Android Test with Jetpack Compose's IdlingResource

ExampleScenarioTest.kt

@LargeTest
@RunWith(AndroidJUnit4::class)
class ExampleScenarioTest {

    @get:Rule
    val composeTestRule = createAndroidComposeRule<MainActivity>()

    private val mainIdlingResource = object : IdlingResource {
        override var isIdleNow: Boolean = true
    }

    @Before
    fun before() {
        composeTestRule.activity.mainIdlingResource.onIdleStateChanged = {
            mainIdlingResource.isIdleNow = it
        }
        composeTestRule.registerIdlingResource(mainIdlingResource)
    }

    @After
    fun after() {
        composeTestRule.unregisterIdlingResource(mainIdlingResource)
    }

    @Test
    fun testExample() {
        // do something 
        composeTestRule.waitForIdle()
        // do something 
    }
}

ExampleViewModel.kt

/**
 * @author Masayuki Higashino
 */
@HiltViewModel
class ExampleViewModel
@Inject constructor(
    private val mainIdlingResource: MainIdlingResource,
) : ViewModel() {
    fun sample() {
        mainIdlingResource.isIdleNow = false
        // do something
        mainIdlingResource.isIdleNow = true
    }
}

MainActivity.kt

/**
 * @author Masayuki Higashino
 */
@AndroidEntryPoint
class MainActivity : ComponentActivity() {

    @Inject
    @VisibleForTesting(otherwise = VisibleForTesting.PRIVATE)
    lateinit var mainIdlingResource: MainIdlingResource

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContent {
            MainScreen()
        }
    }

}

MainIdlingResource.kt

/**
 * @author Masayuki Higashino
 */
@Singleton
class MainIdlingResource @Inject constructor() {
    var isIdleNow: Boolean = true
        set(isIdleNow) {
            field = isIdleNow
            onIdleStateChanged(isIdleNow)
        }
    var onIdleStateChanged: (Boolean) -> Unit = {}
}