tdd

name	tdd
description	Red-green-refactor workflow for this repository: write a failing Swift Testing test in MyAppCoreTests first, prove it fails, implement the minimum in MyAppCore to pass, then refactor and re-verify the coverage floor. Use PROACTIVELY when: implementing a feature, changing behavior, fixing a bug, adding logic, new function, new type, TDD, test-first.

TDD Workflow

TDD is mandatory for all production Swift code (see .claude/rules/testing.md). Never write implementation before a failing test exists.

Step 0: Decide Where the Code Lives

• Logic, state, view models → Packages/MyAppKit/Sources/MyAppCore — this is the default. Core is coverage-gated, so code here is forced to stay tested.
• Rendering only → Packages/MyAppKit/Sources/MyAppUI, as a thin view over a Core view model. If a view needs an if, the condition belongs in Core.
• If you are about to put logic in MyAppUI or App/, stop and move it to Core.

Step 1: RED — Write the Failing Test First

Add the test to Packages/MyAppKit/Tests/MyAppCoreTests using Swift Testing:

• @Test functions inside a @Suite; @MainActor on suites touching view models
• #expect / #require; error paths assert the thrown error's payload with #expect(throws:), not just its type
• Use @Test(arguments:) instead of copy-pasted variations
• Cover the edge-case checklist: boundaries (at / just inside / just outside), repeated operations, both branches of every conditional

Step 2: Prove It Fails

cd Packages/MyAppKit && swift test

Confirm the new test fails (a compile error for a not-yet-existing symbol counts). Do not skip this run — a test that has never failed proves nothing.

Step 3: GREEN — Implement the Minimum

Write the smallest implementation in MyAppCore that makes the test pass. Follow .claude/rules/swift.md: typed errors, no force-unwrap/try!, value types first, /// docs on public API.

cd Packages/MyAppKit && swift test

All tests must pass — the new ones and every existing one.

Step 4: REFACTOR — With the Gate On

Clean up (naming, extraction, dead code) while tests stay green, then run the full local gate including the coverage floor:

just check   # fmt → lint → test (80% floor on MyAppCore) → build

If coverage dropped below the floor, write more tests for the code you added — NEVER lower the floor, exclude paths, or move logic out of Core to dodge it.

Step 5: Commit

Tests and their implementation land in the same commit (use the smart-commit skill). Red tests are never committed alone.

Anti-patterns (hard prohibitions)

• Implementation first, tests "later"
• Weakening an assertion to make a test pass — fix the code
• Testing implementation details instead of behavior and contracts
• sleep-based assertions or order-dependent tests
• Skipping Step 2 because the failure "is obvious"