Is there away to mock ViewModel that is inside of a fragment

Question

Is there a way to mock ViewModel that's built is inside of a fragment? I'm trying to run some tests on a fragment, one of the fragment functions interacts with the ViewModel, I would like to run the test function and provided a mocked result for the ViewModel. Is this even possilbe?

MyFragment

class MyFragment : Fragment() {
    @Inject
    lateinit var viewModel: MyViewModel
    
    override fun onCreate(savedInstanceState: Bundle?) {
        (requireActivity().application as MyApplication).appComponent.inject(this)
        super.onCreate(savedInstanceState)
    }
    
} 

Test

@RunWith(RoboeltricTestRunner::) {
    
    @Before
    fun setup() {
        FragmentScenario.Companion.launchIncontainer(MyFragment::class.java)
    }
}

Answer 1

I thought I would post this for anyone else struggling to find a solution. You'll want to use a Fragment Factory, that has a dependency on the ViewModel. Injecting the ViewModel into the fragments constructor allows the ViewModel to easliy be mocked. There are a few steps that need to be completed for a FragmentFactory but it's not that complicated once you do a couple of them.

1. Fragment Add the ViewModel into the constructor.

class MyFragment(private val viewModel: ViewModel) : Fragment {

    ... 
}

2. FragmentFactory, allows fragments to have dependencies in the constructor.

class MyFragmentFactory(private val viewModel: MyViewModel) : FragmentFactory() {
    override fun instantiate(classLoader: ClassLoader, className: String): Fragment {
        return when(className) {

            MyFirstFragment::class.java.name -> {
                MyFragment(viewModel)
            }

            // You could use this factory for multiple Fragments.
            MySecondFragment::class.java.name -> {
                MySecondFragment(viewModel)
            }

            // You also don't have to pass the dependency
            MyThirdFragment::class.java.name -> {
                MyThirdFragment()
            }
            else -> super.instantiate(classLoader, className)
        }
    }
}

3. Main Activity

class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        // Create your ViewModel
        val viewModel = ViewModelProvider(this).get(MainViewModel::class.java)

        // create the FragmentFactory and the viewModel dependency.
        supportFragmentManager.fragmentFactory = MainFragmentFactory(viewModel)

        // FragmentFactory needs to be created before super in an activity. 
        super.onCreate(savedInstanceState)

    }
}

4. Test

@RunWith(RobolectricTestRunner::class)
class MyFragmentUnitTest {
    
    @Before 
    fun setup() { 
        val viewModel: MainViewModel = mock(MyViewModel::class.java)
        ...
    }
}