How to unit test Retrofit api calls?

I am trying to integrate Unit test cases for every chunk of code possible. But I am facing issues while adding test cases for api calls that are made through retrofit.

The JUnit compiler never executes the code in the CallBack functions.

There is another option of making all the api calls Synchronous for testing purpose, but that's not possible for every case in my app.

How can I sort this out? I have to add test cases in the api calls by any means.

I test my Retrofit callbacks using Mockito, Robolectric and Hamcrest libraries.

First of all, set up lib stack in your module's build.gradle:

dependencies {
    testCompile 'org.robolectric:robolectric:3.0'
    testCompile "org.mockito:mockito-core:1.10.19"
    androidTestCompile 'org.hamcrest:hamcrest-library:1.1'
}

In jour project's global build.gradle add following line to buildscript dependencies:

classpath 'org.robolectric:robolectric-gradle-plugin:1.0.1'

Then enter "Build Variants" menu in Android Studio (to quickly find it, hit Ctrl+Shift+A and search for it), and switch "Test Artifact" option to "Unit Tests". Android studio will switch your test folder to "com.your.package (test)" (instead of androidTest).

Ok. Set-up is done, time to write some tests!

Let's say you've got some retrofit api calls to retrieve a list of objects that need to be put into some adapter for a RecyclerView etc. We would like to test whether adapter gets filled with proper items on successful call. To do this, we'll need to switch your Retrofit interface implementation, that you use to make calls with a mock, and do some fake responses taking advantage of Mockito ArgumentCaptor class.

@Config(constants = BuildConfig.class, sdk = 21,
    manifest = "app/src/main/AndroidManifest.xml")
@RunWith(RobolectricGradleTestRunner.class)
public class RetrofitCallTest {

    private MainActivity mainActivity;

    @Mock
    private RetrofitApi mockRetrofitApiImpl;

    @Captor
    private ArgumentCaptor<Callback<List<YourObject>>> callbackArgumentCaptor;

    @Before
    public void setUp() {            
        MockitoAnnotations.initMocks(this);

        ActivityController<MainActivity> controller = Robolectric.buildActivity(MainActivity.class);
        mainActivity = controller.get();

        // Then we need to swap the retrofit api impl. with a mock one
        // I usually store my Retrofit api impl as a static singleton in class RestClient, hence:
        RestClient.setApi(mockRetrofitApiImpl);

        controller.create();
    }

    @Test
    public void shouldFillAdapter() throws Exception {
        Mockito.verify(mockRetrofitApiImpl)
            .getYourObject(callbackArgumentCaptor.capture());

        int objectsQuantity = 10;
        List<YourObject> list = new ArrayList<YourObject>();
        for(int i = 0; i < objectsQuantity; ++i) {
            list.add(new YourObject());
        }

        callbackArgumentCaptor.getValue().success(list, null);

        YourAdapter yourAdapter = mainActivity.getAdapter(); // Obtain adapter
        // Simple test check if adapter has as many items as put into response
        assertThat(yourAdapter.getItemCount(), equalTo(objectsQuantity));
    }
}

Proceed with the test by right clicking the test class and hitting run.

And that's it. I strongly suggest using Robolectric (with robolectric gradle plugin) and Mockito, these libs make testing android apps whole lotta easier. I've learned this method from the following blog post. Also, refer to this answer.

Update: If you're using Retrofit with RxJava, check out my other answer on that too.

If you use .execute() instead of .enqueue() it makes execution synchron, thus the tests can ran properly without the need of importing 3 different libraries and adding any code or modify the build variants.

Like:

public class LoginAPITest {

    @Test
    public void login_Success() {

        APIEndpoints apiEndpoints = RetrofitHelper.getTesterInstance().create(APIEndpoints.class);

        Call<AuthResponse> call = apiEndpoints.postLogin();

        try {
            //Magic is here at .execute() instead of .enqueue()
            Response<AuthResponse> response = call.execute();
            AuthResponse authResponse = response.body();

            assertTrue(response.isSuccessful() && authResponse.getBearer().startsWith("TestBearer"));

        } catch (IOException e) {
            e.printStackTrace();
        }

    }

}