测试ViewModel与Hilt实现时遇observeForever后台线程调用错误求助

问题描述 #

我正在测试ViewModel与Hilt的实现，已完成代码搭建，但运行测试时抛出异常：Cannot invoke observeForever on a background thread，请求协助排查问题。

测试类代码 #

@HiltAndroidTest
@UninstallModules(AppModule::class)
@OptIn(ExperimentalCoroutinesApi::class)
class SimpleViewModelHiltTest {

    @get:Rule
    var hiltTestRule = HiltAndroidRule(this)

    @Inject
    lateinit var itemsDao: ItemsDao

    @Inject
    lateinit var simpleRepository: SimpleRepository

    lateinit var simpleViewModel: SimpleViewModel

    @Before
    fun setUp(){
        hiltTestRule.inject()
        simpleViewModel = SimpleViewModel(simpleRepository)
    }

    @After
    fun tearDown(){

    }

    @Test
    fun testCaseEmptyItemName_returnsErrorStatus() = runTest{
        val itemModel = ItemsModel("",23F,"California",1)
        simpleViewModel.insertItems(itemModel)
        val result = simpleViewModel.liveData.getOrAwaitValue()
        assertThat(result.getContentIfNotHandled()?.status).isEqualTo(Status.ERROR)
    }

    @Test
    fun testCaseEmptyItemPlace_returnsErrorStatus() = runTest {
        val itemModel = ItemsModel("Banana",23F,"",1)
        simpleViewModel.insertItems(itemModel)
        val result = simpleViewModel.liveData.getOrAwaitValue()
        assertThat(result.getContentIfNotHandled()?.status).isEqualTo(Status.ERROR)
    }

    @Test
    fun testCaseValidValues_returnsSuccessStatus() = runTest {
        val itemModel = ItemsModel("Banana",23F,"California",1)
        simpleViewModel.insertItems(itemModel)
        val result = simpleViewModel.liveData.getOrAwaitValue()
        assertThat(result.getContentIfNotHandled()?.status).isEqualTo(Status.SUCCESS)
    }
}

错误信息 #

Cannot invoke observeForever on a background thread

原因及解决办法 #

原因 #

runTest默认将测试代码调度到后台线程执行，而getOrAwaitValue()内部调用的observeForever方法要求必须在**主线程（UI线程）**执行，因此触发异常。

解决办法 #

方法1：添加InstantTaskExecutorRule

这是最便捷的方案，该规则会强制LiveData的所有操作在测试中同步执行，且默认在主线程处理。

1. 添加依赖：

testImplementation "androidx.arch.core:core-testing:2.2.0"

2. 在测试类中添加规则：

@get:Rule
val instantTaskExecutorRule = InstantTaskExecutorRule()

@get:Rule
var hiltTestRule = HiltAndroidRule(this)

方法2：切换到主线程获取LiveData值

在调用getOrAwaitValue()时，通过withContext切换到主线程调度器：

@Test
fun testCaseEmptyItemName_returnsErrorStatus() = runTest{
    val itemModel = ItemsModel("",23F,"California",1)
    simpleViewModel.insertItems(itemModel)
    val result = withContext(Dispatchers.Main.immediate) {
        simpleViewModel.liveData.getOrAwaitValue()
    }
    assertThat(result.getContentIfNotHandled()?.status).isEqualTo(Status.ERROR)
}

方法3：修改自定义getOrAwaitValue扩展函数

如果getOrAwaitValue是自定义扩展函数，可修改它确保观察操作强制在主线程执行：

import android.os.Handler
import android.os.Looper
import androidx.lifecycle.LiveData
import androidx.lifecycle.Observer
import java.util.concurrent.CountDownLatch
import java.util.concurrent.TimeUnit

fun <T> LiveData<T>.getOrAwaitValue(): T {
    var data: T? = null
    val latch = CountDownLatch(1)

    val observer = object : Observer<T> {
        override fun onChanged(value: T) {
            data = value
            latch.countDown()
            this@getOrAwaitValue.removeObserver(this)
        }
    }

    // 判断当前线程是否为主线程，非主线程则切换执行观察
    if (Looper.myLooper() == Looper.getMainLooper()) {
        observeForever(observer)
    } else {
        Handler(Looper.getMainLooper()).post {
            observeForever(observer)
        }
    }

    // 等待结果，超时时间可按需调整
    latch.await(2, TimeUnit.SECONDS)

    @Suppress("UNCHECKED_CAST")
    return data as T
}

内容的提问来源于stack exchange，提问作者Taki