For 'regular' Java project overriding the dependencies in the unit tests with mock/fake ones is easy. You have to simply build your Dagger component and give it to the 'main' class that drives you application.
For Android things are not that simple and I've searched for a long time for decent example but I was unable to find so I had to created my own implementation and I will really appreciate feedback is this a correct way to use Dagger 2 or there is a simpler/more elegant way to override the dependencies.
Here the explanation (project source can be found on github):
Given we have a simple app that uses Dagger 2 with single dagger component with single module we want to create android unit tests that use JUnit4, Mockito and Espresso:
In the MyApp Application class the component/injector is initialized like this:
public class MyApp extends Application {
private MyDaggerComponent mInjector;
public void onCreate() {
super.onCreate();
initInjector();
}
protected void initInjector() {
mInjector = DaggerMyDaggerComponent.builder().httpModule(new HttpModule(new OkHttpClient())).build();
onInjectorInitialized(mInjector);
}
private void onInjectorInitialized(MyDaggerComponent inj) {
inj.inject(this);
}
public void externalInjectorInitialization(MyDaggerComponent injector) {
mInjector = injector;
onInjectorInitialized(injector);
}
...
In the code above:
Normal application start goes trough onCreate() which calls initInjector() which creates the injector and then calls onInjectorInitialized().
The externalInjectorInitialization() method is ment to be called by the unit tests in order to set the injector from external source, i.e. a unit test.
So far, so good.
Let's see how the things on the unit tests side looks:
We need to create MyTestApp calls which extends MyApp class and overrides initInjector with empty method in order to avoid double injector creation (because we will create a new one in our unit test):
public class MyTestApp extends MyApp {
@Override
protected void initInjector() {
// empty
}
}
Then we have to somehow replace the original MyApp with MyTestApp. This is done via custom test runner:
public class MyTestRunner extends AndroidJUnitRunner {
@Override
public Application newApplication(ClassLoader cl,
String className,
Context context) throws InstantiationException,
IllegalAccessException,
ClassNotFoundException {
return super.newApplication(cl, MyTestApp.class.getName(), context);
}
}
... where in newApplication() we effectively replace the original app class with the test one.
Then we have to tell the testing framework that we have and want to use our custom test runner so in the build.gradle we add:
defaultConfig {
...
testInstrumentationRunner 'com.bolyartech.d2overrides.utils.MyTestRunner'
...
}
When a unit test is run our original MyApp is replaced with MyTestApp. Now we have to create and provide our component/injector with mocks/fakes to the app with externalInjectorInitialization(). For that purpose we extends the normal ActivityTestRule:
@Rule
public ActivityTestRule<Act_Main> mActivityRule = new ActivityTestRule<Act_Main>(
Act_Main.class) {
@Override
protected void beforeActivityLaunched() {
super.beforeActivityLaunched();
OkHttpClient mockHttp = create mock OkHttpClient
MyDaggerComponent injector = DaggerMyDaggerComponent.
builder().httpModule(new HttpModule(mockHttp)).build();
MyApp app = (MyApp) InstrumentationRegistry.getInstrumentation().
getTargetContext().getApplicationContext();
app.externalInjectorInitialization(injector);
}
};
and then we do our test the usual way:
@Test
public void testHttpRequest() throws IOException {
onView(withId(R.id.btn_execute)).perform(click());
onView(withId(R.id.tv_result))
.check(matches(withText(EXPECTED_RESPONSE_BODY)));
}
Above method for (module) overrides works but it requires creating one test class per each test in order to be able to provide separate rule/(mocks setup) per each test. I suspect/guess/hope that there is a easier and more elegant way. Is there?
This method is largely based on the answer of @tomrozb for this question. I just added the logic to avoid double injector creation.
