Testing Android Room with Kotlin Flow

Question

I'm trying to test a Room DAO exposing functions that return Flows. The following test won't pass and I'm struggling to see why :

    @Test
    fun `observeHomeCoursesFeatured() does not return courses that are no longer featured`() = runBlocking {
        val outputList: MutableList<List<HomeCourse>> = mutableListOf()
        launch { subject.observeHomeCoursesFeatured().collect { outputList.add(it) } }

        subject.saveHomeCoursesFeatured(listOf(getHomeCourseFeatured1(), getHomeCourseFeatured2()))
        subject.saveHomeCoursesFeatured(listOf(getHomeCourseFeatured1()))

        assertEquals(2, outputList.size)
        assertEquals(listOf(getHomeCourseFeatured1(), getHomeCourseFeatured2()), outputList[0])
        assertEquals(listOf(getHomeCourseFeatured1()), outputList[1])
    }

It fails at assertEquals(2, outputList.size) saying that outputList is empty.

This test passes :

    @Test
    fun `observeHomeCoursesFeatured() does not return courses that are no longer featured`() = runBlocking {
        subject.saveHomeCoursesFeatured(listOf(getHomeCourseFeatured1(), getHomeCourseFeatured2()))
        assertEquals(listOf(getHomeCourseFeatured1(), getHomeCourseFeatured2()), subject.observeHomeCoursesFeatured().first())

        subject.saveHomeCoursesFeatured(listOf(getHomeCourseFeatured1()))
        assertEquals(listOf(getHomeCourseFeatured1()), subject.observeHomeCoursesFeatured().first())
    }

The second test passing, shows that my DAO is working fine and it is more a question of threading and concurrency between the test thread and the thread that Room uses to trigger Flow changes.

I already added @get:Rule val archRule = InstantTaskExecutorRule() in my test. I also build my test DB with this :

db = Room.inMemoryDatabaseBuilder(ctx, CoreDatabase::class.java)
            .setTransactionExecutor(Executors.newSingleThreadExecutor())
            .allowMainThreadQueries()
            .build()

What am I missing ?

Dominic Fischer · Accepted Answer · 2020-06-28 14:15:03Z

1

launch is asynchronous, so you have a race condition.

@Test
fun `observeHomeCoursesFeatured() does not return courses that are no longer featured`() = runBlocking {
    val job = async { subject.observeHomeCoursesFeatured().take(2).toList() }

    subject.saveHomeCoursesFeatured(listOf(getHomeCourseFeatured1(), getHomeCourseFeatured2()))
    subject.saveHomeCoursesFeatured(listOf(getHomeCourseFeatured1()))

    val outputList = job.await()

    assertEquals(2, outputList.size)
    assertEquals(listOf(getHomeCourseFeatured1(), getHomeCourseFeatured2()), outputList[0])
    assertEquals(listOf(getHomeCourseFeatured1()), outputList[1])
}

edited Jun 28, 2020 at 14:15

answered Jun 28, 2020 at 11:03

Dominic Fischer

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4 Comments

Charly Lafon Over a year ago

Now my test won't finish because runBlocking { subject.observeHomeCoursesFeatured().collect { outputList.add(it) } } runs endlessly. And that's normal, that's why I think I have to launch a coroutine using launch

Charly Lafon Over a year ago

It's the same thing. My test won't end because join() waits for the coroutine to finish which it won't by itself because I'm calling collect() on a never ending Flow generated by Room.

Dominic Fischer Over a year ago

Didn't realise it was a never ending Flow. Another update.

NSaran Over a year ago

Instead of runBlocking, kotlinx.coroutines.test.runTest could be used here. Followed by passing and using the testDispatcher & backgroundScope to collect on the DAO's flow.

gloomyad · Accepted Answer · 2023-05-14 11:12:27Z

1

For this case I recommend to use flow.first()

@Dao 
interface TreeDao{ 

     @Query(Select * from tree) 
     fun getTrees():Flow<List<Tree>>}
}

For example Repository

class Repository(context:Context){
  db = Room.inMemoryDatabaseBuilder(context, TreeDatabase::class.java)
            .build()

fun getTrees():Flow<List<Tree>>
}

Because of any flow is endless than for our testing we can take first value of flow.

@Test
fun GetList()=runTest{
  val list=repository.getDao().getTrees().first()
  Assert.assertEqual(1,list.size)
}

answered May 14, 2023 at 11:12

gloomyad

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