Testing ViewModels with coroutines

Writing tests for ViewModels that use coroutines is trickier than I expected.

The problem is that coroutines run asynchronously, so your test might finish before the coroutine completes. You need to use runBlocking or runTest to wait for coroutines.

@Test
fun testViewModel() = runBlocking {
    val viewModel = MyViewModel(repository)
    viewModel.loadData()
    assertEquals(expectedValue, viewModel.uiState.value)
}

But runBlocking blocks the thread, which can cause issues if you have multiple coroutines or timeouts. Better to use runTest from the coroutines test library:

@Test
fun testViewModel() = runTest {
    val viewModel = MyViewModel(repository)
    viewModel.loadData()
    advanceUntilIdle() // Wait for all coroutines
    assertEquals(expectedValue, viewModel.uiState.value)
}

But here's what got me - if your ViewModel uses viewModelScope, you need to use Dispatchers.setMain to set a test dispatcher, otherwise the coroutines run on the real main thread which can cause issues in tests.

@Before
fun setup() {
    Dispatchers.setMain(UnconfinedTestDispatcher())
}

@After
fun tearDown() {
    Dispatchers.resetMain()
}

Also, if you're testing LiveData or StateFlow, make sure you're observing them properly. LiveData needs an observer to emit values, and StateFlow needs a collector.

One more tip - if your ViewModel has delays or timeouts, use advanceTimeBy() in your tests to control time, rather than actually waiting.

Thanks for reading! Check out more posts on the blog if you'd like.