Test LiveData in Instrumentation tests

The LiveData.observe method should run in main thread, but our test methods run in background thread, so we should use some tricks to avoid it if we want to test LiveData in Instrumentation tests.

There is a method to use InstantTaskExecutorRule in library androidx.arch.core:core-testing to hook DefaultTaskExecutor to a normal TaskExecutor which pretends to run main thread. But it works fine when we use it with testImplementation for unit tests, but not for androidTestImplementation for Instrumentation tests because of test runner(maybe). But we can fix it to imitate InstantTaskExecutorRule, like below code snipped:

public static void hookMainThreadForLiveData() {
    ArchTaskExecutor archTaskExecutor = ArchTaskExecutor.getInstance();
    try {
        Method methodSetDelegate =
                archTaskExecutor.getClass().getDeclaredMethod("setDelegate", TaskExecutor.class);
        methodSetDelegate.setAccessible(true);
        methodSetDelegate.invoke(archTaskExecutor, new TaskExecutor() {
            @Override
            public void executeOnDiskIO(@NonNull Runnable runnable) {
                runnable.run();
            }

            @Override
            public void postToMainThread(@NonNull Runnable runnable) {
                runnable.run();
            }

            @Override
            public boolean isMainThread() {
                return true;
            }
        });
    } catch (Exception e) {
        throw new RuntimeException("Failed to hook main thread for live data", e);
    }
}

public static void resetMainThreadForLiveData() {
    ArchTaskExecutor archTaskExecutor = ArchTaskExecutor.getInstance();
    try {
        Method methodSetDelegate =
                archTaskExecutor.getClass().getDeclaredMethod("setDelegate", TaskExecutor.class);
        methodSetDelegate.setAccessible(true);
        methodSetDelegate.invoke(archTaskExecutor, new Object[]{null});
    } catch (Exception e) {
        throw new RuntimeException("Failed to reset hook thread for live data", e);
    }
}

@Before
public void setUp() {
    TestUtil.hookMainThreadForLiveData();
}

@After
public void tearDown() {
    TestUtil.resetMainThreadForLiveData();
}

To wait and check LiveData value better, we can use jraska’s livedata-testing to simply the work to test LiveData. And we can include to our project with below gralde command:

androidTestImplementation 'com.jraska.livedata:testing:1.1.0'