Coding Standards Enforcer Skill

Coding Standards Enforcer

When to use

Use this skill whenever you add, modify, or review Swift code in this repo to ensure compliance with concurrency, language usage, and Swift 6 migration rules.

Workflow

Identify the files and changes in scope.
Scan for violations of the rules below.
Apply fixes or call out deviations explicitly.

4. Concurrency & Threading

Strict Concurrency: Swift 6.2 defaults to @MainActor isolation for Views and UI logic. Assume strict isolation checks are active. Everything is @MainActor by default.
Background Tasks: Move heavy physics/data work off the main actor using @concurrent functions or dedicated actors.
Task Management: Cancel long-running tasks on teardown.

Swift Concurrency Guidelines

Core mental model

Think in isolation domains rather than threads:

MainActor is the UI lane and must own UI state.
actor types protect their own mutable state.
nonisolated code is shared and cannot touch actor state.
Sendable types are safe to move across domains.

Async and parallel work

func fetchUser(id: Int) async throws -> User {
    let (data, _) = try await URLSession.shared.data(from: url)
    return try JSONDecoder().decode(User.self, from: data)
}

async let avatar = fetchImage("avatar.jpg")
async let banner = fetchImage("banner.jpg")
let profile = Profile(avatar: try await avatar, banner: try await banner)

Tasks and task groups

.task { avatar = await downloadAvatar() }

Task { await saveProfile() }

try await withThrowingTaskGroup(of: Void.self) { group in
    group.addTask { avatar = try await downloadAvatar() }
    group.addTask { bio = try await fetchBio() }
    try await group.waitForAll()
}

Isolation domains

@MainActor
final class ViewModel {
    var items: [Item] = []
}

actor BankAccount {
    var balance: Double = 0
    func deposit(_ amount: Double) { balance += amount }
}

Approachable concurrency settings (Swift 6.2+)

SWIFT_DEFAULT_ACTOR_ISOLATION = MainActor keeps UI code on the main actor by default.
SWIFT_APPROACHABLE_CONCURRENCY = YES keeps nonisolated async on the caller's actor.

@concurrent func processLargeFile() async { }

Sendable

struct User: Sendable {
    let id: Int
    let name: String
}

final class ThreadSafeCache: @unchecked Sendable {
    private let lock = NSLock()
    private var storage: [String: Data] = [:]
}

Isolation inheritance

Task { } inherits the current actor.
Task.detached { } does not inherit isolation and should be rare.

Common mistakes to avoid

Treating async as automatic background work.
Creating many actors when @MainActor is sufficient.
Using MainActor.run when the enclosing function can be annotated.
Blocking async code with DispatchSemaphore or DispatchGroup.wait().
Spawning unstructured Task instances instead of async let or task groups.

Quick reference

async and await for suspension points.
Task { } for structured async work.
actor for isolated mutable state.
@MainActor for UI-bound work.
Sendable for cross-actor data transfer.

7. Swift Language Standards

Observable Classes: @Observable classes are @MainActor by default, so explicit @MainActor annotation is not needed.
Swift-Native APIs: Prefer Swift-native alternatives to Foundation methods where they exist, such as using replacing("hello", with: "world") with strings rather than replacingOccurrences(of: "hello", with: "world").
Modern Foundation API: Prefer modern Foundation API, for example URL.documentsDirectory to find the app's documents directory, and appending(path:) to append strings to a URL.
Number Formatting: Never use C-style number formatting such as Text(String(format: "%.2f", abs(myNumber))); always use Text(abs(change), format: .number.precision(.fractionLength(2))) instead.
Static Member Lookup: Prefer static member lookup to struct instances where possible, such as .circle rather than Circle(), and .borderedProminent rather than BorderedProminentButtonStyle().
Modern Concurrency: Never use old-style Grand Central Dispatch concurrency such as DispatchQueue.main.async(). If behavior like this is needed, always use modern Swift concurrency.
Text Filtering: Filtering text based on user-input must be done using localizedStandardContains() as opposed to contains().
Force Unwraps: Avoid force unwraps and force try unless it is unrecoverable.

Agent Skills: Coding Standards Enforcer

Skill Files