swift

# 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. - **M...