swift-concurrency

name: "Swift Concurrency Guidelines" description: "Swift 6 strict concurrency patterns, actor isolation, @MainActor usage, async/await best practices, structured concurrency, and Sendable conformance. Apply when working with async code, actors, concurrent operations, or resolving concurrency warnings/errors." agent_requested: true applies_to: ["**/*.swift"]

Swift Concurrency Guidelines

Actor Isolation (Swift 6 Strict Mode)

All code must follow Swift 6 strict concurrency rules. The project uses strict concurrency mode as configured in MyProjectPackage/Package.swift.

@MainActor for UI Code

All UI-related code must be isolated to the main actor:

@MainActor
struct ContentView: View {
    @State private var model = DataModel()
    
    var body: some View {
        // All UI code runs on main actor
        List(model.items) { item in
            Text(item.title)
        }
    }
}

@MainActor
class UIModel: ObservableObject {
    @Published var state: ViewState = .idle
    
    func updateUI() {
        // Safe to update UI properties here
        state = .loaded
    }
}

Actors for Shared Mutable State

Use actors for managing shared mutable state safely:

actor DataStore {
    private var items: [Item] = []
    
    func addItem(_ item: Item) {
        items.append(item)
    }
    
    func getItems() -> [Item] {
        return items
    }
}

Async/Await Patterns

Required: Use .task Modifier in Views

Always use .task for async operations tied to view lifecycle:

// ✅ CORRECT: Use .task for view lifecycle
struct DataView: View {
    @State private var items: [Item] = []
    
    var body: some View {
        List(items, id: \.id) { item in
            Text(item.name)
        }
        .task {
            // Automatically cancels when view disappears
            do {
                items = try await loadItems()
            } catch {
                // Handle error
            }
        }
    }
}