swift-swiftui

Swift and SwiftUI

The code is right when it compiles with zero warnings in Swift 6 language mode and the app does what the story says. For how it should look, call the Skill tool with mac-design. For copyable view code, read ../../design/mac-design/patterns.md.

Concurrency

The template sets main-actor default isolation: every type and function runs on the main actor unless you say otherwise. UI code needs no annotations.

Work	Put it in
UI state, views, view models	Default (main actor)
CPU-heavy pure work (parsing, image work, diffing)	A nonisolated function marked @concurrent
Shared mutable state used off the main thread	An actor
A C callback (event tap, Carbon hotkey, CoreAudio listener)	A nonisolated static func that hops back with Task { @MainActor in ... }

• Data that crosses actors is Sendable: structs and enums of Sendable values, or final class with only let properties.
• MainActor.assumeIsolated { } inside a callback you know runs on the main thread (a main run loop Timer, a main queue notification).
• Task { } inherits the actor. Task.detached only when it must not.
• Fix a concurrency error at its type, not with @unchecked Sendable or nonisolated(unsafe). Those two are a last resort, with a comment why.
• Runtime trap the compiler misses. A closure created inside a @MainActor type and passed to an API without a @Sendable closure parameter (DispatchSource.setEventHandler, DispatchWorkItem, addPeriodicTimeObserver, CoreAudio listener blocks) inherits the main actor. When it runs on a background queue, Swift 6 crashes the app. Write it as { @Sendable in ... }, and run every background path once after any concurrency change.

State

• @Observable classes for models. @State in the view that owns one, @Bindable to bind to its properties, .environment(model) to share it.
• @ObservationIgnored on caches, timers, and handles, so they never trigger a redraw.
• @State for view-local values. @AppStorage for small preferences. Keychain for secrets.
• ForEach(items) over values, never ForEach($items) for data the user can delete: a row can read an index that no longer exists and crash. Get a binding by looking the item up by id.

SwiftData

• @Model classes, .modelContainer on the scene, @Query in views, @Environment(\.modelContext) to insert and delete.
• Every new property has a default value, so existing stores still open.
• Store at ~/Library/Application Support/<AppName>/. Allow an override (APP_STORE) so tests never touch the real data.
• Give every model its own uid: UUID. persistentModelID changes on the first save, so it breaks selection and lookups.
• Undo: register the reverse of each action on the window's undo manager. SwiftData's automatic undo is not reliable. Code in ../../design/mac-design/patterns.md (Undo with SwiftData).
• Need SQL, a CLI, or several processes on one file? Use SQLite with WAL through a small library target instead.

Menu commands

SwiftUI Commands do not observe @Observable models, so a title or a .disabled(...) that reads the model goes stale. In testing, neither @FocusedValue (from .focusedSceneValue) nor an @ObservedObject inside the commands updated reliably: the menu stayed disabled with a row selected. What works: mirror the selection into a small ObservableObject, observe it in the App, and pass the values into the commands. The App's body then rebuilds the menus when they change.

@MainActor final class MenuState: ObservableObject {
    static let shared = MenuState()
    @Published private(set) var selection: Set<UUID> = []
    func update(_ s: Set<UUID>) { if selection != s { selection = s } }
}
// AppModel: var selection: Set<UUID> = [] { didSet { MenuState.shared.update(selection) } }

struct MyApp: App {
    @ObservedObject private var menu = MenuState.shared
    var body: some Scene {
        Window("App", id: "main") { ContentView() }
            .commands { AppCommands(model: model, selectedIDs: menu.selection) }
    }
}

Disable items with .disabled(selectedIDs.isEmpty) and resolve the target again when the item fires. Never ship items that stay enabled and do nothing. Check the menu from outside the app (test-mac-app skill, "Menu check").

A menu shortcut also fires while a text field has focus. Never use Delete, Command-Delete, arrows, or Return as menu shortcuts. Handle them on the list with .onDeleteCommand and .onKeyPress.

Names Swift already uses

Never name a type Task, View, Color, Image, Text, Section, Label, Menu, Group, or Settings. Task breaks every Task { }. Use Todo, TaskItem, NoteSection.

AppKit bridges

SwiftUI covers windows, lists, and forms. AppKit is still the answer for:

• Floating panels that do not steal focus: NSPanel with .nonactivatingPanel hosting a SwiftUI view (NSHostingView, with sizingOptions = [] for a fixed frame).
• Text views with custom behavior: NSViewRepresentable around NSTextView.
• Status items with custom click handling: NSStatusItem.
• Key and mouse monitors: NSEvent.addLocalMonitorForEvents. Remove the monitor when the view goes away.

For system hooks (hotkeys, Accessibility, screen capture), call the Skill tool with os-integrations.

Compiler errors

Error	Fix
cannot find 'X' in scope right after adding a file	swift package clean --scratch-path ~/Library/Developer/<AppName>/build
Main actor-isolated ... from a nonisolated context	Make the caller main-actor, or await the call
Sending 'x' risks causing data races	Make the type Sendable, or keep it on one actor
The compiler is unable to type-check this expression	Split the view into smaller subviews or let constants
ambiguous use of 'Task'	A type in your code is named Task; rename it

Fix the first error first. Later errors often come from it.

Swift 6 errors

Moving an existing Swift 5 app? Read ../../ship/build-and-verify/existing-app.md first: expect several build rounds, then hunt the runtime trap above.

Error	Fix
Static let of a non-Sendable type (formatter, NSFont, CFString)	@MainActor on the enclosing type, a computed var, or a commented nonisolated(unsafe)
sending 'note' risks causing data races in a main-queue observer	Copy the Sendable parts (an id, a pid) out of the notification before MainActor.assumeIsolated
Observer token captured before it is assigned	Keep the token in a small @MainActor class
"pattern that the region-based isolation checker does not understand" in a task group	Use an array of Task { @MainActor in ... } and await each one
Vars changed from OperationQueue blocks under NSLock	OSAllocatedUnfairLock(initialState:)
@objc action on an NSMenuItem subclass	@MainActor @objc func

Done when the app builds with zero warnings in Swift 6 language mode and the story's acceptance checks pass.