Unit test ViewModel using repository pattern and LiveData

Question

I want to write a unit test for my viewmodel class :

class MainViewModel(
    repository: ShowRepository
) : ViewModel() {

    private val _shows = repository.shows
    val shows: LiveData<MyResult<List<Show>>>
        get() = _shows
}

Here is my repository class :

class ShowRepository(
    private val dao: ShowDao,
    private val api: TVMazeService,
    private val context: Context
) {

    /**
     * A list of shows that can be shown on the screen.
     */
    val shows = resultLiveData(
        databaseQuery = {
            Transformations.map(dao.getShows()) {
                it.asDomainModel()
            }
        },
        networkCall = { refreshShows() })

    /**
     * Refresh the shows stored in the offline cache.
     */
    suspend fun refreshShows(): MyResult<List<Show>> =
        try {
            if (isNetworkAvailable(context)) {
                val shows = api.fetchShowList().await()
                dao.insertAll(*shows.asDatabaseModel())
                MyResult.success(shows)
            } else {
                MyResult.error(context.getString(R.string.failed_internet_msg))
            }
        } catch (err: HttpException) {
            MyResult.error(context.getString(R.string.failed_loading_msg))
        } catch (err: UnknownHostException) {
            MyResult.error(context.getString(R.string.failed_unknown_host_msg))
        } catch (err: SocketTimeoutException) {
            MyResult.error(context.getString(R.string.failed_socket_timeout_msg))
        }
}

And here is my Dao class :

@Dao
interface ShowDao {

    /**
     * Select all shows from the shows table.
     *
     * @return all shows.
     */
    @Query("SELECT * FROM databaseshow")
    fun getShows(): LiveData<List<DatabaseShow>>
}

Here is my unit test :

@ExperimentalCoroutinesApi
class MainViewModelTest {

    private lateinit var viewModel: MainViewModel
    private lateinit var repository: ShowRepository
    private val api: TVMazeService = mock()
    private val dao: ShowDao = mock()
    private val context: Context = mock()

    @Test
    fun fetch() {
        val observer1: Observer<List<DatabaseShow>> = mock()
        dao.getShows().observeForever(observer1)

        repository = ShowRepository(dao, api, context)

        val observer2: Observer<MyResult<List<Show>>> = mock()
        repository.shows.observeForever(observer2)

        viewModel = MainViewModel(repository)

        val observer3: Observer<MyResult<List<Show>>> = mock()
        viewModel.shows.observeForever(observer3)

        verify(viewModel).shows
    }
}

But I receive following exception :

java.lang.NullPointerException
    at com.android.sample.tvmaze.viewmodel.MainViewModelTest.fetch(MainViewModelTest.kt:39)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
    at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
    at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
    at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
    at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:26)
    at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
    at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
    at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
    at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
    at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
    at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
    at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
    at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
    at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)
    at com.intellij.rt.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:33)
    at com.intellij.rt.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:230)
    at com.intellij.rt.junit.JUnitStarter.main(JUnitStarter.java:58)

I appreciate if you give me any guideline.

Answer 1

I change my Dao method to return Flow instead of LiveData :

@Dao
interface ShowDao {
   /**
   * Select all shows from the shows table.
   *
   * @return all shows.
   */
   @Query("SELECT * FROM databaseshow")
   fun getShows(): Flow<List<DatabaseShow>>
}

And I could successfully run my tests such as :

    @Test
    fun givenServerResponse200_whenFetch_shouldReturnSuccess() {
        mockkStatic("com.android.sample.tvmaze.util.ContextExtKt")
        every {
            context.isNetworkAvailable()
        } returns true
        `when`(api.fetchShowList()).thenReturn(Calls.response(Response.success(emptyList())))
        `when`(dao.getShows()).thenReturn(flowOf(emptyList()))
        val repository = ShowRepository(dao, api, context, TestContextProvider())
        val viewModel = MainViewModel(repository).apply {
            shows.observeForever(resource)
        }
        try {
            verify(resource).onChanged(Resource.loading())
            verify(resource).onChanged(Resource.success(emptyList()))
        } finally {
            viewModel.shows.removeObserver(resource)
        }
    }