Flutter Testing

Testing Strategy

• Test Pyramid: More unit and widget tests, fewer integration tests. Unit tests are fastest and cheapest.
• Mirror Test Rule: 100% logic and widget coverage. No code without a test.
• Mirror Organization: Test files MUST strictly mirror the lib/ directory structure and end with _test.dart.
• Coverage Targets: Target 100% logic coverage for domain and bloc layers.
• Test Independence: Each test MUST be independent. No shared mutable state between tests.

Widget Testing

• Write widget tests for all major UI components.
• Test user interactions and state changes.
• Widget Keys: Use Key('feature_action_id') format on interactive widgets for test access.
• Test Localization: Use AppLocalizations (context.l10n) in widget tests — no hardcoded strings.

Pattern-Based Testing

Adopt these three structural patterns to eliminate boilerplate, enforce complete coverage, and ensure consistency across the test suite. These are conventions — no external package dependency is required.

Golden Variant Testing

When a widget has multiple visual states (primary, disabled, hover, error), test all variants in a single structured group() with a Map<String, Widget Function()>:

• Define a variants map where keys are state names and values are widget builders.
• Each variant MUST be rendered in isolation (fresh pumpWidget call per variant).
• Use deterministic golden file naming: goldens/<component>.<variant>.png.
• Set a consistent surfaceSize via tester.view.physicalSize to avoid flaky pixel diffs.
• When to use: Design system components, widgets with distinct modes/types.
• When NOT to use: Integration tests or animation frame verification.

State Matrix Testing

Every stateful widget MUST be tested against ALL possible UI states. Use a state matrix to separate setup from verification:

• Define a states map with every state the widget can render (loading, error, data, empty).
• Write a single verify(stateName) callback that asserts correct rendering per state using switch expressions.
• This pattern prevents the common mistake of only testing the "happy path".
• When to use: Widgets with complex state machines (e.g., BLoC-driven screens).
• When NOT to use: If verification logic varies wildly between states, write separate tests.

Interaction Contract Testing

Reusable widgets have implicit behavioral rules. Define these as explicit, reusable contracts:

• Create helper functions in test/utils/ for common contracts:
  • verifyTappable(tester, finder, mockCallback) — Tap fires callback exactly once.
  • verifyDisabledNotTappable(tester, finder, mockCallback) — Tap does NOT fire callback when disabled.
  • verifyValidatesOnBlur(tester, finder) — Validation triggers when focus leaves.
• Apply contracts consistently across all widgets sharing the same behavior.
• When to use: Widgets with strictly defined behavioral rules that must hold across refactors.
• When NOT to use: One-off logic unique to a single widget.

Integration Testing

• Use IntegrationTestWidgetsFlutterBinding.ensureInitialized() at the start of integration tests
• Interact with widgets via Key (e.g., find.byKey(const ValueKey('increment')))
• Use pumpAndSettle() to wait for animations and async operations to complete
• Run with: flutter test integration_test/

Test Naming & Structure

• Test Naming: Use string interpolation for test group names: group('$ClassName', not group('ClassName',. This ensures consistency and enables better tooling support.
• Test Grouping: Use group() to organize tests by feature, class, or state for clearer reporting.
• Descriptive Names: Test names should clearly describe what is being tested and why.

Common Test Errors

• A RenderFlex overflowed... — Wrap widget in Expanded or constrain dimensions in test
• Vertical viewport was given unbounded height — Wrap ListView in SizedBox with fixed height in test
• setState called during build — Defer state changes to post-frame callback

Workflow: Testing Execution

Follow this sequential workflow when testing a feature. Copy the checklist to track progress.

Task Progress

• Step 1: Write Widget Tests. Ensure all interactive UI elements have Keys. Apply Golden Variant, State Matrix, or Interaction Contract patterns as appropriate.
• Step 2: Write Integration Tests. Create end-to-end user flows using IntegrationTestWidgetsFlutterBinding.
• Step 3: Check Coverage. Run flutter test --coverage and verify targets are met.
• Step 4: Run Static Analysis. Execute dart analyze to ensure code conforms to linting rules.

Running Tests

• flutter test — Run all unit and widget tests
• flutter test test/path/to/file_test.dart — Run specific test file
• flutter test integration_test/ — Run integration tests
• flutter test --coverage — Run with coverage report

Source: dhruvanbhalara/skills — distributed by TomeVault.