Android RxJava 2 JUnit test - getMainLooper in android.os.Looper not mocked RuntimeException
This error occurs because the default scheduler returned by AndroidSchedulers.mainThread()
is an instance of LooperScheduler
and relies on Android dependencies that are not available in JUnit tests.
We can avoid this issue by initializing RxAndroidPlugins
with a different Scheduler before the tests are run. You can do this inside of a @BeforeClass
method like so:
@BeforeClass
public static void setUpRxSchedulers() {
Scheduler immediate = new Scheduler() {
@Override
public Disposable scheduleDirect(@NonNull Runnable run, long delay, @NonNull TimeUnit unit) {
// this prevents StackOverflowErrors when scheduling with a delay
return super.scheduleDirect(run, 0, unit);
}
@Override
public Worker createWorker() {
return new ExecutorScheduler.ExecutorWorker(Runnable::run);
}
};
RxJavaPlugins.setInitIoSchedulerHandler(scheduler -> immediate);
RxJavaPlugins.setInitComputationSchedulerHandler(scheduler -> immediate);
RxJavaPlugins.setInitNewThreadSchedulerHandler(scheduler -> immediate);
RxJavaPlugins.setInitSingleSchedulerHandler(scheduler -> immediate);
RxAndroidPlugins.setInitMainThreadSchedulerHandler(scheduler -> immediate);
}
Or you can create a custom TestRule
that will allow you to reuse the initialization logic across multiple test classes.
public class RxImmediateSchedulerRule implements TestRule {
private Scheduler immediate = new Scheduler() {
@Override
public Disposable scheduleDirect(@NonNull Runnable run, long delay, @NonNull TimeUnit unit) {
// this prevents StackOverflowErrors when scheduling with a delay
return super.scheduleDirect(run, 0, unit);
}
@Override
public Worker createWorker() {
return new ExecutorScheduler.ExecutorWorker(Runnable::run);
}
};
@Override
public Statement apply(final Statement base, Description description) {
return new Statement() {
@Override
public void evaluate() throws Throwable {
RxJavaPlugins.setInitIoSchedulerHandler(scheduler -> immediate);
RxJavaPlugins.setInitComputationSchedulerHandler(scheduler -> immediate);
RxJavaPlugins.setInitNewThreadSchedulerHandler(scheduler -> immediate);
RxJavaPlugins.setInitSingleSchedulerHandler(scheduler -> immediate);
RxAndroidPlugins.setInitMainThreadSchedulerHandler(scheduler -> immediate);
try {
base.evaluate();
} finally {
RxJavaPlugins.reset();
RxAndroidPlugins.reset();
}
}
};
}
}
Which you can then apply to your test class
public class TestClass {
@ClassRule public static final RxImmediateSchedulerRule schedulers = new RxImmediateSchedulerRule();
@Test
public void testStuff_stuffHappens() {
...
}
}
Both of these methods will ensure that the default schedulers will be overridden before any of the tests execute and before AndroidSchedulers
is accessed.
Overriding the RxJava schedulers with an immediate scheduler for unit testing will also make sure the RxJava usages in the code being tested gets run synchronously, which will make it much easier to write the unit tests.
Sources:
https://www.infoq.com/articles/Testing-RxJava2
https://medium.com/@peter.tackage/overriding-rxandroid-schedulers-in-rxjava-2-5561b3d14212
I just added
RxAndroidPlugins.setInitMainThreadSchedulerHandler(scheduler -> Schedulers.trampoline());
in @Before
annoted method.
I was getting the same error when testing LiveData. When testing LiveData, this InstantTaskExecutorRule is needed in addition to RxImmediateSchedulerRule if the class being tested has both background thread and LiveData.
@RunWith(MockitoJUnitRunner::class)
class MainViewModelTest {
companion object {
@ClassRule @JvmField
val schedulers = RxImmediateSchedulerRule()
}
@Rule
@JvmField
val rule = InstantTaskExecutorRule()
@Mock
lateinit var dataRepository: DataRepository
lateinit var model: MainViewModel
@Before
fun setUp() {
model = MainViewModel(dataRepository)
}
@Test
fun fetchData() {
//given
val returnedItem = createDummyItem()
val observer = mock<Observer<List<Post>>>()
model.getPosts().observeForever(observer)
//when
liveData.value = listOf(returnedItem)
//than
verify(observer).onChanged(listOf(Post(returnedItem.id, returnedItem.title, returnedItem.url)))
}
}
Reference: https://pbochenski.pl/blog/07-12-2017-testing_livedata.html