Coroutines - unit testing viewModelScope.launch methods

Question

I am writing unit tests for my viewModel, but having trouble executing the tests. The runBlocking { ... } block doesn't actually wait for the code inside to finish, which is surprising to me.

The test fails because result is null. Why doesn't runBlocking { ... } run the launch block inside the ViewModel in blocking fashion?

I know if I convert it to a async method that returns a Deferred object, then I can get the object by calling await(), or I can return a Job and call join(). But, I'd like to do this by leaving my ViewModel methods as void functions, is there a way to do this?

// MyViewModel.kt  class MyViewModel(application: Application) : AndroidViewModel(application) {      val logic = Logic()     val myLiveData = MutableLiveData<Result>()      fun doSomething() {         viewModelScope.launch(MyDispatchers.Background) {             System.out.println("Calling work")             val result = logic.doWork()             System.out.println("Got result")             myLiveData.postValue(result)             System.out.println("Posted result")         }     }      private class Logic {         suspend fun doWork(): Result? {           return suspendCoroutine { cont ->               Network.getResultAsync(object : Callback<Result> {                       override fun onSuccess(result: Result) {                           cont.resume(result)                       }                       override fun onError(error: Throwable) {                           cont.resumeWithException(error)                       }                   })           }     } } 

// MyViewModelTest.kt  @RunWith(RobolectricTestRunner::class) class MyViewModelTest {      lateinit var viewModel: MyViewModel      @get:Rule     val rule: TestRule = InstantTaskExecutorRule()      @Before     fun init() {         viewModel = MyViewModel(ApplicationProvider.getApplicationContext())     }      @Test     fun testSomething() {         runBlocking {             System.out.println("Called doSomething")             viewModel.doSomething()         }         System.out.println("Getting result value")         val result = viewModel.myLiveData.value         System.out.println("Result value : $result")         assertNotNull(result) // Fails here     } }  

Answer 1

What you need to do is wrap your launching of a coroutine into a block with given dispatcher.

var ui: CoroutineDispatcher = Dispatchers.Main var io: CoroutineDispatcher =  Dispatchers.IO var background: CoroutineDispatcher = Dispatchers.Default  fun ViewModel.uiJob(block: suspend CoroutineScope.() -> Unit): Job {     return viewModelScope.launch(ui) {         block()     } }  fun ViewModel.ioJob(block: suspend CoroutineScope.() -> Unit): Job {     return viewModelScope.launch(io) {         block()     } }  fun ViewModel.backgroundJob(block: suspend CoroutineScope.() -> Unit): Job {     return viewModelScope.launch(background) {         block()     } } 

Notice ui, io and background at the top. Everything here is top-level + extension functions.

Then in viewModel you start your coroutine like this:

uiJob {     when (val result = fetchRubyContributorsUseCase.execute()) {     // ... handle result of suspend fun execute() here          } 

And in test you need to call this method in @Before block:

@ExperimentalCoroutinesApi private fun unconfinifyTestScope() {     ui = Dispatchers.Unconfined     io = Dispatchers.Unconfined     background = Dispatchers.Unconfined } 

(Which is much nicer to add to some base class like BaseViewModelTest)