Android Hilt example and a unit test (with Robolectric)

Android Hilt has been released and I must say I'm quite impressed. I absolutely hate Dagger which I think comprises of far too much boiler plate code, its badly documented and incredibly confusing. Hilt is of course built on top of dagger and I think this is a huge win for the Android community. Hilt decreases confusion, increases code generation and is nice and simple.

I don't want to go into what Dependency Injection (DI) is, that's been done to death.

There are two resources I found really useful to get started with Hilt:

The Hilt documentation - https://dagger.dev/hilt/

Coding with Mitch videos: https://www.youtube.com/watch?v=zTpM2olXCok

There are two things I want to talk about in this blog post:

1. A really quick intro and setup of a Hilt module.
2. A unit test using Robolectic.

Before I go much further I would discourage you from using Robolectric. It's become a real pain to work with, requiring Java 9 for its latest version and regularly deprecating things without clear documentation on how to migrate. That said I've been using Robolectic for a while so I'm stuck with it until I can remove it from my code. I'm hoping Hilt and ViewModels will help me do this.

The first thing I wanted to look at is (I think) a fairly standard use case where a class has some dependencies and we want to use that class in our activity without creating it every time.

Step 1 - The application

@HiltAndroidApp
class MyApplication : Application()

Step 2 - The Class

class Petrol(val input: String) {

    fun getTheInfo(): String {
        return input
    }
}

Step 3 - The Module

Now this is the interesting bit, here we define a "factory" where we list how the class is to be created. Obviously this is a very simple example, but I think that's a good way to start.

@InstallIn(ApplicationComponent::class)
@Module
class PetrolFactory {

    @Provides
    fun getPetrol(): Petrol {
        return Petrol("Petroll")
    }
}

Step 4 - The Activity

Now we setup the activity and you'll note how little code we need to write to create and start using the class

@AndroidEntryPoint
class MainActivity : AppCompatActivity() {

    @Inject
    lateinit var petrol: Petrol

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        setSupportActionBar(findViewById(R.id.toolbar))
        
        findViewById<TextView>(R.id.main_text).text = petrol.getTheInfo()
    }
}

I think this is fantastic and hopefully it's clear to see how easy this is and how quick you can make complex interactions really simple.

Step 5 - The Tests

With the tests I want to show how you would pass in something different for the purpose of testing. 

@UninstallModules(PetrolFactory::class)
@HiltAndroidTest
class MainActivityTest : RobolectricTestConfig() {

    @get:Rule
    var hiltRule = HiltAndroidRule(this)

    @Module
    @InstallIn(ApplicationComponent::class)
    class TestEngine {
        @Provides
        fun getPetrol(): Petrol {
            return Petrol("TestPetrol")
        }
    }


    @Before
    fun init() {
        hiltRule.inject()
    }

    @Test
    fun testActivityMainText() {
        val scenario = ActivityScenario.launch(MainActivity::class.java)

        scenario.onActivity {
            assertEquals("TestPetrol", it.findViewById<TextView>(R.id.main_text).text)
        }
    }
}

One thing that's really important to remember is you can't run the test with the green arrow in Android Studio. You need to use gradle. So just run the gradle task instead. That's it I hope that helps demonstrate how you would create a DI app with Hilt in a few easy steps. I also hope you can see how easy that makes testing.