ViewModel Unit testing multiple view states with LiveData, Coroutines and MockK

Question

I have a function in ViewModel with 2 states, first state is always LOADING, second state depends on result of api or db interactions.

This is the function

fun getPostWithSuspend() {

    myCoroutineScope.launch {

        // Set current state to LOADING
        _postStateWithSuspend.value = ViewState(LOADING)

        val result = postsUseCase.getPosts()

        // Check and assign result to UI
        val resultViewState = if (result.status == SUCCESS) {
            ViewState(SUCCESS, data = result.data?.get(0)?.title)
        } else {
            ViewState(ERROR, error = result.error)
        }

        _postStateWithSuspend.value = resultViewState
    }
}

And no error, test works fine for checking final result of ERROR or SUCCESS

   @Test
    fun `Given DataResult Error returned from useCase, should result error`() =
        testCoroutineRule.runBlockingTest {

            // GIVEN
            coEvery {
                useCase.getPosts()
            } returns DataResult.Error(Exception("Network error occurred."))

            // WHEN
            viewModel.getPostWithSuspend()

            // THEN
            val expected = viewModel.postStateWithSuspend.getOrAwaitMultipleValues(dataCount = 2)

//            Truth.assertThat("Network error occurred.").isEqualTo(expected?.error?.message)
//            Truth.assertThat(expected?.error).isInstanceOf(Exception::class.java)
            coVerify(atMost = 1) { useCase.getPosts() }
        }

But i couldn't find a way to test whether LOADING state has occurred or not, so i modified existing extension function to

fun <T> LiveData<T>.getOrAwaitMultipleValues(
    time: Long = 2,
    dataCount: Int = 1,
    timeUnit: TimeUnit = TimeUnit.SECONDS,
    afterObserve: () -> Unit = {}
): List<T?> {

    val data = mutableListOf<T?>()
    val latch = CountDownLatch(dataCount)

    val observer = object : Observer<T> {
        override fun onChanged(o: T?) {
            data.add(o)
            latch.countDown()
            [email protected](this)
        }
    }
    this.observeForever(observer)

    afterObserve.invoke()

    // Don't wait indefinitely if the LiveData is not set.
    if (!latch.await(time, timeUnit)) {
        this.removeObserver(observer)
        throw TimeoutException("LiveData value was never set.")
    }

    @Suppress("UNCHECKED_CAST")
    return data.toList()
}

To add data to a list when LiveData changes and store states in that list but it never returns LOADING state because it happens before observe starts. Is there a way to test multiple values of LiveData?

Answer 1

• Observes [LiveData] and captures latest value and all subsequent values, returning them in ordered list.

inline fun <reified T > LiveData<T>.captureValues(): List<T?> {
    val mockObserver = mockk<Observer<T>>()
    val list = mutableListOf<T?>()
    every { mockObserver.onChanged(captureNullable(list))} just runs
    this.observeForever(mockObserver)
    return list
}

Answer 2

I assume you are using mockk library

1. First you need to create observer object

    val observer = mockk<Observer<ViewState<YourObject>>> { every { onChanged(any()) } just Runs }
   

2. Observe your livedata using previous observer object

   viewModel.postStateWithSuspend.observeForever(observer)
   

3. Call your getPostWithSuspend() function

   viewModel.getPostWithSuspend()
   

4. Verify it

    verifySequence {
           observer.onChanged(yourExpectedValue1)
           observer.onChanged(yourExpectedValue2)
       }