Recipe: Test a presenter that navigates¶

Problem: verify that a presenter emits the right states and navigates where you expect, in response to events.

Use Presenter.test {} from circuit-test and a FakeNavigator. test {} gives you a Turbine for presenter states. Drive the presenter by invoking the eventSink on an emitted state.

@Test
fun `clicking an item navigates to its detail`() = runTest {
  val navigator = FakeNavigator(FeedScreen)
  val presenter = FeedPresenter(FeedScreen, navigator, FakeFeedRepository(items = listOf(item1)))

  presenter.test {
    // Consume the initial loading emission before asserting on loaded state.
    assertEquals(FeedState.Loading, awaitItem())

    val loaded = assertIs<FeedState.Loaded>(awaitItem())
    loaded.eventSink(FeedEvent.OpenItem(item1.id))     // simulate the tap

    assertEquals(DetailScreen(item1.id), navigator.awaitNextScreen())
  }
}

Common FakeNavigator assertions:

awaitNextScreen() — the next screen pushed via goTo
awaitPop() — a pop() happened
awaitResetRoot() — a resetRoot() happened
assertGoToIsEmpty() — assert no navigation occurred

Things to watch for:

Consume Loading first. A presenter that starts in Loading emits it before the loaded state. assertIs<FeedState.Loaded>(awaitItem()) on the first item will fail — await the loading emission first.
awaitItem() is distinct-until-changed. Identical consecutive states collapse into one, so you assert state changes, not every recomposition. When you specifically want to assert a recomposition produced no change, use the escape hatch awaitUnchanged() — it awaits the next emission and fails if it differs from the previous one.

For UI-level event assertions, render the composable with a TestEventSink and assert the events it emits — see the testing doc.