Kotlin Coroutine Unit Test Flow collection with viewModelScope

Question

I want to test a method of my ViewModel that collects a Flow. Inside the collector a LiveData object is mutated, which I want to check in the end. This is roughly how the setup looks:

//Outside viewmodel
val f = flow { emit("Test") }.flowOn(Dispatchers.IO)

//Inside viewmodel
val liveData = MutableLiveData<String>()

fun action() {
    viewModelScope.launch { privateAction() }
}

suspend fun privateAction() {
    f.collect {
        liveData.value = it
    }
}

When I now call the action() method in my unit test, the test finishes before the flow is collected. This is how the test might look:

@Test
fun example() = runBlockingTest {
    viewModel.action()
    assertEquals(viewModel.liveData.value, "Test")
}

I am using the TestCoroutineDispatcher via this Junit5 extension and also the instant executor extension for LiveData:

    class TestCoroutineDispatcherExtension : BeforeEachCallback, AfterEachCallback, ParameterResolver {
    @SuppressLint("NewApi") // Only used in unit tests
    override fun supportsParameter(parameterContext: ParameterContext?, extensionContext: ExtensionContext?): Boolean {
        return parameterContext?.parameter?.type === testDispatcher.javaClass
    }

    override fun resolveParameter(parameterContext: ParameterContext?, extensionContext: ExtensionContext?): Any {
        return testDispatcher
    }

    private val testDispatcher = TestCoroutineDispatcher()

    override fun beforeEach(context: ExtensionContext?) {
        Dispatchers.setMain(testDispatcher)
    }

    override fun afterEach(context: ExtensionContext?) {
        Dispatchers.resetMain()
        testDispatcher.cleanupTestCoroutines()
    }
}

    class InstantExecutorExtension : BeforeEachCallback, AfterEachCallback {
    override fun beforeEach(context: ExtensionContext?) {
        ArchTaskExecutor.getInstance()
            .setDelegate(object : TaskExecutor() {
                override fun executeOnDiskIO(runnable: Runnable) = runnable.run()
                override fun postToMainThread(runnable: Runnable) = runnable.run()
                override fun isMainThread(): Boolean = true
            })
    }

    override fun afterEach(context: ExtensionContext?) {
        ArchTaskExecutor.getInstance().setDelegate(null)
    }
}

Answer 1

You can try either,

fun action() = viewModelScope.launch { privateAction() }

suspend fun privateAction() {
    f.collect {
        liveData.value = it
    }
}

@Test
fun example() = runBlockingTest {
    viewModel.action().join()
    assertEquals(viewModel.liveData.value, "Test")
}

or

fun action() {
    viewModelScope.launch { privateAction()
}

suspend fun privateAction() {
    f.collect {
        liveData.value = it
    }
}

@Test
fun example() = runBlockingTest {
    viewModel.action()
    viewModel.viewModelScope.coroutineContext[Job]!!.join()
    assertEquals(viewModel.liveData.value, "Test")
}

You could also try this,

suspend fun <T> LiveData<T>.awaitValue(): T? {
    return suspendCoroutine { cont ->
        val observer = object : Observer<T> {
            override fun onChanged(t: T?) {
                removeObserver(this)
                cont.resume(t)
            }
        }
        observeForever(observer)
    }
}

@Test
fun example() = runBlockingTest {
    viewModel.action()
    assertEquals(viewModel.liveData.awaitValue(), "Test")
}