A working version of the test with LiveData

PostDaoReadWriteTest.kt

@RunWith(AndroidJUnit4::class)
class PostDaoReadWriteTest {
    private lateinit var postDao: PostDao
    private lateinit var db: TestDatabase
  
    @get:Rule
    val instantTaskExecutorRule = InstantTaskExecutorRule()

    @Before
    fun createDb() {
        val context = ApplicationProvider.getApplicationContext<Context>()
        db = Room.inMemoryDatabaseBuilder(
                context, TestDatabase::class.java).build()
        postDao = db.getPostDao()
    }

    @After
    @Throws(IOException::class)
    fun closeDb() {
        db.close()
    }

    @Test
    @Throws(Exception::class)
    fun writePostAndReadInList() {
        postDao.getAll().observeOnce {
          assertEquals(0, it.size)
        }
      
        val post = TestUtil.createPost(id=42)
        postDao.insert(post)
      
        postDao.getAll().observeOnce {
          assertEquals(1, it.size)
        }
    }
}