Unit Testing Skill

Mock Builders (ADR-0002)

The project follows ADR-0002: Reusable Mock Builders for Testing — a Builder pattern that centralizes test data creation across unit, integration, and E2E tests.

Why Builders?

• Reusability: Same builders used in unit, integration, and E2E tests
• Consistency: All tests get valid, well-formed test data by default
• Flexibility: Override only the fields relevant to the test, rest auto-populated
• Readability: Tests focus on behavior, not data setup boilerplate

Architecture

@lichtblick/test-builders          (external published npm dependency, not a workspace package)
├── BasicBuilder                    (primitives: string, number, boolean, date, lists, etc.)
└── defaults<T>(overrides, defaults) (utility to merge partial overrides with defaults)

@lichtblick/suite-base/testing/builders/  (domain builders)
├── PlayerBuilder.ts                (PlayerState, Topic, TopicStats, ActiveData)
├── MessageEventBuilder.ts          (MessageEvent with topic + data)
├── RosTimeBuilder.ts               (Time { sec, nsec })
├── RosDatatypesBuilder.ts          (MessageDefinition, datatypes maps)
├── LayoutBuilder.ts                (Layout configs, global variables)
├── RenderStateBuilder.ts           (Panel renderState)
├── ExtensionBuilder.ts             (Extension metadata)
├── PlotBuilder.ts                  (Plot panel data)
├── DiagnosticsBuilder.ts           (DiagnosticStatusArray)
├── SettingsTreeNodeBuilder.ts      (Settings tree nodes)
├── InitializationSourceBuilder.ts   (Data source initialization)
└── ... (more domain-specific builders)

Usage Pattern

import { BasicBuilder } from "@lichtblick/test-builders";
import PlayerBuilder from "@lichtblick/suite-base/testing/builders/PlayerBuilder";

// Create with all defaults — valid data without specifying anything
const topic = PlayerBuilder.topic();

// Override only what matters for this test
const customTopic = PlayerBuilder.topic({ name: "/camera/image" });

// Generate multiple instances
const topics = PlayerBuilder.topics(5);

// Compose builders for complex objects
const state = PlayerBuilder.playerState({
  activeData: PlayerBuilder.activeData({
    topics: [PlayerBuilder.topic({ name: "/lidar" })],
    currentTime: RosTimeBuilder.time({ sec: 100, nsec: 0 }),
  }),
});

Creating a New Builder

Follow the existing pattern when adding builders for new domain types:

import { BasicBuilder, defaults } from "@lichtblick/test-builders";

class MyEntityBuilder {
  // Static factory method with partial overrides
  public static myEntity(props: Partial<MyEntity> = {}): MyEntity {
    return defaults<MyEntity>(props, {
      id: BasicBuilder.string(),
      name: BasicBuilder.string(),
      count: BasicBuilder.number(),
      enabled: BasicBuilder.boolean(),
      createdAt: BasicBuilder.date(),
    });
  }

  // Plural helper for generating lists
  public static myEntities(count = 3): MyEntity[] {
    return BasicBuilder.multiple(MyEntityBuilder.myEntity, count);
  }
}

export default MyEntityBuilder;

Key Principles

1. Default values must produce valid objects — a builder with no overrides should return a structurally correct instance
2. Use defaults<T>() helper — merges partial overrides with generated defaults
3. Static methods only — builders are stateless utility classes, no instances needed
4. Compose builders — entity builders call other builders for nested types (e.g., PlayerBuilder calls RosTimeBuilder)
5. Place builders in correct scope:
   • Shared primitives → @lichtblick/test-builders (BasicBuilder)
   • Domain entities → @lichtblick/suite-base/testing/builders/
   • Component-specific test data → colocated builders/ directory within the component folder

BasicBuilder API

The BasicBuilder class (from @lichtblick/test-builders) provides:

Anti-patterns

• Don't inline test data when a builder exists — use PlayerBuilder.topic() instead of { name: "/foo", schemaName: "bar" }
• Don't copy-paste builder data between tests — extract to a builder method
• Don't assert on random builder values — override the specific field you're testing, then assert on that known value
• Don't use builders to test the builder output — test actual behavior

Test Structure Reference

See the Unit Test Agent (.github/agents/lb-unit-test.agent.md) for:

• Given-When-Then (GWT) pattern
• Naming conventions
• Mocking patterns (modules, workers, timers)
• Test quality rules