Platform Notes

• Optional helper plugins may help in some environments, but they must not be treated as required for this skill.

iOS Test-Driven Development (TDD)

Use When

• iOS Test-Driven Development standards. Enforces Red-Green-Refactor cycle, test pyramid (70/20/10), layer-specific testing strategies with XCTest and Swift Testing, and CI integration. Use when building or reviewing iOS apps with TDD methodology.
• The task needs reusable judgment, domain constraints, or a proven workflow rather than ad hoc advice.

Do Not Use When

• The task is unrelated to ios-tdd or would be better handled by a more specific companion skill.
• The request only needs a trivial answer and none of this skill's constraints or references materially help.

Required Inputs

• Gather relevant project context, constraints, and the concrete problem to solve; load references only as needed.
• Confirm the desired deliverable: design, code, review, migration plan, audit, or documentation.

Workflow

• Read this SKILL.md first, then load only the referenced deep-dive files that are necessary for the task.
• Apply the ordered guidance, checklists, and decision rules in this skill instead of cherry-picking isolated snippets.
• Produce the deliverable with assumptions, risks, and follow-up work made explicit when they matter.

Quality Standards

• Keep outputs execution-oriented, concise, and aligned with the repository's baseline engineering standards.
• Preserve compatibility with existing project conventions unless the skill explicitly requires a stronger standard.
• Prefer deterministic, reviewable steps over vague advice or tool-specific magic.

Anti-Patterns

• Treating examples as copy-paste truth without checking fit, constraints, or failure modes.
• Loading every reference file by default instead of using progressive disclosure.

Outputs

• A concrete result that fits the task: implementation guidance, review findings, architecture decisions, templates, or generated artifacts.
• Clear assumptions, tradeoffs, or unresolved gaps when the task cannot be completed from available context alone.
• References used, companion skills, or follow-up actions when they materially improve execution.

Evidence Produced

Category	Artifact	Format	Example
Correctness	iOS TDD test plan	Markdown doc per skill-composition-standards/references/test-plan-template.md covering Red-Green-Refactor cycles per layer	docs/ios/tdd-plan-checkout.md
Correctness	Test pyramid coverage report	Markdown doc showing 70/20/10 distribution and per-layer coverage	docs/ios/tdd-coverage-2026-04-16.md

References

• Use the references/ directory for deep detail after reading the core workflow below.

Overview

TDD is a development process where you write tests before feature code, following the Red-Green-Refactor cycle. Every feature starts with a failing test, gets minimal implementation, then is refined.

Core Principle: No production code without a failing test first.

Preferred Framework: Swift Testing (@Test macro) for all new unit and integration tests. Use XCTest only for UI tests (XCUITest) and legacy test suites.

Dependency Injection: Protocol-based injection — no DI framework required. Define protocols for all external dependencies, inject via initialiser parameters.

Quick Reference

Topic	Section	Covers
TDD Workflow	Red-Green-Refactor	Step-by-step cycle with Swift examples
Test Pyramid	Test Pyramid	Unit, integration, UI split
Swift Testing	Swift Testing	@Test macro, #expect, async testing
Mocking	Protocol Mocking	Protocol-based mocks, no library needed
Network Mocking	URLProtocol Mock	URLProtocol subclass for API tests
UI Tests	XCUITest	End-to-end UI testing
CI Setup	CI Pipeline	Xcode Cloud, GitHub Actions

Additional Guidance

Extended guidance for ios-tdd was moved to references/skill-deep-dive.md to keep this entrypoint compact and fast to load.

Use that deep dive for:

• The Red-Green-Refactor Cycle
• Test Pyramid (70/20/10)
• TDD Workflow for iOS Features
• Swift Testing Framework
• Protocol-Based Mocking
• Testing @Observable ViewModels
• URLProtocol for Network Mocking
• SwiftData / Core Data Testing
• XCUITest for Critical Flows
• Test Naming Convention
• Patterns and Anti-Patterns
• Integration with Other Skills
• Additional deep-dive sections continue in the reference file.