android - Unit testing Room and LiveData

Question

I'm currently developing an app using the newly Android Architecture Components. Specifically, I'm implementing a Room Database that returns a LiveData object on one of its queries. Insertion and querying work as expected, however I have an issue testing the query method using a unit test.

Here is the DAO I'm trying to test:

NotificationDao.kt

@Dao
interface NotificationDao {

    @Insert
    fun insertNotifications(vararg notifications: Notification): List<Long>

    @Query("SELECT * FROM notifications")
    fun getNotifications(): LiveData<List<Notification>>
}

As you can tell, the query function returns a LiveData object, if I change this to be just a List, Cursor, or basically whatever then I get the expected result, which is the data inserted in the Database.

The issue is that the following test will always fail because the value of the LiveData object is always null:

NotificationDaoTest.kt

lateinit var db: SosafeDatabase
lateinit var notificationDao: NotificationDao

@Before
fun setUp() {
    val context = InstrumentationRegistry.getTargetContext()
    db = Room.inMemoryDatabaseBuilder(context, SosafeDatabase::class.java).build()
    notificationDao = db.notificationDao()
}

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

@Test
fun getNotifications_IfNotificationsInserted_ReturnsAListOfNotifications() {
    val NUMBER_OF_NOTIFICATIONS = 5
    val notifications = Array(NUMBER_OF_NOTIFICATIONS, { i -> createTestNotification(i) })
    notificationDao.insertNotifications(*notifications)

    val liveData = notificationDao.getNotifications()
    val queriedNotifications = liveData.value
    if (queriedNotifications != null) {
        assertEquals(queriedNotifications.size, NUMBER_OF_NOTIFICATIONS)
    } else {
        fail()
    }
}

private fun createTestNotification(id: Int): Notification {
    //method omitted for brevity 
}

So the question is: Does anyone knows of a better way to perform unit tests that involve LiveData objects?

Answer 1

Room calculates the LiveData's value lazily when there is an observer.

You can check the sample app.

It uses a getValue utility method which adds an observer to get the value:

public static <T> T getOrAwaitValue(final LiveData<T> liveData) throws InterruptedException {
    final Object[] data = new Object[1];
    final CountDownLatch latch = new CountDownLatch(1);
    Observer<T> observer = new Observer<T>() {
        @Override
        public void onChanged(@Nullable T o) {
            data[0] = o;
            latch.countDown();
            liveData.removeObserver(this);
        }
    };
    liveData.observeForever(observer);
    latch.await(2, TimeUnit.SECONDS);
    //noinspection unchecked
    return (T) data[0];
}

Better w/ kotlin, you can make it an extensions function :).