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.

Test Types Overview

Pattern-Based Testing

These three patterns cut repetitive test setup, cover all visual states, and keep widget behavior consistent. They're conventions, not packages.

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.

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
• No MediaQuery widget ancestor: Always wrap test widget in MaterialApp

Running Tests (Quick Reference)

• 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
• dart test: Pure Dart unit tests