使用LiveData进行Android ViewModel单元测试

Question

我正在对我的视图模型进行单元测试，但是我总是得到 NullPointerException。

这是我的视图模型代码 -

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)
            }
        }
}

以下是MyUseCase接口的代码 -

interface MyUseCase {

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

以下是相同的单元测试代码示例 -

@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))
    }
}

以下是我得到的输出 -

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)

- Ma2340

---

你能发布空指针异常的完整堆栈跟踪吗？ - Kushal

@Kushal在我的问题中进行了更新。 - Ma2340

@Maria 你解决了吗？ - Akbolat SSS

Answer 1

看起来是关于存根（when/thenReturn）时机问题的。 这对于大多数顺序语句并不是问题， 但是当存根LiveData时，情况比较棘手， 因为当您设置ViewModel和模拟MyUseCase时， 模拟的MyUseCase已经被评估了。 但是，在测试方法中，

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

以下方法不受上面的存根方法影响：

viewModel.onLoginClicked()

因此返回null，因此'observeForever'引发NPE。

我的良好实践思路是： 1. ViewModel的初始化在存根之后进行。

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

或者在您的源代码中使用liveData协程构建器，请参考： https://github.com/android/architecture-components-samples/tree/master/LiveDataSample

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

- AnonyKnow

---

我的良好实践是，1. ViewModel的初始化在存根之后。这就是我的想法！++ - JacksOnF1re