Android单元测试LiveData.value返回null

我正在尝试使用MVVM架构为我的应用程序示例创建单元测试。在这个场景中，我试图为执行插入的方法创建一个测试，在本例中，我调用ViewModel方法，该方法调用存储库，将文本传递给插入，我的存储库反过来调用Dao类的插入，然后返回一个新列表，其中插入了新项。但是在我的测试中，我的LiveData对象是空的

ViewModel类

class MainViewModel(private val todoRepository: TodoRepository) : ViewModel() {
    private val _todos = MutableLiveData<List<Todo>>()
    fun observeTodos() = _todos

    fun insertNewTask(text: String){
        viewModelScope.launch {
            val result  = todoRepository.insertTask(text)
            _todos.value = result
        }
    }
}

---

为了解决这个问题，我只错过了一件事，我的方法viewmodel.insertTask（）不调用repository.getAllTodos（），那么我的方法就错了！！！正确答案是：

@Test
fun should_insertNewTodo() = runBlockingTest {

    //given
    val returnedList = createTodoList()
    whenever(repository.insertTask(anyString())).thenReturn(returnedList)

    //when
    viewModel.insertNewTask("newTask")
    val result = viewModel.observeTodos().value

    //than
    verify(repository).insertTask(anyString())
    assertEquals(returnedList, result)

}

@ExperimentalCoroutinesApi
@RunWith(JUnit4::class)
class MainViewModelTest {

@Rule
@JvmField
val rule = InstantTaskExecutorRule()

private val repository = mock(TodoRepository::class.java)

private lateinit var viewModel: MainViewModel

@Before
fun setUp() {
    viewModel = MainViewModel(repository)
}

@Test
fun should_insertNewTodo() = runBlockingTest {

    //given
    val result = MutableLiveData<List<Todo>>()
    val returnedList = createTodoList()
    whenever(repository.getAllTodos()).thenReturn(returnedList)

    //when
    viewModel.insertNewTask("newTask")
    result.value = viewModel.observeTodos().value


    //than
    verify(repository).insertTask(anyString())
    assertEquals(returnedList, result.value)

}

fun createTodoList(): List<Todo>{
    val mockList = listOf(Todo(id = 1, text = "FIRST", completed = false),
        Todo(id = 2, text = "SECOND", completed = false),
        Todo(id = 3, text = "THIRD", completed = true)
    )
    return mockList
}

}

Expected :[Todo(id=1, text=FIRST, completed=false), Todo(id=2, text=SECOND, completed=false), Todo(id=3, text=THIRD, completed=true)]
Actual   :null

@Test
fun should_insertNewTodo() = runBlockingTest {

    //given
    val returnedList = createTodoList()
    whenever(repository.insertTask(anyString())).thenReturn(returnedList)

    //when
    viewModel.insertNewTask("newTask")
    val result = viewModel.observeTodos().value

    //than
    verify(repository).insertTask(anyString())
    assertEquals(returnedList, result)

}