/swift

name: swift
description: "Swift development: concurrency patterns, async/await, actors, testing with XCTest and Swift Testing framework."
user-invocable: false
context: fork
agent: swift-general-engineer
routing:
  triggers:
    - "swift concurrency"
    - "swift async await"
    - "Swift Actor"
    - "Swift Task group"
    - "swift testing"
    - "XCTest"
    - "Swift Testing framework"
    - "async test swift"
  category: swift
  pairs_with:
    - test-driven-development
    - code-linting

Swift Skill

Swift concurrency and testing: async/await, Actors, TaskGroups, Sendable, structured concurrency, XCTest, Swift Testing framework, and async test patterns.

Reference Loading Table

| Signal | Reference | Size | |--------|-----------|------| | async/await, Task, Sendable | `references/fundamentals.md` | ~20 lines | | Actor, @MainActor, nonisolated | `references/actor-isolation.md` | ~20 lines | | TaskGroup, AsyncSequence, AsyncStream, cancellation | `references/task-patterns.md` | ~20 lines | | Failure modes, common mistakes | `references/preferred-patterns.md` | ~20 lines | | concurrency overview, structured concurrency patterns | `references/swift-concurrency.md` | ~30 lines | | XCTest, Swift Testing, test doubles, async tests, UI tests | `references/swift-testing.md` | ~250 lines |

**Loading rule.** Read the references whose signals match the task before responding.

---

Core Rules (Always Apply)

Concurrency

• **Prefer structured concurrency** -- use `TaskGroup` over loose `Task { }` whenever possible; structured tasks propagate cancellation and errors automatically.
• **Mark types Sendable** -- enable strict concurrency checking (`-strict-concurrency=complete`) and resolve all warnings before they become errors in Swift 6.
• **Use actors for shared mutable state** -- avoid manual locks; actors provide compiler-verified safety.
• **Cancel what you create** -- every `Task` stored in a property should have a corresponding cancellation path.
• **Minimize @MainActor surface** -- isolate only the UI layer; keep business logic and networking off the main actor.

Testing

• **One assertion per concept** -- a single test can have multiple assertions if they verify the same logical behavior, but avoid testing unrelated things together.
• **Arrange-Act-Assert** -- structure every test into setup, execution, and verification phases.
• **Name tests descriptively** -- `testFetchUser_withExpiredToken_throwsAuthError` is better than `testFetch2`.
• **Prefer Swift Testing for new code** -- use `@Test` and `#expect` when targeting Swift 5.9+; fall back to XCTest for older targets or UI tests.
• **Ensure test independence** -- each test must be runnable in isolation; always produce self-contained test state.

---

Phase 1: ASSESS

Determine what kind of Swift work is needed:

| Request type | Load references | Action | |-------------|----------------|--------| | Concurrency patterns | fundamentals, actor-isolation, task-patterns | Pattern guidance | | Concurrency mistakes | preferred-patterns | Failure mode detection | | Write tests | swift-testing | Test authoring | | Async test patterns | swift-testing + fundamentals | Async test guidance | | Full concurrency review | swift-concurrency + all concurrency refs | Full review pass |

**Gate**: Request classified and relevant references loaded.

---

Phase 2: EXECUTE

Apply loaded reference knowledge to the user's code or question.

For concurrency work: 1. Verify structured concurrency used where possible 2. Check Sendable conformance 3. Validate actor isolation boundaries 4. Confirm cancellation paths exist

For testing work: 1. Use Swift Testing (`@Test`, `#expect`) for new code on Swift 5.9+ 2. Use `@dataProvider`-style parameterized tests with `arguments:` 3. Use protocol-based mock injection 4. Use XCTest for UI tests and older targets

**Gate**: Specific, reference-backed feedback or code provided.

---

Phase 3: VERIFY

Run the test suite and confirm:

swift test --enable-code-coverage
swift build

**Gate**: All tests pass. Build succeeds with strict concurrency checking.