Exercise: BaseViewModel
Implement a BaseViewModel class that is a base class for view models in MVVM architecture. It should have a scope property that provides a scope for starting coroutines. It should also have an onCleared function that cancels all coroutines started in this scope. In the case of an exception in a coroutine, other coroutines should not be interrupted, but this error should be emitted from exceptions using trySendBlocking function. All coroutines should run on the main thread by default.
abstract class BaseViewModel : ViewModel() {
private val _exceptions = Channel<Throwable>(Channel.UNLIMITED)
val exceptions: Flow<Throwable> = _exceptions.receiveAsFlow()
val scope: CoroutineScope = TODO()
}
Example usage:
class MainViewModel(
private val userRepo: UserRepository,
private val newsRepo: NewsRepository
) : BaseViewModel() {
private val _userData = MutableStateFlow<UserData?>(null)
val userData: StateFlow<UserData?> = _userData
private val _news = MutableStateFlow(emptyList<News>())
val news: StateFlow<List<News>> = _news
init {
scope.launch {
_userData.value = userRepo.getUser()
}
scope.launch {
_news.value = newsRepo.getNews()
.sortedByDescending { it.date }
}
}
}
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/scope/BaseViewModel.kt. You can find there starting code, example usage and unit tests.
Once you are done with the exercise, you can check your solution here.
Playground
import kotlinx.coroutines.*
import kotlinx.coroutines.channels.Channel
import kotlinx.coroutines.channels.trySendBlocking
import kotlinx.coroutines.flow.*
import kotlinx.coroutines.test.runTest
import kotlinx.coroutines.test.setMain
import org.junit.Before
import org.junit.Test
import java.util.*
import kotlin.test.assertEquals
import kotlin.test.assertTrue
abstract class BaseViewModel : ViewModel() {
private val _exceptions = Channel<Throwable>(Channel.UNLIMITED)
val exceptions: Flow<Throwable> = _exceptions.receiveAsFlow()
val scope: CoroutineScope = TODO()
}
class MainViewModel(
private val userRepo: UserRepository,
private val newsRepo: NewsRepository
) : BaseViewModel() {
private val _userData = MutableStateFlow<UserData?>(null)
val userData: StateFlow<UserData?> = _userData
private val _news = MutableStateFlow(emptyList<News>())
val news: StateFlow<List<News>> = _news
init {
scope.launch {
_userData.value = userRepo.getUser()
}
scope.launch {
_news.value = newsRepo.getNews()
.sortedByDescending { it.date }
}
}
}
abstract class ViewModel {
open fun onCleared() {}
}
interface UserRepository {
suspend fun getUser(): UserData
}
interface NewsRepository {
suspend fun getNews(): List<News>
}
data class UserData(val name: String)
data class News(val date: Date)
@Suppress("FunctionName")
class BaseViewModelTests {
private val UI = newSingleThreadContext("UIThread") // Normally it will be Dispatchers.Main
@Before
fun setUp() {
Dispatchers.setMain(UI)
}
@Test
fun `onDestroy cancels all jobs`() = runTest {
var jobs = listOf<Job>()
val viewModel = object : BaseViewModel() {
init {
jobs += scope.launch {
delay(Long.MAX_VALUE)
}
jobs += scope.launch {
delay(Long.MAX_VALUE)
}
}
}
delay(200)
viewModel.onCleared()
delay(200)
assertEquals(listOf(true, true), jobs.map { it.isCancelled })
}
@Test
fun `Coroutines run on main thread`() = runTest {
var threads = listOf<Thread>()
val viewModel = object : BaseViewModel() {
init {
scope.launch {
threads += Thread.currentThread()
}
}
}
delay(100)
viewModel.onCleared()
delay(100)
threads.forEach {
assert(it.name.startsWith("UIThread")) { "We should switch to UI thread, and now we are on ${it.name}" }
}
assert(threads.isNotEmpty())
}
@Test
fun `When a job throws an error, it is handled`(): Unit = runTest {
val error1 = Error()
val error2 = Error()
val vm = object : BaseViewModel() {
init {
scope.launch {
throw error1
}
scope.launch {
throw error2
}
}
}
var exceptions = setOf<Throwable>()
vm.exceptions.onEach { exceptions += it }.launchIn(backgroundScope)
delay(200)
assertEquals(setOf(error1, error2), exceptions)
}
class FakeViewModelForSingleExceptionHandling(val onSecondAction: () -> Unit) : BaseViewModel() {
fun onCreate() {
scope.launch {
delay(100)
throw Error()
}
scope.launch {
delay(200)
onSecondAction()
}
}
}
@Test
fun `Error on a single coroutine, does not cancel others`() = runBlocking {
var called = false
var started = false
object : BaseViewModel() {
init {
scope.launch {
delay(100)
throw Error()
}
scope.launch {
started = true
delay(200)
called = true
}
}
}
delay(300)
assertTrue(started)
assertTrue(called)
}
}
