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.

swift

@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.

swift-swiftui

NPX Install

SKILL.md Content