MVVM on Android

In this video, we’ll save new creatures into the repository we previously setup. We don’t want the user to be able to save creatures with blank attributes, for example, a blank name. Let’s write a view model test to make sure that creatures with blank names can’t be saved.

  @Test
  fun testCantSaveCreatureWithBlankName() {
    creatureViewModel.intelligence = 10
    creatureViewModel.strength = 3
    creatureViewModel.endurance = 7
    creatureViewModel.drawable = 1
    creatureViewModel.name = ""
     val canSaveCreature = creatureViewModel.canSaveCreature()
     assertEquals(false, canSaveCreature)
  }

The canSaveCreature method on the viewmodel does not exist yet, but we’ll add it soon. In CreatureViewModel, add a repository property to connect the viewmodel to the creature repository.

private val repository: CreatureRepository = RoomRepository()

The type on the repository property is the CreatureRepository interface, but the default value for it is the concrete RoomRepository.

If your app is sufficiently complex, you may want to use a dependency injection framework like Dagger 2 or Koin to inject the repository into the viewmodel.

Having the repository be passed in to the viewmodel as a constructor property is enough injection for our case. In CreatureViewModelTest, add a mock for the creature repository.

  @Mock
  lateinit var repository: CreatureRepository

And update the setup method to add the mock repository into the constructor call for the view model.

creatureViewModel = CreatureViewModel(mockGenerator, repository)

Now we add canSaveCreature() into the viewmodel.

   fun canSaveCreature(): Boolean {
    return intelligence != 0 && strength != 0 && endurance != 0 &&
        name.isNotEmpty() && drawable != 0
  }

We have a condition that none of intelligence, strength, endurance or drawable can be zero, and the name cannot be empty.

Add a saveCreature() method to the viewmodel that calls canSaveCreature() and saves the creature to the repository if the creature can be saved.

  fun saveCreature(): Boolean {
    return if (canSaveCreature()) {
      repository.saveCreature(creature)
      true
    } else {
      false
    }
  }

Now we’ll run our new test for the viewmodel and see that it passes. I’ll leave writing more tests for other empty properties in the viewmodel as a challenge for you in the next video. In CreatureActivity, in the click listener for the save button, we can now ask the view model to save the creature.

      if (viewModel.saveCreature()) {
        Toast.makeText(this, getString(R.string.creature_saved), Toast.LENGTH_SHORT).show()
        finish()
      } else {
        Toast.makeText(this, getString(R.string.error_saving_creature), Toast.LENGTH_SHORT).show()
      }

If the creature saves, we show a toast and finish, otherwise, we show an error toast.

Now we build and run the app. I’ll create and save a couple of new creatures. If I try saving a creature with no name, I see the error toast telling me to fill out all fields.

We can use Device File Explorer to see that the Room database file for storing creatures has been created. In a later video, we’ll pull creatures out of the database for display.