考林克斯-协同试验。withContext {.}在liveData {.}完成后不会切换回以前的上下文

Question

我正在为我的Android应用程序编写一个集成测试。我在liveData {...}协同构建器中遇到了一个问题:当我在其中调用withContext {...}函数时，它会切换到给定的上下文(例如Dispatchers.IO)，但在返回(到Dispatchers.Main.immediate)之后不会切换回来。

测试结果如下：

WordFragmentIntegrationTest:

@RunWith(RobolectricTestRunner::class)
@Config(application = TestAppWithDaggerComponent::class)
class WordFragmentIntegrationTest {

    @get:Rule
    val coroutinesRule = CoroutinesRule()

    private val mockWebServer = MockWebServer()

    @After
    fun teardown() {
        mockWebServer.shutdown()
    }

    @Test
    fun `should fetch a word from api and populate the view`() = runBlockingTest {
        // Mock api response, it works fine.
        val response = MockResponse().setResponseCode(HttpURLConnection.HTTP_OK)
                                     .setBody(SAMPLE_API_WORD_JSON)
                                     .setBodyDelay(0, TimeUnit.MILLISECONDS)
        mockWebServer.enqueue(response)
        mockWebServer.start(8080)

        // Launch a Fragment under test. It triggers an api call.
        val fragmentScenario = launchFragmentInContainer<WordFragment>()

        // Wait for the Fragment to do its work.
        fragmentScenario.onFragment { fragmentUnderTest ->
            runBlocking {
                while (fragmentUnderTest.isLoading) { yield() }
                
                // Do some assertions...
            }
        }
    }
}

CoroutinesRule:

class CoroutinesRule : ExternalResource() {

    override fun before() {
        Dispatchers.setMain(TestCoroutineDispatcher())
    }

    override fun after() {
        Dispatchers.resetMain()
    }
}

和一段代码，这会导致测试失败。

WordViewModel:

val wordLiveData = liveData {
    printCurrentThread("Emitting the first value")
    emit(UIState.ShowLoading)

    val value = withContext(Dispatchers.IO) {
        printCurrentThread("Fetching a value")
        loadWordUseCase(wordId)
    }

    printCurrentThread("Emitting the second value")
    emit(value)
}

private fun printCurrentThread(message: String) {
    val threadInfo = "Thread: ${Thread.currentThread().id}. UI thread: ${Looper.getMainLooper().thread.id}"
    println("$message. $threadInfo")
}

它在生产环境中运行良好：

Emitting the first value. Thread: 1. UI thread: 1  # First emitting is on the UI thread.
Fetching a value. Thread: 365. UI thread: 1        # Fetching is on some IO thread.
Emitting the second value. Thread: 1. UI thread: 1 # Switched back to the UI thread.

但是在Dispatchers.Main被替换的测试环境中，withContext {...}不会切换回liveData {...}的Dispatchers.Main.immediate，它会导致CoroutineLiveData崩溃，因为它的emit()应该从UI线程中调用。

Emitting the first value. Thread: 11. UI thread: 11  # Emitted on the UI thread.
Fetching a value. Thread: 19. UI thread: 11          # Switched to IO thread.
Emitting the second value. Thread: 19. UI thread: 11 # Didn't switch back, which caused:

Exception in thread "DefaultDispatcher-worker-1 @coroutine#2"
 java.lang.IllegalStateException: Cannot invoke setValue on a background thread
    at androidx.lifecycle.LiveData.assertMainThread(LiveData.java:462)
    at androidx.lifecycle.LiveData.setValue(LiveData.java:304)
...

是窃听器还是我做错了什么？也许我搞错了CoroutinesRule

Answer 1

这是因为TestCoroutineDispatcher没有像android的Dispatchers.Main那样切换回特定的线程。相反，它的行为类似于Dispatchers.Unconfined。

我不确定这是否适用于Robolectric，但是在单元测试中起作用的是：

将InstantTaskExecutorRule添加到您的测试中，就像您对CoroutinesRule做的那样。例如：

@get:Rule
val executorRule = InstantTaskExecutorRule()

这将允许在任何线程上调用LiveData.setValue(...)。

由于emit最终只是委托给setValue，所以不需要调整ViewModel代码。

如果该规则尚未可用，则添加此依赖项：

testImplementation 'androidx.arch.core:core-testing:2.1.0'

如果这仍然没有帮助，我想知道如果你不替换主调度员会发生什么