Android Espresso: Testing error message is shown?

Question

I have a test that is supposed to test the visibility of an error msg when there is no network connection, but I dunno how to get it to show?

@RunWith(AndroidJUnit4.class)
@LargeTest
public class PostsActivityTest {

@Rule
public final ActivityRule<PostsActivity> rule = new ActivityRule<>(PostsActivity.class);

PostsActivity postsActivity;

@Before
public void init() {
    postsActivity = rule.get();
}

@Test
public void testShouldShowErrorViewOnNetworkError() {
    postsActivity.showErrorMsg(); // doesn't work
    onView(withId(R.id.error_view)).check(matches(isDisplayed()));
}

When calling postsActivity.showErrorMsg(); I get a crash stating android.view.ViewRootImpl$CalledFromWrongThreadException: Only the original thread that created a view hierarchy can touch its views.

This is the showErrorMsg() in PostsActivity:

public void showErrorMsg() {
    errorView.setVisibility(View.VISIBLE);
}

Any help appreciated. Thanks!

Edit: Using runOnUiThread seems to be working. I dunno if this is the preferred way though.

@Test
public void testShouldShowErrorViewOnNetworkError() {
    postsActivity.runOnUiThread(new Runnable() {
        @Override
        public void run() {
            postsActivity.showErrorMsg();
        }
    });
    onView(withId(R.id.error_view)).check(matches(isDisplayed()));
}

Answer 1

In your Espresso tests you should not directly call methods of your Activity. You should use Espresso to interact with the UI like a real user.

Why runOnUiThread is required:

When running instrumentation tests (e.g. Espresso tests) there are actually two apps involved. First the app that is being tested and the app that executes the tests. Both use different main threads.

If you call PostsActivity#showErrorMsg from your test, it is running in the testing apps thread. But manipulating views in Android is only allowed from the apps UI Thread. If you really have to call PostsActivity#showErrorMsg from your test, runOnUiThread is the right choice.

How to improve

So instead of calling PostsActivity#showErrorMsg directly from your test, use Espresso to click() e.g. on a Button that will call that method for you:

@Test
public void testShouldShowErrorViewOnNetworkError() {
    onView(withId(R.id.post_button)).perform(click());
    onView(withId(R.id.error_view)).check(matches(isDisplayed()));
}