Unit Testing

Kotlin Unit Testing Guidelines & Best Practices for Flow

This skill outlines the standards and conventions for writing unit tests in the Flow codebase (io.github.aedev.flow).

1. Naming Conventions

Test Class Naming

• Test class names MUST mirror the target class with a Test suffix.
  • Example: LikedVideosViewModel → LikedVideosViewModelTest
  • Example: HistoryViewModel → HistoryViewModelTest
  • Example: SearchViewModel → SearchViewModelTest

Test Method Naming

• Use backtick-quoted descriptive phrases outlining the scenario and expected outcome:
  • Format: `[given condition or action] [expected result]`
  • Good: `initial state loads liked videos from repository`
  • Good: `removeLike calls repository and updates state`
  • Good: `search with query updates results flow`
  • Bad: testRemoveLike(), test1()

2. Mocking Framework & Cleanup Rules

• Framework: Use MockK (io.mockk.*).
• Initialization: Create mocks in @Before or as class properties via mockk(relaxed = true) or mockk().
• Teardown / Cleanup: ALWAYS clean up mocks in an @After method using unmockkAll() or clearAllMocks() to ensure isolated test execution and prevent memory leaks/polluted mock states across tests.

@Before
fun setUp() {
    repository = mockk(relaxed = true)
}

@After
fun tearDown() {
    unmockkAll()
}

3. Coroutines & ViewModel Testing

ViewModel unit tests run on the JVM without an Android Main Looper. Since viewModelScope uses Dispatchers.Main, you MUST replace the Main dispatcher during tests:

• Use Dispatchers.setMain(testDispatcher) in @Before (or via a JUnit TestRule).
• Use Dispatchers.resetMain() in @After.
• Wrap suspend test logic inside runTest from kotlinx.coroutines.test.

@OptIn(ExperimentalCoroutinesApi::class)
class MyViewModelTest {

    private val testDispatcher = StandardTestDispatcher()

    @Before
    fun setUp() {
        Dispatchers.setMain(testDispatcher)
    }

    @After
    fun tearDown() {
        Dispatchers.resetMain()
        unmockkAll()
    }
}

4. Assertions & AAA Pattern

• Assertion Library: Use Google Truth (com.google.common.truth.Truth.assertThat).
• AAA Pattern: Structure tests clearly with Arrange, Act, and Assert:

@Test
fun `removeLike calls repository to remove item`() = runTest {
    // Arrange
    val videoId = "test_id_123"
    coEvery { repository.removeLikeState(videoId) } returns Unit

    // Act
    viewModel.removeLike(videoId)
    testScheduler.advanceUntilIdle()

    // Assert
    coVerify(exactly = 1) { repository.removeLikeState(videoId) }
}

5. Summary Checklist Before Shipping Tests

1. ✅ Test file resides under app/src/test/java/... matching package structure of source class.
2. ✅ Method names use backtick-quoted descriptive sentences.
3. ✅ MockK is used for dependencies with unmockkAll() in @After.
4. ✅ Main dispatcher rule/setup is present for ViewModel tests.
5. ✅ Google Truth (assertThat) is used for assertions.
6. ✅ Test passes cleanly via ./gradlew :app:testGithubDebugUnitTest.