Android Unit testing dataStore

Question

I'm trying to write a test for my view model that gets the data from datastore but I can't figure it out. this is my data store implementation. I save the user data such as email and token when user is sign up or sign in :

    class FlowAuthenticationDataStore(context: Context) : AuthenticationDataStore {
    private val dataStore = context.createDataStore(name = "user_auth")
    private val userEmailKey = preferencesKey<String>(name = "USER_EMAIL")
    private val userTokenKey = preferencesKey<String>(name = "USER_TOKEN")
    private val userNameKey = preferencesKey<String>(name = "USER_NAME")
    private val userIsLoginKey = preferencesKey<Boolean>(name = "USER_IS_LOGIN")

    override suspend fun updateUser(userDataStore: UserDataStoreModel) {
        dataStore.edit {
            it[userEmailKey] = userDataStore.email
            it[userTokenKey] = userDataStore.token
            it[userNameKey] = userDataStore.name
            it[userIsLoginKey] = userDataStore.isLogin
        }
    }

    override fun observeUser(): Flow<UserDataStoreModel> {
        return dataStore.data.catch {
            if (it is IOException) {
                emit(emptyPreferences())
            } else {
                throw it
            }
        }.map {
            UserDataStoreModel(
                it[userIsLoginKey]!!,
                it[userNameKey]!!,
                it[userEmailKey]!!,
                it[userTokenKey]!!
            )
        }
    }
}

and this is my view model. I observe the user data store and if its success and has a data then I update my live data. If there is not data and user is first time to register then my live data is equal to default value from data class :

    class SplashScreenViewModel(
    private val flowOnBoardingDataStore: FlowAuthenticationDataStore,
    private val contextProvider: CoroutineContextProvider,
) :
    ViewModel() {
    private val _submitState = MutableLiveData<UserDataStoreModel>()
    val submitState: LiveData<UserDataStoreModel> = _submitState

    fun checkUserLogin() {
        viewModelScope.launch {
            kotlin.runCatching {
                withContext(contextProvider.io) {
                        flowOnBoardingDataStore.observeUser().collect {
                            _submitState.value = it
                        }
                }
            }.onFailure {
                _submitState.value = UserDataStoreModel()
            }
        }

    }
}

and this is my test class:

    @ExperimentalCoroutinesApi
class SplashScreenViewModelTest {
    private val dispatcher = TestCoroutineDispatcher()

    @get:Rule
    val rule = InstantTaskExecutorRule()

    @get:Rule
    val coroutineTestRule = CoroutineTestRule(dispatcher)

    @RelaxedMockK
    lateinit var flowOnBoardingDataStore: FlowAuthenticationDataStore

    private fun createViewModel()=SplashScreenViewModel(flowOnBoardingDataStore,
        CoroutineContextProvider(dispatcher,dispatcher)
    )

    @Before
    fun setup() {
        MockKAnnotations.init(this)
    }

    @After
    fun tearDown() {
        unmockkAll()
    }

    @Test
    fun `when user is already sign in, then state should return model`()=dispatcher.runBlockingTest {
        val viewModel=createViewModel()
        val userDataStoreModel= UserDataStoreModel(true,"test","test","test")
        flowOnBoardingDataStore.updateUser(userDataStoreModel)
        viewModel.checkUserLogin()
        assertEquals(userDataStoreModel,viewModel.submitState.value)
    }
}

This is the result of my test function:

    junit.framework.AssertionFailedError: 
Expected :UserDataStoreModel(isLogin=true, name=test, email=test, token=test)
Actual   :null

Answer 1

I find the solution and I posted here Incase anybody needs it. The solution is using coEvery to return a fake data with flowOf from the usecase( you don't need to use flowOf , its based on your return data from your use case, in my case it's return a flow):

     @Test
    fun `when user is already sign in, then state should return user data`()=dispatcher.runBlockingTest {
        val userData=UserDataStoreModel(true, Name,
            Email,"","")
        coEvery { authenticationDataStore.observeUser() }returns flowOf(userData)
        val viewModel=createViewModel()
        viewModel.checkUserLogin()
        assertEquals(userData,viewModel.submitState.value)
    }

This is the full test class:

    @ExperimentalCoroutinesApi
class SplashScreenViewModelTest {
    private val dispatcher = TestCoroutineDispatcher()

    @get:Rule
    val rule = InstantTaskExecutorRule()

    @get:Rule
    val coroutineTestRule = CoroutineTestRule(dispatcher)

    @RelaxedMockK
    lateinit var authenticationDataStore: AuthenticationDataStore

    private fun createViewModel()=SplashScreenViewModel(authenticationDataStore,
        CoroutineContextProvider(dispatcher,dispatcher)
    )

    @Before
    fun setup() {
        MockKAnnotations.init(this)
    }

    @After
    fun tearDown() {
        unmockkAll()
    }

    @Test
    fun `when user is already sign in, then state should return user data`()=dispatcher.runBlockingTest {
        val userData=UserDataStoreModel(true, Name,
            Email,"","")
        coEvery { authenticationDataStore.observeUser() }returns flowOf(userData)
        val viewModel=createViewModel()
        viewModel.checkUserLogin()
        assertEquals(userData,viewModel.submitState.value)
    }
}