PureYAML/docs/rules-swift/ui/appkit-views.md

AppKit Views

How a Tiledown-style native editor writes its AppKit renderer: NSViewController and NSView subclasses that are purely presentational over the framework-free model (see pre-ui-layer.md). The renderer reads Surface values, forwards every user action as perform(_ intent:), and re-renders on the model's change signal, observed through withObservationTracking. It holds ZERO business logic and owns ZERO state that is not view-local.

Core rules

1. The renderer renders Surface state only: no business logic, no API calls, no domain decisions.
2. Every user action (target/action, menu item, delegate callback) becomes a perform(_ intent:) call; the renderer reports, it never decides.
3. Observe the model with withObservationTracking (the AppKit idiom from pre-ui-layer.md), re-registering after each change, then re-pull the dirty Set<SurfaceArea>.
4. All layout and menus in code; no Storyboards, no XIBs, no MainMenu.xib (build NSMenu in the app delegate).
5. Use a diffable data source (NSCollectionViewDiffableDataSource / NSTableViewDiffableDataSource) keyed by stable Surface identifiers.
6. Every control sets an accessibility label (setAccessibilityLabel(_:)).
7. Appearance happens in one color(for: token) map returning NSColor; never hard-code inline.
8. AppKit views recycle (makeView(withIdentifier:), itemForRepresentedObjectAt:); reconfigure fully, stash no per-row state.

Decision tree: code belongs in the AppKit renderer?

Visual presentation (views, layout, animation) → YES
View-local UI state (drag in progress, split position) → YES
Reading a Surface to populate views → YES
Translating an action/menu/delegate callback into an Intent → YES (perform(intent))
Data transformation → NO (domain service, in the model)
Business logic, validation, enable/disable → NO (engine; reflect the surface flag)
Navigation / window routing decision → NO (engine sets a destination surface; the controller reflects it)
Network / persistence → NO (data layer, behind a seam)

Patterns

Entry point and menus in code

A code-only AppKit app has no storyboard and no @main, so the entry point is main.swift and several steps the Xcode template normally hides must be written by hand. This is the cumbersome part; you write it once per app.

// main.swift: AppKit with no storyboard needs an explicit entry point (no @main).
import AppKit

let app = NSApplication.shared
app.setActivationPolicy(.regular)       // without this a tool-launched app has no dock icon or menu bar
app.mainMenu = buildMainMenu()          // programmatic NSMenu; there is no MainMenu.xib

let window = TileListWindow.make(model: model)
window.makeKeyAndOrderFront(nil)
model.perform(.appDidLaunch)            // lifecycle is an intent, not entry-point logic

app.activate()                          // bring the app to the front
app.run()

For lifecycle hooks (applicationDidFinishLaunching, termination) set an NSApplicationDelegate via app.delegate before run(); keep it thin, it raises intents and owns no behavior.

Binding the model with withObservationTracking

final class TileListViewController: NSViewController {
    private let model: TileListRendererModel
    private lazy var dataSource = makeDataSource()

    init(model: TileListRendererModel) {
        self.model = model
        super.init(nibName: nil, bundle: nil)
    }

    @available(*, unavailable) required init?(coder: NSCoder) { fatalError("code-only UI") }

    override func loadView() { view = NSView() }        // all hierarchy built in code

    override func viewDidLoad() {
        super.viewDidLoad()
        configureHierarchy()
        observe()
    }

    // Re-register after every change: withObservationTracking fires once per registration.
    private func observe() {
        withObservationTracking {
            render(model.listSurface, model.statusSurface)
        } onChange: { [weak self] in
            DispatchQueue.main.async { self?.observe() }
        }
    }
}

Action / menu / delegate to Intent

// CORRECT: target/action reports an intent
@objc private func reload(_ sender: NSToolbarItem) {
    model.perform(.run(.reload))
}

// Menu validation reflects a surface flag the engine computed; it does not decide.
func validateMenuItem(_ item: NSMenuItem) -> Bool {
    model.menuSurface.isEnabled(item.action)
}

// NSTableView selection to intent
func tableViewSelectionDidChange(_ notification: Notification) {
    guard let id = dataSource.itemIdentifier(for: tableView.selectedRow) else { return }
    model.perform(.select(id))
}

