Swift Standards

Swift Coding Standards

Authoritative for Swift work. The non-negotiables are inline below; each area has a reference with the full rules, rationale, and patterns — read the relevant reference before working in that area, and when reviewing code against it.

Non-negotiables (always apply)

Modern Swift — @Observable (+ @State in views), never ObservableObject/@Published/@StateObject except when bridging old code. Actors + await MainActor.run {} over DispatchQueue. async/await over completion handlers and Combine (adapters for legacy APIs). Views take data

• action callbacks, not ViewModel blobs. Typed throws where they help. SDK 27: @State is now a macro — for any post-SDK-update compile error in a view using @State ("used before being initialized", "invalid redeclaration of synthesized property", "extraneous argument label"), consult the swiftui-whats-new-27 skill before fixing; the obvious fix (reordering init assignments) is wrong.

Concurrency — UI writes on MainActor only. Stateful services are actors; no shared mutable state outside them. No Task.detached without a documented reason. Check Task.isCancelled in loops/streams. SwiftData: pass Model.ID across actors and refetch in the local ModelContext (consider @ModelActor); never pass live models.

Reactive over imperative — derive, don't duplicate: computed properties for derivations, no stored copies, no manual notify/refresh calls. Stateless logic = free/static functions, not wrapper objects. Cache only after profiling.

SwiftUI performance — state as low in the tree as practical; split @Observable models so high-churn fields don't invalidate the world; stable ForEach IDs (never indices or \.self on mutables); no filter/sort or blocking work in body; no AnyView without need; one lazy container per scroll hierarchy; Equatable-gate expensive subtrees; debounce high-frequency inputs.

Apple agent conventions — PascalCase types / camelCase members; @State private var; no force unwraps; Swift Testing for unit tests, XCUIAutomation for UI tests; 4-space indent; limit changes to the requested task; verify cheapest-first (per-file diagnostics → snippet → build); never guess at post-cutoff APIs — look them up (apple-docs, apple-api-updates, or Xcode's DocumentationSearch per its routing rules).

References

• references/old-swift-patterns.md — the replace-at-a-glance table for legacy patterns (ObservableObject, DispatchQueue, completion handlers, SwiftData model passing), with adapters and snippets. Read when modernizing or reviewing older code.
• references/concurrency.md — the strict-concurrency rule set (MainActor, actors, cancellation, timeouts, Sendable). Read when writing or reviewing concurrent code.
• references/imperative-patterns.md — reactive-first rules, smells, and examples (derived-not-stored, minimal stores). Read when you see manual sync/notify patterns or stored derivations.
• references/swiftui-performance.md — full invalidation/identity/layout rules with checklist and patterns (debounce via .task(id:), guarded measurements, .id() state scoping, lazy-container layout). Read before performance work or SwiftUI review; for deeper dives the vendored swiftui-specialist skill goes further.
• references/preview-pitfalls.md — fast-compiling, working previews. Read when writing #Preview blocks.
• references/apple-agent-conventions.md — the conventions Apple steers its Xcode agents with, including the verification ladder. Read when unsure about style or testing-framework choices.