Add initial support for closing windows with dismiss / dismissWindow

Examples/Sources/WindowingExample/WindowingApp.swift

@@ -157,6 +157,21 @@ struct SheetDemo: View {
     }
 }
 
+struct TertiaryWindowView: View {
+    @Environment(\.dismissWindow) private var dismissWindow
+
+    var body: some View {
+        VStack {
+            Text("This a tertiary window!")
+                .padding(10)
+
+            Button("Close window") {
+                dismissWindow()
+            }
+        }
+    }
+}
+
 @main
 @HotReloadable
 struct WindowingApp: App {
@@ -220,8 +235,7 @@ struct WindowingApp: App {
 
             WindowGroup("Tertiary window") {
                 #hotReloadable {
-                    Text("This a tertiary window!")
-                        .padding(10)
+                    TertiaryWindowView()
                 }
             }
             .defaultSize(width: 200, height: 200)

Sources/AppKitBackend/AppKitBackend.swift

@@ -123,6 +123,10 @@ public final class AppKitBackend: AppBackend {
         window.makeKeyAndOrderFront(nil)
     }
 
+    public func close(window: Window) {
+        window.close()
+    }
+
     public func openExternalURL(_ url: URL) throws {
         NSWorkspace.shared.open(url)
     }

Sources/SwiftCrossUI/Backend/AppBackend.swift

@@ -167,6 +167,8 @@ public protocol AppBackend: Sendable {
     /// receives an external URL or file to handle from the desktop environment.
     /// May be used in other circumstances eventually.
     func activate(window: Window)
+    /// Closes a window.
+    func close(window: Window)
 
     /// Sets the application's global menu. Some backends may make use of the host
     /// platform's global menu bar (such as macOS's menu bar), and others may render their
@@ -825,6 +827,10 @@ extension AppBackend {
 
     // MARK: Application
 
+    public func close(window: Window) {
+        todo()
+    }
+
     public func setApplicationMenu(_ submenus: [ResolvedMenu.Submenu]) {
         todo()
     }

Sources/SwiftCrossUI/Environment/Actions/DismissAction.swift

@@ -1,7 +1,7 @@
 /// An action that dismisses the current presentation context.
 ///
 /// Use the `dismiss` environment value to get an instance of this action,
-/// then call it to dismiss the current sheet.
+/// then call it to dismiss (close) the current window or sheet.
 ///
 /// Example usage:
 /// ```swift
@@ -48,7 +48,8 @@ extension EnvironmentValues {
     /// An action that dismisses the current presentation context.
     ///
     /// Use this environment value to get a dismiss action that can be called
-    /// to dismiss the current sheet, popover, or other presentation.
+    /// to dismiss (close) the current window, sheet, popover, or other
+    /// presentation.
     ///
     /// Example:
     /// ```swift

Sources/SwiftCrossUI/Environment/Actions/DismissWindowAction.swift

@@ -0,0 +1,38 @@
+/// An action that closes the current window.
+///
+/// Use the `dismissWindow` environment value to get an instance of this action,
+/// then call it to close the current window .
+///
+/// Example usage:
+/// ```swift
+/// struct ContentView: View {
+///     @Environment(\.dismissWindow) var dismissWindow
+///
+///     var body: some View {
+///         VStack {
+///             Text("Window Content")
+///             Button("Close") {
+///                 dismissWindow()
+///             }
+///         }
+///     }
+/// }
+/// ```
+@MainActor
+public struct DismissWindowAction {
+    let backend: any AppBackend
+    let window: MainActorBox<Any?>
+
+    /// Dismisses the current presentation context.
+    public func callAsFunction() {
+        func closeWindow<Backend: AppBackend>(backend: Backend) {
+            guard let window = window.value else {
+                print("warning: dismissWindow() called outside of a window")
+                return
+            }
+            backend.close(window: window as! Backend.Window)
+        }
+
+        closeWindow(backend: backend)
+    }
+}

Sources/SwiftCrossUI/Environment/EnvironmentValues.swift

@@ -183,6 +183,15 @@ public struct EnvironmentValues {
         )
     }
 
+    /// Closes the current window.
+    @MainActor
+    public var dismissWindow: DismissWindowAction {
+        return DismissWindowAction(
+            backend: backend,
+            window: .init(value: window)
+        )
+    }
+
     /// Reveals a file in the system's file manager. This opens
     /// the file's enclosing directory and highlighting the file.
     ///

Sources/SwiftCrossUI/Scenes/WindowGroupNode.swift

@@ -29,7 +29,11 @@ public final class WindowGroupNode<Content: View>: SceneGraphNode {
         viewGraph = ViewGraph(
             for: scene.body,
             backend: backend,
-            environment: environment.with(\.window, window)
+            environment: environment
+                .with(\.window, window)
+                .with(\.dismiss, DismissAction {
+                    backend.close(window: window)
+                })
         )
         let rootWidget = viewGraph.rootNode.concreteNode(for: Backend.self).widget
 
@@ -135,6 +139,7 @@ public final class WindowGroupNode<Content: View>: SceneGraphNode {
                 )
             }
             .with(\.window, window)
+            .with(\.dismiss, DismissAction { backend.close(window: window) })
 
         let dryRunResult: ViewUpdateResult?
         if !windowSizeIsFinal {