with_test_coroutine_dispatcher.kt

class MainViewModel : ViewModel() {

    private var userData: Any? = null
    fun getUserData(): Any? = userData

    suspend fun saveSessionData() {
        viewModelScope.launch {
            userData = "some_user_data"
        }
    }
}

@ExperimentalCoroutinesApi
class MainViewModelTest {
  
    @ExperimentalCoroutinesApi
    private val testDispatcher = TestCoroutineDispatcher()
  
    @Before
    fun setup() {
        // 1
        Dispatchers.setMain(testDispatcher)
    }

    @After
    fun tearDown() {
        // 2
        Dispatchers.resetMain()
        // 3
        testDispatcher.cleanupTestCoroutines()
    }

    @ExperimentalCoroutinesApi
    @Test
    fun testsSaveSessionData() = runBlockingTest {
        val mainViewModel = MainViewModel()
      
        mainViewModel.saveSessionData()
      
        val userData = mainViewModel.getUserData()
        assertEquals("some_user_data", userData)
    }

}