使用 livedata 進行 Android 視圖模型單元測試

Question

我正在對我的視圖模型進行單元測試，並且我總是不斷收到NullPointerException 。

這是我的視圖模型代碼 -

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)
            }
        }
}

這是 MyUseCase 接口代碼 -

interface MyUseCase {

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

這是相同的單元測試 -

@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))
    }
}

這是我得到的輸出 -

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

這似乎是關於 stubbing(when/thenReturn) 計時問題。 對於大多數順序語句來說這不是問題，但是當存根 LiveData 時，這很棘手，因為
當您的 ViewModel 和模擬的 MyUseCase 被設置時，模擬的 MyUseCase 已經被評估。 然而，在測試方法中，

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

下面的方法不受上面的存根方法的影響，

viewModel.onLoginClicked()

所以返回 null，因此 'observeForever` 提高 NPE。

我的想法的好做法是 1. ViewModel 的初始化是在 stubbing 之后

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

或在您的源代碼中使用 liveData 協程構建器參考： https : //github.com/android/architecture-components-samples/tree/master/LiveDataSample

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