How can I test debounce with turbine?

[CharAlex]

Hi, I am wondering how can I test the debounce with turbine.
I have the following ViewModel:

private val _stepDataChange = MutableSharedFlow<String>()
  val stepDataChange = _stepDataChange.asSharedFlow()

  val performSearchFlow = MutableSharedFlow<String>()

  init {
    viewModelScope.launch(dispatcher) {
      bindPerformSearchIntent()
    }
  }
  
  fun onTextChange(term: String) {
    viewModelScope.launch(dispatcher) {
      performSearchFlow.emit(term)
    }
  }

  private suspend fun bindPerformSearchIntent() {
    performSearchFlow
      .debounce(300)
      .collectLatest { titleText ->
        _stepDataChange.emit(titleText)
      }
  }

Here is the test:

@OptIn(ExperimentalCoroutinesApi::class)
  @get: Rule
  var coroutinesTestRule = CoroutineTestRule()

  @ExperimentalCoroutinesApi
  @Test
  fun testDebounce() = runTest {
    val vm = MainViewModel(
      dispatcher = UnconfinedTestDispatcher(TestCoroutineScheduler())
    )

    val flow = vm.stepDataChange.testIn(this)

    vm.onTextChange("Test")

    advanceTimeBy(300)

    assertEquals("Test", flow.awaitItem())

    flow.cancel()
  }

where CoroutineTestRule:

@ExperimentalCoroutinesApi
class CoroutineTestRule() : TestWatcher() {

  override fun starting(description: Description) {
    super.starting(description)
    Dispatchers.setMain(UnconfinedTestDispatcher(TestCoroutineScheduler()))
  }

  override fun finished(description: Description) {
    super.finished(description)
    Dispatchers.resetMain()
  }
}

My test is getting timed out, not sure what am I doing wrong here?
While debugging the breakpoint inside the collectLatest block is never triggered.
If I remove the debounce or set it to 0 the test passes.

Versions:
kotlin: 1.5.31
coroutines: 1.6.1
turbine: 0.8.0

[mhernand40]

Looking at your ViewModel, it appears you are already passing in a CoroutineDispatcher via the constructor. To rule out any unnecessary factors, your tests don't need to use CoroutineTestRule.

As for the test itself, all test dispatchers should be sharing the same scheduler. According to https://developer.android.com/kotlin/coroutines/test#testdispatchers

Key Point: runTest runs the test coroutine in a TestScope, using a TestDispatcher. TestDispatchers use a TestCoroutineScheduler to control virtual time and schedule new coroutines in a test. All TestDispatchers in a test must use the same scheduler instance.

Hence, I'd update

    val vm = MainViewModel(
      dispatcher = UnconfinedTestDispatcher(TestCoroutineScheduler())
    )

to

    val vm = MainViewModel(
      dispatcher = UnconfinedTestDispatcher(testScheduler)
    )

Where testScheduler comes from the this receiver of runTest.

Also, make sure to update to the latest version of Turbine, since there have been some significant changes and fixes lately. Not sure if your tests will suddenly start passing with these changes, but at least you can troubleshoot from there.

[alexandroschar]

Thank you for your explanation! Tests are passing now.