Unit Testing ViewModel with Kotlin Coroutines and LiveData

I am Amit Shekhar, Co-Founder @ Outcome School, I have taught and mentored many developers, and their efforts landed them high-paying tech jobs, helped many tech companies in solving their unique problems, and created many open-source libraries being used by top companies. I am passionate about sharing knowledge through open-source, blogs, and videos.

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In this blog, we are going to learn how to write the unit test for ViewModel with Kotlin Coroutines and LiveData that follows a basic MVVM Architecture. We will write the unit-test for the ViewModel which makes a network call and then, validate if our ViewModel is working as expected or not.

I will be using this project for the implementation part. If you have not gone through the project, you should go through it and then come back. The project follows a basic MVVM Architecture for simplicity. You can find the complete code for unit testing mentioned in the blog in the project itself.

We will take the example of SingleNetworkCallViewModel which is present in the project.

Basically, this SingleNetworkCallViewModel is a ViewModel that is associated with SingleNetworkCallActivity which triggers the ViewModel to fetch the list of users to render into the UI. The SingleNetworkCallViewModel, then asks the data layer for the list of users using the ApiHelper. As you can see below, the ViewModel uses Kotlin Coroutines and LiveData.

class SingleNetworkCallViewModel(
    private val apiHelper: ApiHelper,
    private val dbHelper: DatabaseHelper
) : ViewModel() {

    private val uiState = MutableLiveData<UiState<List<ApiUser>>>()

    init {
        fetchUsers()
    }

    private fun fetchUsers() {
        viewModelScope.launch {
            uiState.postValue(UiState.Loading)
            try {
                val usersFromApi = apiHelper.getUsers()
                uiState.postValue(UiState.Success(usersFromApi))
            } catch (e: Exception) {
                uiState.postValue(UiState.Error(e.toString()))
            }
        }
    }

    fun getUiState(): LiveData<UiState<List<ApiUser>>> {
        return uiState
    }

}

Now, we have to write the unit-test for this ViewModel which uses Kotlin Coroutines and LiveData.

First, we need to set up our dependencies for the test like below:

testImplementation 'junit:junit:4.12'
testImplementation "org.mockito:mockito-core:3.3.3"
testImplementation 'androidx.arch.core:core-testing:2.1.0'
testImplementation 'org.jetbrains.kotlinx:kotlinx-coroutines-test:1.3.4'

Make sure to use the latest version which is applicable when you are reading this article. This is important as there are many bug fixes coming along with every release.

Let's move to the test package where we will be writing the unit-test for the ViewModel.

Now, we need to create the TestRule, we will name it as TestCoroutineRule and put that inside the utils package.

@ExperimentalCoroutinesApi
class TestCoroutineRule : TestRule {

    private val testCoroutineDispatcher = TestCoroutineDispatcher()

    private val testCoroutineScope = TestCoroutineScope(testCoroutineDispatcher)

    override fun apply(base: Statement, description: Description?) = object : Statement() {
        @Throws(Throwable::class)
        override fun evaluate() {
            Dispatchers.setMain(testCoroutineDispatcher)

            base.evaluate()

            Dispatchers.resetMain()
            testCoroutineScope.cleanupTestCoroutines()
        }
    }

    fun runBlockingTest(block: suspend TestCoroutineScope.() -> Unit) =
        testCoroutineScope.runBlockingTest { block() }

}

Why the above TestRule?

During the unit-test, it enables the main dispatcher to use TestCoroutineDispatcher.
After the test, it resets and cleanup.

Now, we will create SingleNetworkCallViewModelTest at the appropriate place inside the test package like below:

@ExperimentalCoroutinesApi
@RunWith(MockitoJUnitRunner::class)
class SingleNetworkCallViewModelTest {

    @get:Rule
    val testInstantTaskExecutorRule: TestRule = InstantTaskExecutorRule()

    @get:Rule
    val testCoroutineRule = TestCoroutineRule()

    @Mock
    private lateinit var apiHelper: ApiHelper

    @Mock
    private lateinit var databaseHelper: DatabaseHelper

    @Mock
    private lateinit var uiStateObserver: Observer<UiState<List<ApiUser>>>

    @Before
    fun setUp() {
        // do something if required
    }

    @Test
    fun givenServerResponse200_whenFetch_shouldReturnSuccess() {
        testCoroutineRule.runBlockingTest {
            doReturn(emptyList<ApiUser>())
                .`when`(apiHelper)
                .getUsers()
            val viewModel = SingleNetworkCallViewModel(apiHelper, databaseHelper)
            viewModel.getUiState().observeForever(uiStateObserver)
            verify(apiHelper).getUsers()
            verify(uiStateObserver).onChanged(UiState.Success(emptyList()))
            viewModel.getUiState().removeObserver(uiStateObserver)
        }
    }

    @Test
    fun givenServerResponseError_whenFetch_shouldReturnError() {
        testCoroutineRule.runBlockingTest {
            val errorMessage = "Error Message For You"
            doThrow(RuntimeException(errorMessage))
                .`when`(apiHelper)
                .getUsers()
            val viewModel = SingleNetworkCallViewModel(apiHelper, databaseHelper)
            viewModel.getUiState().observeForever(uiStateObserver)
            verify(apiHelper).getUsers()
            verify(uiStateObserver).onChanged(
                UiState.Error(RuntimeException(errorMessage).toString())
            )
            viewModel.getUiState().removeObserver(uiStateObserver)
        }
    }

    @After
    fun tearDown() {
        // do something if required
    }

}

Here we have used the InstantTaskExecutorRule which is needed to test code with LiveData. If we do not use this, we will get the RuntimeException related to Looper in Android.

Here, We have mocked ApiHelper, DatabaseHelper, and etc and written two tests:

When the server gives 200, it should return success to the UI layer.
When the server gives an error, it should return an error to the UI layer.

In the first one, we have mocked the ApiHelper to return the success with an empty list. Then, we fetch and verify.

Similarly in the second one, we have mocked the ApiHelper to return the error. Then, we fetch and verify.

This way, we are able to write the unit test for ViewModel with Kotlin Coroutines and LiveData that follows a basic MVVM Architecture.

You can run and test the end-to-end implementation in this project.

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That's it for now.

Thanks

Amit Shekhar
Co-Founder @ Outcome School

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