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