Android架构组件ViewModel – 如何在测试Activity上模拟ViewModel？

这是我的代码,我试图通过模拟viewmodel来测试Activity.但viewmodel未在Activity中的onCreate()之前设置.

@RunWith(AndroidJUnit4::class)
class MainActivityTest {

    val viewmodel = mock(Mainviewmodel::class.java)

    @Rule
    @JvmField
    val activityRule = ActivityTestRule<MainActivity>(MainActivity::class.java,true,truE)

    private val liveData = mutablelivedata<resource<Object>>()

    @Before
    open fun setUp() {
        activityRule.activity.viewmodelFactory = createviewmodelFor(viewmodel)
        `when`(viewmodel.liveData).thenReturn(liveData)
        viewmodel.liveData?.observeForever(mock(Observer::class.java) as Observer<resource<Object>>)
        liveData.postValue(resource.success(Object()))
    }

    fun <T : viewmodel> createviewmodelFor(model: T): viewmodelProvider.Factory =
        object : viewmodelProvider.Factory {
            override fun <T : viewmodel> create(modelClass: Class<T>): T {
                if (modelClass.isAssignableFrom(model.javaClass)) {
                    return model as T
                }
                throw IllegalArgumentexception("unexpected model class " + modelClass)
            }
        }
}

有人可以帮我解决这个问题吗？