after_dispatcher_injection.kt

class MainViewModel (
    private val dispatcher: CoroutineDispatcher
) : ViewModel() {

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

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

@ExperimentalCoroutinesApi
class MainViewModelTest {

    private val testDispatcher = TestCoroutineDispatcher()

    @ExperimentalCoroutinesApi
    @get:Rule
    var mainCoroutineRule = MainCoroutineRule()

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

}