Android viewmodel unit test with livedata

Question

I am unit testing my viewmodel and I always keep getting NullPointerException.

Here is my viewmodel code -

class LoginViewModel(private val myUseCase: MyUseCase) :BaseViewModel() {

    private val viewState = LoginViewState()

    fun onLoginClicked() =
        Transformations.map(
            myUseCase.performUseCaseAction(
                MyAction.LoginUser(
                    email,password)
            )
        ) {
            when (it) {
                is MyResult.Loading -> viewState.copy(loading = true)
                is MyResult.UserLoggedIn -> viewState.copy(
                    loading = false,
                    userLoggedIn = true
                )
                is MyResult.Error -> viewState.copy(loading = false, error = it.error)
            }
        }
}

Here is the MyUseCase interface code -

interface MyUseCase {

    fun performUseCaseAction(action: MyAction): LiveData<MyResult>
}

Here is the unit test for the same -

@RunWith(MockitoJUnitRunner::class)
class LoginViewModelTest {

    @get:Rule
    val instantExecutorRule = InstantTaskExecutorRule()

    private lateinit var viewModel: LoginViewModel

    @Mock
    private lateinit var myUseCase: MyUseCase

    @Mock
    private lateinit var observer: Observer<LoginViewState>

    @Before
    fun setUp() {
        MockitoAnnotations.initMocks(this)
        viewModel = LoginViewModel(useCase)
    }

    @Test
    fun login_loginClicked_userLoggedInExpected() {
        //Given
        val viewState = LoginViewState()

        //When
        val liveData1 = MutableLiveData<MyResult>()
        `when`(useCase.performUseCaseAction(
            MyAction.LoginUser("email","password")
        )).thenReturn(liveData1)
        liveData1.postValue(MyResult.UserLoggedIn)

        viewModel.onLoginClicked().observeForever(observer)

        //Then
        verify(observer).onChanged(viewState.copy(loading = true))
        verify(observer).onChanged(viewState.copy(loading = false, userLoggedIn = true))
    }
}

Here is the output I get -

java.lang.NullPointerException
at androidx.lifecycle.MediatorLiveData$Source.plug(MediatorLiveData.java:141)
at androidx.lifecycle.MediatorLiveData.onActive(MediatorLiveData.java:118)
at androidx.lifecycle.LiveData$ObserverWrapper.activeStateChanged(LiveData.java:437)
at androidx.lifecycle.LiveData.observeForever(LiveData.java:232)
at com.client.personaldiary.view.viewmodel.LoginViewModelTest.onLoginClicked(LoginViewModelTest.kt:88)
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.mockito.internal.runners.DefaultInternalRunner$1$1.evaluate(DefaultInternalRunner.java:44)
at org.junit.rules.TestWatcher$1.evaluate(TestWatcher.java:55)
at org.junit.rules.RunRules.evaluate(RunRules.java:20)
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.mockito.internal.runners.DefaultInternalRunner$1.run(DefaultInternalRunner.java:74)
at org.mockito.internal.runners.DefaultInternalRunner.run(DefaultInternalRunner.java:80)
at org.mockito.internal.runners.StrictRunner.run(StrictRunner.java:39)
at org.mockito.junit.MockitoJUnitRunner.run(MockitoJUnitRunner.java:163)
at org.junit.runners.Suite.runChild(Suite.java:128)
at org.junit.runners.Suite.runChild(Suite.java:27)
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$2.evaluate(ParentRunner.java:268)
at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)
at com.intellij.rt.execution.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:47)
at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:242)
at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:70)

Answer 1

It seems about stubbing(when/thenReturn) timing problem. This is not a problem for most sequential statement, but when stubbing LiveData, it is tricky since
When your ViewModel and mocked MyUseCase are set up, mocked MyUseCase is already evaluated. However, in test method,

`when`(useCase.performUseCaseAction(
            MyAction.LoginUser("email","password")
        )).thenReturn(liveData1)

below method is not affected from above stubbing method,

viewModel.onLoginClicked()

so return null, therefore 'observeForever` raise the NPE.

The good practice of my thinking is that 1. ViewModel's initialization is after stubbing

`when`()...
viewModel = LoginViewModel(useCase)

or using liveData coroutine builder in your source code refer to: https://github.com/android/architecture-components-samples/tree/master/LiveDataSample

liveData { emitSource(....map { ... }) }