import AppKit
import Observation

// MARK: - AppAppearance

/// The app-wide appearance override — Auto (follow system) / Light / Dark (11-command-nexus.md ▸
/// View ▸ Appearance; 03-board-ui.md ▸ Toolbar).
///
/// There is no `.auto` case: following the system is the *absence* of an override, which is what makes
/// "no stored key" the one honest spelling of it (`AppearanceStore`, `AppPreferences.appearanceKey`) —
/// a third case would need its own reading of what it means to override with "don't override".
public enum AppAppearance: String, CaseIterable, Sendable {
    case light
    case dark
}

// MARK: - AppearanceStore

/// The app's one appearance override, app-wide and persisted (11-command-nexus.md ▸ View ▸ Appearance;
/// 03-board-ui.md ▸ Toolbar).
///
/// `BoardZoomStore`'s shape exactly, and for its reasons. Two consumers need change notification a
/// property wrapper in a view cannot give them: the board toolbar's picker item, whose checkmarks are
/// read fresh whenever AppKit opens its menu rather than polled (`WindowToolbarController`), and the
/// View-menu picker, which lives outside every scene's environment and reaches `AppModel` as a plain
/// `let` the same way `ZoomCommands` reaches `zoom`. An `@Observable` object over an injectable
/// `UserDefaults` is what serves both without either one mirroring the other's state.
///
/// ### Why not `@AppStorage`, like Show Comments
///
/// Show Comments has exactly one write path and exactly one thing reading it back — the checkbox
/// itself. This preference has two independent controls that must never drift the way the toolbar's
/// zoom buttons and the View-menu zoom rows must not (`BoardZoomStore.setLevel`'s rule), and the
/// toolbar's is AppKit underneath — an `NSMenuToolbarItem` cannot bind to `@AppStorage` at all.
///
/// ### Why the write path applies live, unlike zoom's
///
/// A zoom level only ever feeds a board's own drawing, so persisting it is enough — the board reads it
/// back through the environment. An appearance override is a statement about the whole app's chrome,
/// every open window included, so the setter both persists *and* calls the apply seam in the same
/// beat: there is no reload and no window that has to be told twice.
@MainActor
@Observable
public final class AppearanceStore {

    /// The current override. `nil` is Auto — the app follows the system appearance.
    public private(set) var override: AppAppearance?

    @ObservationIgnored
    private let defaults: UserDefaults

    /// The one seam that touches `NSApp` — injected so a test can prove the setter's whole contract
    /// (persist, then apply) without a live application object, `BoardZoomStore.defaults`'s reason
    /// turned toward AppKit rather than `UserDefaults`.
    @ObservationIgnored
    private let apply: (NSAppearance.Name?) -> Void

    /// - Parameters:
    ///   - defaults: the domain to persist in — injected for `BoardZoomStore`'s reason: a test holds
    ///     its own rather than touching the developer's real appearance.
    ///   - apply: what "make it so" means. Defaulted to the real thing; a test hands in a recording
    ///     closure instead so it never touches `NSApp`.
    public init(
        defaults: UserDefaults = .standard,
        apply: @escaping (NSAppearance.Name?) -> Void = { name in
            NSApp.appearance = name.map { NSAppearance(named: $0) } ?? nil
        }
    ) {
        self.defaults = defaults
        self.apply = apply
        // A string, not an enum-backed scalar: the key is absent for Auto (the remove-at-default
        // idiom a default lane width and an empty rename already use), and any value that survives to
        // here but is neither "light" nor "dark" — a hand edit, a future build's spelling read by an
        // older one — degrades to Auto rather than refusing to resolve. `AppAppearance.init(rawValue:)`
        // already answers `nil` for anything it does not recognise, so the lenient read costs nothing
        // beyond the `flatMap`.
        override = defaults.string(forKey: AppPreferences.appearanceKey).flatMap(AppAppearance.init(rawValue:))
    }

    // MARK: - The pure resolver

    /// What an override means to AppKit — no `NSApp`, no live application, provable with nothing but
    /// the enum (`BoardZoom.normalize`'s reason: the rule is a function, and the object around it is
    /// only that function's persistence and observability).
    ///
    /// `nonisolated`, unlike everything else here: it touches no actor-isolated state, and marking it
    /// so is what lets a plain (non-`@MainActor`) test call it directly, the same freedom
    /// `BoardZoom.normalize` has by living outside `BoardZoomStore` entirely.
    public nonisolated static func appearanceName(for override: AppAppearance?) -> NSAppearance.Name? {
        switch override {
        case .light: .aqua
        case .dark: .darkAqua
        case nil: nil
        }
    }

    // MARK: - Writing

    /// Sets the override, persists it, and applies it — the single write path the View-menu picker and
    /// the board-toolbar item share (`BoardZoomStore.setLevel`'s rule: the two faces of one command
    /// must never become two implementations of it).
    ///
    /// **Auto removes the key** rather than writing a third spelling of it: the preference is meant to
    /// read as "no override on file" to anyone who inspects it, the same bargain a default lane width
    /// and an empty rename already keep.
    ///
    /// **An unchanged value writes and applies nothing**, `BoardZoomStore.setLevel`'s own guard and for
    /// the same load-bearing reason: `@Observable` notifies on every assignment, equal or not, so an
    /// ungated write would invalidate every observer of `override` — the picker's checkmarks, the
    /// toolbar controller's tracked validation — on a no-op, and hand the apply seam a repeat call for
    /// nothing every one of its callers would have to tolerate.
    public func setOverride(_ newValue: AppAppearance?) {
        guard newValue != override else { return }
        override = newValue
        if let newValue {
            defaults.set(newValue.rawValue, forKey: AppPreferences.appearanceKey)
        } else {
            defaults.removeObject(forKey: AppPreferences.appearanceKey)
        }
        apply(Self.appearanceName(for: newValue))
    }

    /// Re-applies the stored override — launch's whole job
    /// (`AppDelegate.applicationDidFinishLaunching`). `init` above already read the value; this is the
    /// method that hands it to AppKit, kept separate from `init` so building a store — including in a
    /// test, including as `AppModel`'s own construction — is never itself a global side effect.
    public func applyCurrent() {
        apply(Self.appearanceName(for: override))
    }
}