test-driven-development

Test-Driven Development

Overview

Write tests before implementation. The Red-Green-Refactor cycle — write a failing test (RED), write minimal code to pass (GREEN), improve the code without changing behavior (REFACTOR) — produces code that is correct by construction and has comprehensive test coverage from the start.

When to Use

• Implementing any new feature or behavior
• Fixing any bug (use the Prove-It pattern)
• Adding new business logic to ViewModels, use cases, or repositories
• Before refactoring (establish a test safety net first)

Skip when: Purely cosmetic UI changes with no behavioral logic.

Core Process

Red-Green-Refactor Cycle

RED: Write a Failing Test

1. Write the test first — describe what the code should do:

class TaskListViewModelTest {
    private val repository = mockk<TaskRepository>()
    private lateinit var viewModel: TaskListViewModel

    @Test
    fun `initial state is Loading`() {
        every { repository.getTasks() } returns flowOf(emptyList())

        viewModel = TaskListViewModel(GetTasksUseCase(repository))

        assertEquals(TaskListUiState.Loading, viewModel.uiState.value)
    }

    @Test
    fun `emits Success with tasks when repository returns data`() = runTest {
        val tasks = listOf(Task("1", "Buy groceries", false))
        every { repository.getTasks() } returns flowOf(tasks)

        viewModel = TaskListViewModel(GetTasksUseCase(repository))
        advanceUntilIdle()

        assertEquals(TaskListUiState.Success(tasks), viewModel.uiState.value)
    }

    @Test
    fun `emits Error when repository throws`() = runTest {
        every { repository.getTasks() } returns flow { throw IOException("Network error") }

        viewModel = TaskListViewModel(GetTasksUseCase(repository))
        advanceUntilIdle()

        val state = viewModel.uiState.value
        assertIs<TaskListUiState.Error>(state)
        assertEquals("Network error", state.message)
    }
}

1. Run the test — it must fail (./gradlew test)
   • If it passes immediately, the test doesn't test what you think