Android Mocking a Dagger2 injected dependency for a Espresso test

Question

I have a heavily dependency injected (dagger2) application. I would like to run an espresso test without having the test navigate through the whole application

Answer 1

First, your decision to use dependency injection (Dagger2) is a very good one and will indeed make your tests easier to write.

You have to override dependency injection configuration (module) and inject a mock. Here is a simple example of how it can be done.

First you need a mock:

LoginStateManager lsmMock = mock(LoginStateManager.class);

Now override the DI config to use this mock:

//Extend your TelePresenterModule, override provider method
public class TestTelePresenterModule extends TelePresenterModule{
    @Override
    public LoginStateManager getLoginStateManager() {
        //simply return the mock here
        return lsmMock;
    }
}

Now to the test:

@Test
//this is an espresso test
public void withAMock() {
    //build a new Dagger2 component using the test override
    TelePresenterComponent componentWithOverride = DaggerTelePresenterComponent.builder()
            //mind the Test in the class name, see a class above
            .telePresenterModule(new TestTelePresenterModule())
            .build();

    //now we initialize the dependency injector with this new config
    DaggerInjectorTele.set(componentWithOverride);

    mActivityRule.launchActivity(null);

    //verify that injected mock was interacted with
    verify(lsmMock).whatever();
}

Example from: https://github.com/yuriykulikov/DIComparison/blob/master/app/src/androidTest/java/com/example/yuriy/dependencyinjectioncomparison/Dagger2Test.java