Add AlertScene

Examples/Sources/WindowingExample/WindowingApp.swift

@@ -59,7 +59,7 @@ struct AlertDemo: View {
 
         Button("Present error") {
             Task {
-                await presentAlert("Failed to succeed")
+                await presentAlert("Failed to succeed") {}
             }
         }
     }
@@ -162,6 +162,7 @@ struct SheetDemo: View {
 struct WindowingApp: App {
     @State var title = "My window"
     @State var resizable = false
+    @State var isAlertSceneShown = false
     @State var toggle = false
 
     var body: some Scene {
@@ -190,6 +191,9 @@ struct WindowingApp: App {
                     #endif
 
                     AlertDemo()
+                    Button("Show alert scene") {
+                        isAlertSceneShown = true
+                    }
 
                     Divider()
 
@@ -213,6 +217,9 @@ struct WindowingApp: App {
                 }
             }
         }
+
+        AlertScene("Alert scene", isPresented: $isAlertSceneShown) {}
+
         #if !(os(iOS) || os(tvOS) || os(Windows))
             WindowGroup("Secondary window") {
                 #hotReloadable {

Sources/SwiftCrossUI/Builders/AlertActionsBuilder.swift

@@ -1,6 +1,11 @@
 /// A builder for `[AlertAction]`.
 @resultBuilder
 public struct AlertActionsBuilder {
+    /// If no actions are provided, return a default "OK" action.
+    public static func buildBlock() -> [AlertAction] {
+        [.default]
+    }
+
     public static func buildPartialBlock(first: Button) -> [AlertAction] {
         [
             AlertAction(

Sources/SwiftCrossUI/Environment/Actions/AlertAction.swift

@@ -5,7 +5,7 @@
 /// breaking ``Button``'s API would have much more wide-reaching impacts than
 /// breaking this single-purpose API.
 public struct AlertAction: Sendable {
-    public static let ok = AlertAction(label: "Ok", action: {})
+    public static let `default` = AlertAction(label: "OK", action: {})
 
     public var label: String
     public var action: @MainActor @Sendable () -> Void

Sources/SwiftCrossUI/Environment/Actions/PresentAlertAction.swift

@@ -10,7 +10,7 @@ public struct PresentAlertAction {
     @discardableResult
     public func callAsFunction(
         _ title: String,
-        @AlertActionsBuilder actions: () -> [AlertAction] = { [.ok] }
+        @AlertActionsBuilder actions: () -> [AlertAction] = { [.default] }
     ) async -> Int {
         let actions = actions()
 

Sources/SwiftCrossUI/Scenes/AlertScene.swift

@@ -0,0 +1,81 @@
+/// A scene that shows a standalone alert.
+///
+/// The exact behavior of the alert is backend-dependent, but it typically
+/// shows up as an application modal, or attaches itself to the app's main
+/// window.
+public struct AlertScene: Scene {
+    public typealias Node = AlertSceneNode
+
+    var title: String
+    @Binding var isPresented: Bool
+    var actions: [AlertAction]
+
+    public let commands = Commands.empty
+
+    /// Creates an alert scene.
+    ///
+    /// The exact behavior of the alert is backend-dependent, but it typically
+    /// shows up as an application modal, or attaches itself to the app's main
+    /// window.
+    ///
+    /// - Parameters:
+    ///   - title: The alert's title.
+    ///   - isPresented: A binding to a `Bool` that controls whether the alert
+    ///     is presented.
+    ///   - actions: The alert's actions.
+    public init(
+        _ title: String,
+        isPresented: Binding<Bool>,
+        @AlertActionsBuilder actions: () -> [AlertAction]
+    ) {
+        self.title = title
+        self._isPresented = isPresented
+        self.actions = actions()
+    }
+}
+
+/// The scene graph node for ``AlertScene``.
+public final class AlertSceneNode: SceneGraphNode {
+    public typealias NodeScene = AlertScene
+
+    private var scene: AlertScene
+    private var alert: Any?
+
+    public init<Backend: AppBackend>(
+        from scene: AlertScene,
+        backend: Backend,
+        environment: EnvironmentValues
+    ) {
+        self.scene = scene
+    }
+
+    public func update<Backend: AppBackend>(
+        _ newScene: AlertScene?,
+        backend: Backend,
+        environment: EnvironmentValues
+    ) {
+        if let newScene {
+            self.scene = newScene
+        }
+
+        if scene.isPresented, alert == nil {
+            let alert = backend.createAlert()
+            backend.updateAlert(
+                alert,
+                title: scene.title,
+                actionLabels: scene.actions.map(\.label),
+                environment: environment
+            )
+            backend.showAlert(alert, window: nil) { responseId in
+                self.alert = nil
+                self.scene.isPresented = false
+                self.scene.actions[responseId].action()
+            }
+
+            self.alert = alert
+        } else if !scene.isPresented, let alert  {
+            backend.dismissAlert(alert as! Backend.Alert, window: nil)
+            self.alert = nil
+        }
+    }
+}

Sources/UIKitBackend/UIKitBackend+Window.swift

@@ -70,11 +70,16 @@ extension UIKitBackend {
     public typealias Window = UIWindow
 
     public func createWindow(withDefaultSize _: SIMD2<Int>?) -> Window {
-        var window: UIWindow
+        let window: UIWindow
 
         if !Self.hasReturnedAWindow {
+            if let mainWindow = Self.mainWindow {
+                window = mainWindow
+            } else {
+                window = UIWindow()
+                Self.mainWindow = window
+            }
             Self.hasReturnedAWindow = true
-            window = Self.mainWindow ?? UIWindow()
         } else {
             window = UIWindow()
         }