MVVM on Android

When testing the model layer, we used TDD. For the view model, we’ll now add an after-the-fact test for the view model we’ve just created.

Since the viewmodel depends on CreatureGenerator in the model layer, we’ll create a mock creature generator in our view model test, to ensure that we’re isolating the test to the view model. We’ll create the mock creature generator using the Mockito library. In the app module build.gradle file, add the Mockito dependency to the testImplementation:

  testImplementation "org.mockito:mockito-core:2.11.0"

Since we’re using ViewModel and LiveData from the Android Architecture components, we also need to add the associated testing dependency.

testImplementation "android.arch.core:core-testing:$arch_comp_version"

Now open CreatureViewModel, select the class name, and hit option+return on Mac or alt+enter on PC to create an associated test class.

class CreatureViewModelTest {
}

We confirm that the new file is in the viewmodel subpackage of the test package. Add a property for the viewmodel being tested.

  private lateinit var creatureViewModel: CreatureViewModel

Since our ViewModel uses LiveData, we need to add a test rule that swaps out the background thread executor normally used with a synchronous thread executor.

  @get:Rule
  var rule: TestRule = InstantTaskExecutorRule()

Next, we add a property for the mock creature generator for the view model.

  @Mock
  lateinit var mockGenerator: CreatureGenerator
  

We add a setup method annotated with @Before, and in the method we init the mocks and setup the creatureViewModel to be tested.

  @Before
  fun setup() {
    MockitoAnnotations.initMocks(this)
    creatureViewModel = CreatureViewModel(mockGenerator)
  }

Now we’ll add a test for setting up the creature being generated in the view model, testSetupCreature(). First we arrange the expected value as a stubCreature.

  @Test
  fun testSetupCreature() {
    val attributes = CreatureAttributes(10, 3, 7)
    val stubCreature = Creature(attributes, 87, "Test Creature")
    `when`(mockGenerator.generateCreature(attributes)).thenReturn(stubCreature)
    
    creatureViewModel.intelligence = 10
    creatureViewModel.strength = 3
    creatureViewModel.endurance = 7
  }

We use the when method for the mock generator to return stubCreature. Now we perform the action being tested, which is a call to updateCreature on the viewmodel.

 
    creatureViewModel.updateCreature()
   

And then we assert that the viewmodel creature is the stubCreature we’re expecting.

    assertEquals(`stubCreature`, creatureViewModel.creature)

We can go ahead and run the tests and see that the view model updateCreature method is working correctly.