
assertEquals(0, errors.size)
assertEquals(newsRepository.newsList, newsShown)
assertEquals(newsRepository.fetchNewsStartDelay * (exceptionsNum + 1), dispatcher.scheduler.currentTime)
}
@Test
fun `should catch exceptions`() {
val exception = Exception()
newsRepository.failWith.add(exception)
newsRepository.fetchNewsStartDelay = 1000
val viewModel = NewsViewModel(newsRepository)
var errors = listOf<Throwable>()
viewModel.errors.onEach {
errors += it
}.launchIn(CoroutineScope(dispatcher))
dispatcher.scheduler.advanceUntilIdle()
assertEquals(listOf(exception), errors)
assertEquals(false, viewModel.progressVisible.value)
assertEquals(1000, dispatcher.scheduler.currentTime)
}
}
