Android unit testing view model that receives flow

Question

I have a ViewModel that talks to a use case and gets a flow back i.e Flow<MyResult>. I want to unit test my ViewModel. I am new to using the flow. Need help pls. Here is the viewModel below -

class MyViewModel(private val handle: SavedStateHandle, private val useCase: MyUseCase) : ViewModel() {

        private val viewState = MyViewState()

        fun onOptionsSelected() =
            useCase.getListOfChocolates(MyAction.GetChocolateList).map {
                when (it) {
                    is MyResult.Loading -> viewState.copy(loading = true)
                    is MyResult.ChocolateList -> viewState.copy(loading = false, data = it.choclateList)
                    is MyResult.Error -> viewState.copy(loading = false, error = "Error")
                }
            }.asLiveData(Dispatchers.Default + viewModelScope.coroutineContext)

MyViewState looks like this -

 data class MyViewState(
        val loading: Boolean = false,
        val data: List<ChocolateModel> = emptyList(),
        val error: String? = null
    )

The unit test looks like below. The assert fails always don't know what I am doing wrong there.

class MyViewModelTest {

    @get:Rule
    val instantExecutorRule = InstantTaskExecutorRule()

    private val mainThreadSurrogate = newSingleThreadContext("UI thread")

    private lateinit var myViewModel: MyViewModel

    @Mock
    private lateinit var useCase: MyUseCase

    @Mock
    private lateinit var handle: SavedStateHandle

    @Mock
    private lateinit var chocolateList: List<ChocolateModel>

    private lateinit var viewState: MyViewState


    @Before
    fun setup() {
        MockitoAnnotations.initMocks(this)
        Dispatchers.setMain(mainThreadSurrogate)
        viewState = MyViewState()
        myViewModel = MyViewModel(handle, useCase)
    }

    @After
    fun tearDown() {
        Dispatchers.resetMain() // reset main dispatcher to the original Main dispatcher
        mainThreadSurrogate.close()
    }

    @Test
    fun onOptionsSelected() {
        runBlocking {
            val flow = flow {
                emit(MyResult.Loading)
                emit(MyResult.ChocolateList(chocolateList))
            }

            Mockito.`when`(useCase.getListOfChocolates(MyAction.GetChocolateList)).thenReturn(flow)
            myViewModel.onOptionsSelected().observeForever {}

            viewState.copy(loading = true)
            assertEquals(viewState.loading, true)

            viewState.copy(loading = false, data = chocolateList)
            assertEquals(viewState.data.isEmpty(), false)
            assertEquals(viewState.loading, true)
        }
    }
}

Answer 1

There are few issues in this testing environment as:

1. The flow builder will emit the result instantly so always the last value will be received.
2. The viewState holder has no link with our mocks hence is useless.
3. To test the actual flow with multiple values, delay and fast-forward control is required.
4. The response values need to be collected for assertion

Solution:

1. Use delay to process both values in the flow builder
2. Remove viewState.
3. Use MainCoroutineScopeRule to control the execution flow with delay
4. To collect observer values for assertion, use ArgumentCaptor.

Source-code:

1. MyViewModelTest.kt

   import androidx.arch.core.executor.testing.InstantTaskExecutorRule
   import androidx.lifecycle.Observer
   import androidx.lifecycle.SavedStateHandle
   import com.pavneet_singh.temp.ui.main.testflow.*
   import org.junit.Assert.assertEquals
   import kotlinx.coroutines.delay
   import kotlinx.coroutines.flow.flow
   import kotlinx.coroutines.runBlocking
   import org.junit.Before
   import org.junit.Rule
   import org.junit.Test
   import org.mockito.ArgumentCaptor
   import org.mockito.Captor
   import org.mockito.Mock
   import org.mockito.Mockito.*
   import org.mockito.MockitoAnnotations
   
   class MyViewModelTest {
   
       @get:Rule
       val instantExecutorRule = InstantTaskExecutorRule()
   
       @get:Rule
       val coroutineScope = MainCoroutineScopeRule()
   
       @Mock
       private lateinit var mockObserver: Observer<MyViewState>
   
       private lateinit var myViewModel: MyViewModel
   
       @Mock
       private lateinit var useCase: MyUseCase
   
       @Mock
       private lateinit var handle: SavedStateHandle
   
       @Mock
       private lateinit var chocolateList: List<ChocolateModel>
   
       private lateinit var viewState: MyViewState
   
       @Captor
       private lateinit var captor: ArgumentCaptor<MyViewState>
   
   
       @Before
       fun setup() {
           MockitoAnnotations.initMocks(this)
           viewState = MyViewState()
           myViewModel = MyViewModel(handle, useCase)
       }
   
       @Test
       fun onOptionsSelected() {
           runBlocking {
               val flow = flow {
                   emit(MyResult.Loading)
                   delay(10)
                   emit(MyResult.ChocolateList(chocolateList))
               }
   
               `when`(useCase.getListOfChocolates(MyAction.GetChocolateList)).thenReturn(flow)
               `when`(chocolateList.get(0)).thenReturn(ChocolateModel("Pavneet", 1))
               val liveData = myViewModel.onOptionsSelected()
               liveData.observeForever(mockObserver)
   
               verify(mockObserver).onChanged(captor.capture())
               assertEquals(true, captor.value.loading)
               coroutineScope.advanceTimeBy(10)
               verify(mockObserver, times(2)).onChanged(captor.capture())
               assertEquals("Pavneet", captor.value.data[0].name)// name is custom implementaiton field of `ChocolateModel` class
           }
       }
   }
   

2. MainCoroutineScopeRule.kt source to copy the file

3. List of dependencies

   dependencies {
       implementation fileTree(dir: 'libs', include: ['*.jar'])
       implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"
       implementation 'androidx.appcompat:appcompat:1.1.0'
       implementation 'androidx.core:core-ktx:1.2.0'
       implementation 'androidx.constraintlayout:constraintlayout:1.1.3'
       implementation 'androidx.lifecycle:lifecycle-extensions:2.2.0'
       implementation 'androidx.lifecycle:lifecycle-viewmodel-ktx:2.2.0'
       testImplementation 'junit:junit:4.12'
       androidTestImplementation 'androidx.test.ext:junit:1.1.1'
       androidTestImplementation 'androidx.test.espresso:espresso-core:3.2.0'
       implementation 'androidx.lifecycle:lifecycle-livedata-ktx:2.3.0-alpha01'
       implementation 'org.mockito:mockito-core:2.16.0'
       testImplementation 'androidx.arch.core:core-testing:2.1.0'
       testImplementation 'org.jetbrains.kotlinx:kotlinx-coroutines-test:1.3.5'
       testImplementation 'org.mockito:mockito-inline:2.13.0'
   }
   

Output (gif is optimized by removing frames so bit laggy):

View mvvm-flow-coroutine-testing repo on Github for complete implementaion.