🏌A Simplified Android Ktx ViewModel Test ⛳️

You cannot test Ktx ViewModel directly if you switch from val to var. The system will give you a ❌ RED flag at once. Unless we wrap it up with a lazy function. Here is my process with my example:

— === Menu === —

🔌1. Gradle.Module
🔑2. Add ProvideModel Function
⚜………………………..⭐ProvideViewModel.kt
🔬 3. Test the ViewModel
⚜………………………..🏭 Fragment Factory
⚜………………………..✔️ LiveData Support Function
⚜………………………..✅ Test the ViewModel

🔌1. Gradle.Module …… → Menu

Add this line to your Gradle.module:

// Kotlin Reflect
implementation "org.jetbrains.kotlin:kotlin-reflect:$kotlin_version"

🔑2. Add ProvideModel Function …… → Menu

With Google ViewModel Ktx version, for example,

val viewModel: ShoppingViewModel by viewModels()

You can not test this viewModel in the test case. Unless you go back to the old way:

lateinit var viewModel: ShoppingViewModel
onCreate() {
    viewModel = ViewModelProvider(...)
}

Let’s add a function to make Ktx be testable by adding ⭐ ProvideViewModel.kt: …… → Menu

inline fun <reified VM : ViewModel> Fragment.provideViewModel(
    noinline ownerProducer: () -> ViewModelStoreOwner = { this },
    noinline factoryProducer: (() -> ViewModelProvider.Factory)? = null
): Lazy<VM> =
    OverridableLazy(viewModels(ownerProducer, factoryProducer))

// wrapper
class OverridableLazy<T>(var implementation: Lazy<T>): Lazy<T> {
    override val value
        get() = implementation.value
    override fun isInitialized() = implementation.isInitialized()
}

// fragment extended function
fun <VM: ViewModel, T> T.replace(
    viewModelDelegate: KProperty1<T, VM>, viewModel: VM) {
    viewModelDelegate.isAccessible = true
    (viewModelDelegate.getDelegate(this) as
            OverridableLazy<VM>).implementation = lazy { viewModel }
}

Replace the viewModels() in the Fragment:

val viewModel: ShoppingViewModel by provideViewModel()

🔬 3. Test the ViewModel …… → Menu

🏭 Fragment Factory …… → Menu

Let’s add a Fragment Factory to instantiate the Fragment.

⭐ CoolFragmentFactory.kt

class CoolFragmentFactory @Inject constructor(): FragmentFactory() {

    override fun instantiate(classLoader: ClassLoader, className: String): Fragment {
        // dispatch by className
        return when (className) {
            YourFragment::class.java.name -> YourFragment()
            else -> super.instantiate(classLoader, className)
        }
    }
}

Inject the factory into the YourFragmentTest.kt.

@MediumTest
@HiltAndroidTest
@ExperimentalCoroutinesApi
class YourFragmentTest {

    @get:Rule
    var hiltRule = HiltAndroidRule(this)

    @get:Rule
    var instantTaskExecutorRule = InstantTaskExecutorRule()

    @Inject
    lateinit var fragmentFactory: CoolFragmentFactory

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

✔️ LiveData Support Function …… → Menu

Copied from Google,

@VisibleForTesting(otherwise = VisibleForTesting.NONE)
fun <T> LiveData<T>.getOrAwaitValue(
    time: Long = 2,
    timeUnit: TimeUnit = TimeUnit.SECONDS,
    afterObserve: () -> Unit = {}
): T {
    var data: T? = null
    val latch = CountDownLatch(1)
    val observer = object : Observer<T> {
        override fun onChanged(o: T?) {
            data = o
            latch.countDown()
            this@getOrAwaitValue.removeObserver(this)
        }
    }
    this.observeForever(observer)

    try {
        afterObserve.invoke()

        // Don't wait indefinitely if the LiveData is not set.
        if (!latch.await(time, timeUnit)) {
            throw TimeoutException("LiveData value was never set.")
        }

    } finally {
        this.removeObserver(observer)
    }

    @Suppress("UNCHECKED_CAST")
    return data as T
}

✅ Test the ViewModel …… → Menu

For example,

@Test
fun backpressed_setImageUrl_blank_test() {
    val imageUrl = "Test"
    val testViewModel = YourViewModel(...)
    
    testViewModel.setCurImageUrl(imageUrl)
   
    launchFragmentInHiltContainer<YourFragment>(
        fragmentFactory = fragmentFactory
    ) {
        navController.setGraph(R.navigation.cool_nav)
        Navigation.setViewNavController(
            requireView(), 
            navController)
        navController.navigate(R.id.add_item)
   
        this.replace(
            YourFragment::viewModel, 
            testViewModel)
    }
    
    Espresso.pressBack()
    runOnUiThread {
        Truth.assertThat(testViewModel.curImgUrl.getOrAwaitValue())
            .isEqualTo("")
    }
}

In this example, you’ll found that is very simple to replace the Ktx viewModels() with testViewModel. And I can test its LiveData, too. The LiveData has a preset URL as “Test”. When I press the back button, the LiveData shall be reset to blank.

The test has passed! Isn’t that great?

🥳Try it yourself! Have fun! …… → Menu