swift-swiftui

1# Swift and SwiftUI
2 
3The code is right when it compiles with zero warnings in Swift 6 language
4mode and the app does what the story says. For how it should look, call the
5Skill tool with `mac-design`. For copyable view code, read
6[../../design/mac-design/patterns.md](../../design/mac-design/patterns.md).
7 
8## Concurrency
9 
10The template sets main-actor default isolation: every type and function runs
11on the main actor unless you say otherwise. UI code needs no annotations.
12 
13| Work | Put it in |
14| --- | --- |
15| UI state, views, view models | Default (main actor) |
16| CPU-heavy pure work (parsing, image work, diffing) | A `nonisolated` function marked `@concurrent` |
17| Shared mutable state used off the main thread | An `actor` |
18| A C callback (event tap, Carbon hotkey, CoreAudio listener) | A `nonisolated` static func that hops back with `Task { @MainActor in ... }` |
19 
20- Data that crosses actors is `Sendable`: structs and enums of `Sendable`
21  values, or `final class` with only `let` properties.
22- `MainActor.assumeIsolated { }` inside a callback you know runs on the main
23  thread (a main run loop `Timer`, a main queue notification).
24- `Task { }` inherits the actor. `Task.detached` only when it must not.
25- Fix a concurrency error at its type, not with `@unchecked Sendable` or
26  `nonisolated(unsafe)`. Those two are a last resort, with a comment why.
27- **Runtime trap the compiler misses.** A closure created inside a
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