kotlinx-coroutines-test. withContext {...} in liveData {...} doesn't switch back to the previous context after its finish

[a-kari]

Hello! I'm writing an integration test for my Android app. And I've faced an issue with liveData {...} coroutine builder: when I call withContext {...} function inside of it, it switches to a given context (e.g. Dispatchers.IO), but does not switch back after return (to Dispatchers.Main.immediate).

The test looks like:

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()
    }
}

---

And a piece of code, which causes test failure.

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")
}

It works fine in production environment:

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.

But in the test environment (where Dispatchers.Main is replaced) withContext {...} does not switch back to liveData {...}'s Dispatchers.Main.immediate, and it causes CoroutineLiveData crash, because its emit() should be called from the UI thread.

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)
...

---

Is it a bug or am I doing something wrong? E.g. maybe I have a wrong CoroutinesRule?

[elizarov]

It looks like some problem in TestCoroutineDispatcher (cc @qwwdfsad ) that sometimes seems to behave like an unconfined dispatcher. I struggle to reproduce your specific setup, though.

[a-kari]

Updated: There is a minimal reproducing project.

---

My real project setup is in this branch.

Files to check:

• build.gradle of the module under test
• Versions of used libraries
• WordViewModel - a class where the error occurs
• WordFragmentIntegrationTest
• CoroutinesRule

[allanconda-mercari]

Is there an update to this?