Hi :android-wave: , I am stuck in choosing the cor...

# coroutines

[Abhimanyu]

Hi 👋 ,
I am stuck in choosing the correct test dispatcher - StandardTestDispatcher vs UnconfinedTestDispatcher .

In short, StandardTestDispatcher - does not start the coroutines eagerly. - I am stuck on how to start the coroutines when using StandardTestDispatcher .
For UnconfinedTestDispatcher - This starts the coroutines eagerly, but if I have multiple continuous coroutines collected in my ViewModel using collectLatest , only the first coroutines is launched and the subsequent coroutines are not launched as the test has only one thread.

Can anyone help share how to decide the correct test dispatcher and how to resolved these issues?
I have gone through the docs and some resources, but I am still not clear on testing them. thank you color

[Abhimanyu]

Note: All coroutines creation and emitting are handled by the ViewModel, but most of the examples I could find show how to test when collection and emission is handled within the tests.

[Zach Klippenstein (he/him) [MOD]]

I would avoid UnconfinedTestDispatcher if at all possible. It's a huge behavior change from how stuff runs in production and will cause headaches down the road, if not right away.

[Abhimanyu]

Hi @Zach Klippenstein (he/him) [MOD],
Thanks for responding.
I see this mentioned everywhere to prefer StandardTestDispatcher .
But I am not able to understand how to launch the coroutines.

[Abhimanyu]

Can you please help?

A minified example,

class MyViewModel(
  coroutineScope: CoroutineScope,
) : ViewModel(
  viewModelScope = coroutineScope,
) {
  val stateFlow1 = MutableStateFlow(1)
  val stateFlow2 = MutableStateFlow(2)

  fun testMethod() {
    viewModelScope.launch {
      getFlow1UseCase().collectLatest {
        stateFlow1.value = 10
      }
    }
    viewModelScope.launch {
      getFlow2UseCase().collectLatest {
        stateFlow2.value = 20
      }
    }
  }
}

Test

class MyViewModelTest {
    private lateinit var testDispatcher: TestDispatcher
    private lateinit var testScope: TestScope

    private lateinit var viewModel: MyViewModel

    @Before
    fun setUp() = runTest {
        testDispatcher = StandardTestDispatcher(
            scheduler = testScheduler,
        )
        testScope = TestScope(
            context = testDispatcher,
        )
    }

    @Test
    fun `when flows emits data, then state flows are updated accordingly`() = runTestWithTimeout {
            whenever(
                methodCall = getFlow1UseCase(),
            ).thenReturn(flowOf(10))
            whenever(
                methodCall = getFlow2UseCase(),
            ).thenReturn(flowOf(20))
            initViewModel(
                coroutineScope = testScope,
            )

            viewModel.testMethod()

            assertEquals(10, viewModel.stateFlow1.value)
            assertEquals(20, viewModel.stateFlow2.value)
        }

    private fun initViewModel(
        coroutineScope: CoroutineScope,
    ) {
        viewModel = MyViewModel(
            coroutineScope = coroutineScope,
        )
    }

    private fun runTestWithTimeout(
        block: suspend TestScope.() -> Unit,
    ) = runTest(
        timeout = 3.seconds,
        testBody = block,
    )
}

[jw]

Your test shows you injecting the scope but your actual view model shows you using viewModelScope. Which are you actually doing?

[jw]

If you are actually using viewModelScope you'll need to use Dispatchers.setPain Dispatchers.setMain to replace the main dispatcher with the test one. Be sure to set it back in @After.

[Abhimanyu]

Sorry, missed to add that part.
I am overriding the viewModelScope in my ViewModel constructor.

[Abhimanyu]

I am following this to inject the Coroutine scope in the view model and it overrides the viewModelScope
https://developer.android.com/jetpack/androidx/releases/lifecycle#2.5.0

[Abhimanyu]

I am referring this talk by Márton Braun .

https://youtu.be/nKCsIHWircA?si=cfDjjS9CzDIjfvuh▾

Does this mean async is always preferred over launch even when fire and forget works for my functionality?

[Zach Klippenstein (he/him) [MOD]]

I don't think this is the cause of whatever issue you're seeing, but this test class is very weird. Why are you calling runTest in setUp? That doesn't make any sense since the setUp method returns before your actual test runs. Then you're also calling runTest for your actual test method, but then not using any of the coroutine machinery provided by the test's runTest, instead using the stuff from the runTest in setUp.

[Zach Klippenstein (he/him) [MOD]]

And I'm not clear what actual issue you're seeing – you said a coroutine isn't being launched? Why do you think that?

[Francesc]

I am overriding the viewModelScope in my ViewModel constructor.

providing a coroutine scope to the viewmodel constructor does not override the existing viewmodelScope, you have to explicitly use the scope that you pass in the constructor to launch your coroutines

[Abhimanyu]

Hi Zach,

And I'm not clear what actual issue you're seeing – you said a coroutine isn't being launched? Why do you think that?

I have added println() like this,

fun testMethod() {
    println("getFlow1UseCase before launch")
    viewModelScope.launch {
      println("getFlow1UseCase launch")
      getFlow1UseCase().collectLatest {
        stateFlow1.value = 10
      }
    }
    println("getFlow2UseCase before launch")
    viewModelScope.launch {
    println("getFlow2UseCase launch")
      getFlow2UseCase().collectLatest {
        stateFlow2.value = 20
      }
    }
}

I don't see getFlow2UseCase launch being printed, hence I infer that the coroutines is not launched.

Regarding the setup,
Thanks for pointing it out, I was trying to reuse the TestDispatcher initialization code which required runTest .
I will do some refactoring to see if that is the root cause of my issue.

[Abhimanyu]

Hi @Francesc,
I am overriding the viewModelScope as mentioned here.
I am following this to inject the Coroutine scope in the view model and it overrides the viewModelScope
https://developer.android.com/jetpack/androidx/releases/lifecycle#2.5.0

It is mentioned here that we can provide CoroutineScope as a parameter to ViewModel to override the viewModelScope .

[Zach Klippenstein (he/him) [MOD]]

Your assertions are being executed immediately after you call launch, so on the standard dispatcher they might not have actually started yet. You’ll want to do something like advance the test scheduler until idle before asserting.