Lists via a diffable data source

private func makeDataSource() -> NSCollectionViewDiffableDataSource<Section, SurfaceID> {
    NSCollectionViewDiffableDataSource(collectionView: collectionView) { [model] cv, indexPath, id in
        let item = cv.makeItem(withIdentifier: .tile, for: indexPath) as! TileItem
        item.render(model.tileSurface(id))     // item reflects a surface, owns no state
        return item
    }
}

private func applyListSnapshot(_ surface: ListSurface) {
    var snapshot = NSDiffableDataSourceSnapshot<Section, SurfaceID>()
    snapshot.appendSections([.main])
    snapshot.appendItems(surface.orderedIDs, toSection: .main)
    dataSource.apply(snapshot, animatingDifferences: true)
}

Navigation / windows reflect a destination surface

private func render(_ list: ListSurface, _ status: StatusSurface) {
    applyListSnapshot(list)
    statusLabel.stringValue = status.message
    switch model.navigationSurface.destination {
    case .none: detailWindow?.close()
    case .detail(let detail): presentDetail(detail)    // reflect, do not decide
    }
}

Accessibility

reloadButton.setAccessibilityLabel("Reload tiles")
reloadButton.setAccessibilityHelp("Fetches the latest tiles")

App setup (no storyboard, no XIB, and the network entitlement)

A code-only app drops the template scaffolding, so do by hand what the storyboard did implicitly, and grant network access in the right place:

• No storyboard, no XIB, no MainMenu.xib. Remove the storyboard from the target and build both the window and the main menu in code (app.mainMenu = buildMainMenu()). UI is created in code, always.

• main.swift, not @main. With no @NSApplicationMain / @main, the entry point is main.swift: share the application, set the activation policy to .regular, install the menu, make the window key, activate(), then run() (see the entry-point pattern above).

• Network access is an entitlement, not Info.plist. If the app enables App Sandbox, network is denied by default. Grant it in the app's .entitlements file, which Xcode's "Signing & Capabilities → App Sandbox → Network" checkboxes write, never in Info.plist:

  <key>com.apple.security.app-sandbox</key>
  <true/>
  <key>com.apple.security.network.client</key>   <!-- outgoing: you make requests -->
  <true/>
  <key>com.apple.security.network.server</key>   <!-- incoming: you listen / accept -->
  <true/>
  

  network.client is outgoing, network.server is incoming; add only the direction the app actually uses. A non-sandboxed Mac app needs neither key.

Responder chain (route, do not decide)

AppKit's responder chain is plumbing, not logic. A first-responder handler translates the action into an intent and forwards it; it does not implement the behavior.

• An @objc action reaching a controller via the responder chain calls model.perform(...) and returns; the engine owns what happens.
• Use validateMenuItem / validateUserInterfaceItem to reflect engine-computed enablement from a surface flag, never to compute it inline.
• Field-editor edits (NSTextField / NSText) forward each commit as an .edit intent over the document surface the model owns; the renderer keeps no separate copy of the text.

Identity and reuse (the AppKit trap)

NSTableView / NSCollectionView recycle views and items; treat each as a pure function of the surface it renders.

• An item/cell MUST fully reconfigure from its surface in render(_:); never assume residual state.
• Cancel in-flight async in the reuse hook before the view is handed out again.
• Drive identity from stable SurfaceIDs in the diffable snapshot, never from row indices.
• Keep view-local state (a drag in progress, a split-view position) on the view, never in the model.

Validation checklist

• Zero business logic; every decision reflects a surface the engine set
• Every action/menu/delegate callback forwarded as perform(_ intent:)
• Updates driven by withObservationTracking re-registration over the change signal
• All layout and menus in code; no Storyboards, XIBs, or MainMenu.xib
• main.swift entry point (not @main) with setActivationPolicy(.regular) and activate()
• If sandboxed, network granted via .entitlements (network.client / network.server), not Info.plist
• Lists use a diffable data source keyed by stable Surface IDs
• Items/cells reconfigure fully on reuse; async cancelled before reuse
• Accessibility labels on every control
• Appearance only through the color(for:) map (NSColor)
• This is one of several renderers over the same model (see pre-ui-layer.md)

Companion rules

• pre-ui-layer.md: the framework-free model this renderer adapts; AppKit observes the change signal via withObservationTracking.
• swiftui-views.md, uikit-views.md: the sibling renderers over the same model; build all three and compare (see ../domain-first.md).
• view-models.md: responsibilities of the presentation layer the renderer binds to.
• colors.md, fonts.md: the color(for:) and font sources the renderer reflects.