Exercise: NewsViewModel
You are implementing a news application. You have a NewsRepository that exposes a fetchNews function that returns a Flow of news. You want to implement a NewsViewModel that should start fetching news when initialized. Each fetched news item should be emitted to _newsToShow using the update function (_newsToShow.update { it + news }). When fetching starts, _progressVisible should be set to true using value property; when fetching ends, _progressVisible should be set to false. When an error occurs, it should be sent to _errors using the send function. However, if the error is ApiException, this flow should always start again. Start this flow in the init block in the viewModelScope scope.
Starting code:
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 {
// TODO
}
}
class ApiException : Exception()
interface NewsRepository {
fun fetchNews(): Flow<News>
}
abstract class BaseViewModel {
protected val viewModelScope = CoroutineScope(
Dispatchers.Main.immediate + SupervisorJob()
)
}
This problem can either be solved in the below playground or you can clone kotlin-exercises project and solve it locally. In the project, you can find code template for this exercise in coroutines/flow/NewsViewModel.kt. You can find there starting code and unit tests.
Once you are done with the exercise, you can check your solution here.
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 {
// TODO
}
}
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)
}
}
