Room and RxJava

A normal Dao

@Query(“SELECT * FROM Users WHERE id = :userId”)
User getUserById(String userId);

This has two disadvantages:

• Code is blocking, synchronous call
• Must manually call method everytime user data is changed

We can fix this with RxJava by returning a Maybe, Single, or Flowable object to make the call asynchronous and observable.

For queries that return Maybe or Single, make sure you’re calling subscribeOn with a different Scheduler than AndroidSchedulers.mainThread().

Build.gradle

// RxJava support for Room
implementation “android.arch.persistence.room:rxjava2:1.0.0-alpha5”

// Testing support
androidTestImplementation “android.arch.core:core-testing:1.0.0-alpha5”

Maybe

@Query(“SELECT * FROM Users WHERE id = :userId”)
Maybe<User> getUserById(String userId);

Single

@Query(“SELECT * FROM Users WHERE id = :userId”)
Single<User> getUserById(String userId);

Flowable

@Query(“SELECT * FROM Users WHERE id = :userId”)
Flowable<User> getUserById(String userId);

Testing

@RunWith(AndroidJUnit4.class)
public class UserDaoTest {

@Rule
public InstantTaskExecutorRule instantTaskExecutorRule = 
                                      new InstantTaskExecutorRule();
…
private UsersDatabase mDatabase;

@Before
public void initDb() throws Exception {
    mDatabase = Room.inMemoryDatabaseBuilder(
                     InstrumentationRegistry.getContext(),
                     UsersDatabase.class)
            // allowing main thread queries, just for testing
            .allowMainThreadQueries()
            .build();
}

@After
public void closeDb() throws Exception {
    mDatabase.close();
}

@Test
public void insertAndGetUserById() {
    // Given that we have a user in the data source
    mDatabase.userDao().insertUser(USER);

    // When subscribing to the emissions of user
    mDatabase.userDao()
             .getUserById(USER.getId())
             .test()
             // assertValue asserts that there was only one emission
             .assertValue(new Predicate<User>() {
                @Override
                public boolean test(User user) throws Exception {
                    // The emitted user is the expected one
                    return user.getId().equals(USER.getId()) &&
                      user.getUserName().equals(USER.getUserName());
                }
            });
}