Wnb Viewmodel Test

Android ViewModel — unit-test skeleton

Every ViewModel that follows the UDF contract ships with a matching unit test in this shape. Same imports, same rule ordering, same factory pattern. If a rule below blocks the task, stop and surface the conflict — don't silently break the contract.

Pairs with [[wnb-viewmodel-udf]].

Non-negotiables

1. @get:Rule val mainDispatcherRule = MainDispatcherRule() at the top of every test class. Replaces Dispatchers.Main with a StandardTestDispatcher for the duration of each test.
2. private val dispatcherProvider = TestDispatcherProvider(mainDispatcherRule.testDispatcher) — reuses the rule's dispatcher so all IO work runs on the same virtual clock as the main dispatcher. Never construct a second TestDispatcher.
3. Factory function newViewModel(...) at the top of the class. Every dependency has a default FakeXxxRepository(). Individual tests override only what they need. Keeps each test independent, no shared mutable state.
4. @OptIn(ExperimentalCoroutinesApi::class) on the class (or per-test) — runTest, advanceUntilIdle, TestDispatcher require it.
5. runTest { ... } wraps every coroutine test. advanceUntilIdle() after any launch { } in the SUT to drain the queue. Use runCurrent() when you need a single step and want to inspect intermediate state.
6. Truth assertions (assertThat(x).isEqualTo(y)), not JUnit assertEquals. Clearer failure messages, chained matchers.
7. Fakes, never mocks, for repository interfaces. Fakes live in app/src/test/java/.../testing/fakes/ (or the test source set your project uses). A mocked interface is a lie — a fake exercises the real contract.
8. No Thread.sleep. Use advanceUntilIdle(), advanceTimeBy(ms), or Turbine's awaitItem().
9. Turbine for Flow / SharedFlow emissions. Snapshot with state.value is fine when asserting a single final state, but any sequence-of-emissions assertion (loading → loaded, one-shot events) uses state.test { ... } / navigationEvent.test { ... }.
10. Backtick test names describe the behavior, not the method under test: `unauthenticated user routes to SignUp and marks initialized`(). Given–When–Then is fine but keep it terse.

Canonical example

@OptIn(ExperimentalCoroutinesApi::class)
class FooViewModelTest {

    @get:Rule
    val mainDispatcherRule = MainDispatcherRule()

    private val dispatcherProvider = TestDispatcherProvider(mainDispatcherRule.testDispatcher)

    private fun newViewModel(
        repo: FakeFooRepository = FakeFooRepository(),
    ) = FooViewModel(repo, dispatcherProvider)

    @Test
    fun `initial state is loading, no foos, no error`() = runTest {
        val vm = newViewModel()

        // Observe the first emission before letting coroutines run
        assertThat(vm.state.value.isLoading).isTrue()
        assertThat(vm.state.value.foos).isEmpty()
        assertThat(vm.state.value.error).isNull()
    }

    @Test
    fun `observeFoos emits foos and clears loading`() = runTest {
        val repo = FakeFooRepository(foos = listOf(Foo("1"), Foo("2")))
        val vm = newViewModel(repo = repo)

        advanceUntilIdle()

        assertThat(vm.state.value.foos).hasSize(2)
        assertThat(vm.state.value.isLoading).isFalse()
        assertThat(vm.state.value.error).isNull()
    }

    @Test
    fun `refresh failure surfaces error and clears loading`() = runTest {
        val repo = FakeFooRepository(refreshResult = Result.failure(RuntimeException("nope")))
        val vm = newViewModel(repo = repo)
        advanceUntilIdle()

        vm.onAction(FooAction.Refresh)
        advanceUntilIdle()

        assertThat(vm.state.value.error).isEqualTo("nope")
        assertThat(vm.state.value.isLoading).isFalse()
    }

    @Test
    fun `Select action updates selected state`() = runTest {
        val foo = Foo("1")
        val vm = newViewModel(repo = FakeFooRepository(foos = listOf(foo)))
        advanceUntilIdle()

        vm.onAction(FooAction.Select(foo))

        assertThat(vm.state.value.selected).isEqualTo(foo)
    }

    @Test
    fun `NavigateToDetail emits a one-shot navigation event`() = runTest {
        val vm = newViewModel()
        advanceUntilIdle()

        vm.navigationEvent.test {
            vm.onAction(FooAction.NavigateToDetail("foo-1"))
            assertThat(awaitItem()).isEqualTo("foo-1")
            cancelAndConsumeRemainingEvents()
        }
    }

    @Test
    fun `state emits loading then loaded when observing repo`() = runTest {
        val repo = FakeFooRepository(foos = listOf(Foo("1")))

        newViewModel(repo = repo).state.test {
            assertThat(awaitItem().isLoading).isTrue()          // initial
            advanceUntilIdle()
            val settled = expectMostRecentItem()
            assertThat(settled.isLoading).isFalse()
            assertThat(settled.foos).hasSize(1)
            cancelAndConsumeRemainingEvents()
        }
    }
}

Common mistakes

• Constructing a new UnconfinedTestDispatcher() inside the test → the SUT and the test now use different schedulers; advanceUntilIdle() won't drive the SUT. Always reuse mainDispatcherRule.testDispatcher.
• Thread.sleep(500) to wait for a coroutine → flaky on CI, doesn't advance the virtual clock. Use advanceUntilIdle() or advanceTimeBy(ms).
• Mocking FooRepository interface → a mock lies about the contract. Write a FakeFooRepository that implements the real interface with in-memory state.
• Reusing a single ViewModel instance across tests as a class field → cross-test bleed. Always construct via newViewModel(...) inside the test.
• Asserting SharedFlow with state.value → SharedFlow has no .value. Use Turbine (event.test { assertThat(awaitItem())... }).
• Calling runBlocking instead of runTest → skips the test-dispatcher machinery, hits real time. Always runTest.
• Forgetting advanceUntilIdle() after onAction → assertion runs before the launched coroutine mutates state; test fails intermittently.
• Asserting on _state (private backing field) via reflection → fragile and tells you nothing the public state doesn't. Assert on state.value or via Turbine.
• One giant test asserting five unrelated things → when it fails, you don't know which behavior broke. One test = one behavior.
• Silent advanceUntilIdle() inside a state.test { } block — Turbine already collects; you rarely need to advance again unless the SUT is doing timed work.

When to move a test to :domain

If the assertion is about rules (discount math, coupon eligibility, cart total) and the ViewModel is just plumbing, extract to a use case in the domain module and test the use case with pure JUnit — no dispatcher, no Turbine, no fakes. Faster to write, faster to run, catches the real bug.

Related skills

• [[wnb-viewmodel-udf]] — the ViewModel shape this test targets.
• [[wnb-compose-ui-test]] — the UI-side test that verifies the state renders correctly.