Create presentAlert env action, don't allow env action access from App

The @Environment property wrapper shouldn't allow environment actions
(or at least ones that require a window handle) to be accessed from
an App's top-level as the property wrapper needs to be inside a scene's
view graph to have a window handle.

Author: stackotter

Sources/SwiftCrossUI/Environment/Actions/PresentAlertAction.swift

@@ -0,0 +1,39 @@
+/// Presents an alert to the user. Returns once an action has been selected and
+/// the corresponding action handler has been run. Returns the index of the
+/// selected action. By default, the alert will have a single button labelled
+/// `OK`. All buttons will dismiss the alert even if you provide your own
+/// actions.
+public struct PresentAlertAction {
+    let environment: EnvironmentValues
+
+    @discardableResult
+    public func callAsFunction(
+        _ title: String,
+        @AlertActionsBuilder actions: () -> [AlertAction] = { [.ok] }
+    ) async -> Int {
+        let actions = actions()
+
+        func presentAlert<Backend: AppBackend>(backend: Backend) async -> Int {
+            await withCheckedContinuation { continuation in
+                backend.runInMainThread {
+                    let alert = backend.createAlert()
+                    backend.updateAlert(
+                        alert,
+                        title: title,
+                        actionLabels: actions.map(\.label),
+                        environment: environment
+                    )
+                    backend.showAlert(
+                        alert,
+                        window: environment.window! as! Backend.Window
+                    ) { actionIndex in
+                        actions[actionIndex].action()
+                        continuation.resume(returning: actionIndex)
+                    }
+                }
+            }
+        }
+
+        return await presentAlert(backend: environment.backend)
+    }
+}

Sources/SwiftCrossUI/Environment/Actions/PresentFileSaveDialogAction.swift

@@ -4,7 +4,7 @@ import Foundation
 /// `nil` if the user cancels the operation.
 public struct PresentFileSaveDialogAction {
     let backend: any AppBackend
-    let window: Any?
+    let window: Any
 
     public func callAsFunction(
         title: String = "Save",

Sources/SwiftCrossUI/Environment/Actions/PresentSingleFileOpenDialogAction.swift

@@ -5,7 +5,7 @@ import Foundation
 /// in a single dialog. Returns `nil` if the user cancels the operation.
 public struct PresentSingleFileOpenDialogAction {
     let backend: any AppBackend
-    let window: Any?
+    let window: Any
 
     public func callAsFunction(
         title: String = "Open",

Sources/SwiftCrossUI/Environment/EnvironmentValues.swift

@@ -47,22 +47,66 @@ public struct EnvironmentValues {
     /// Presents an 'Open file' dialog fit for selecting a single file. Some
     /// backends only allow selecting either files or directories but not both
     /// in a single dialog. Returns `nil` if the user cancels the operation.
+    /// Must not be accessed via the ``Environment`` property wrapper at the
+    /// top-level of an ``App``. Only access it from ``View``s.
     public var chooseFile: PresentSingleFileOpenDialogAction {
+        guard let window else {
+            fatalError(
+                """
+                chooseFile cannot be accessed at the top level of an app as \
+                it must know which scene it's acting within. Access it from a \
+                View implementation or use the `.alert` view modifier if you \
+                want to present an alert directly within an App's body.
+                """
+            )
+        }
         return PresentSingleFileOpenDialogAction(
             backend: backend,
             window: window
         )
     }
 
     /// Presents a 'Save file' dialog fit for selecting a save destination.
-    /// Returns `nil` if the user cancels the operation.
+    /// Returns `nil` if the user cancels the operation. Must not be accessed
+    /// via the ``Environment`` property wrapper at the top-level of an ``App``.
+    /// Only access it from ``View``s.
     public var chooseFileSaveDestination: PresentFileSaveDialogAction {
+        guard let window else {
+            fatalError(
+                """
+                chooseFileSaveDestination cannot be accessed at the top level \
+                of an app as it must know which scene it's acting within. \
+                Access it from a View implementation or use the `.alert` view \
+                modifier if you want to present an alert directly within an \
+                App's body.
+                """
+            )
+        }
         return PresentFileSaveDialogAction(
             backend: backend,
             window: window
         )
     }
 
+    /// Presents an alert for the current window. Must not be accessed via
+    /// the ``Environment`` property wrapper at the top-level of an ``App``.
+    /// Only access it from ``View``s.
+    public var presentAlert: PresentAlertAction {
+        guard window != nil else {
+            fatalError(
+                """
+                presentAlert cannot be accessed at the top level of an app as \
+                it must know which scene it's acting within. Access it from a \
+                View implementation or use the `.alert` view modifier if you \
+                want to present an alert directly within an App's body.
+                """
+            )
+        }
+        return PresentAlertAction(
+            environment: self
+        )
+    }
+
     /// Creates the default environment.
     init<Backend: AppBackend>(backend: Backend) {
         self.backend = backend

Sources/SwiftCrossUI/Modifiers/AlertAction.swift

@@ -5,6 +5,8 @@
 /// breaking ``Button``'s API would have much more wide-reaching impacts than
 /// breaking this single-purpose API.
 public struct AlertAction {
+    public static let ok = AlertAction(label: "Ok", action: {})
+
     public var label: String
     public var action: () -> Void
 }

Sources/SwiftCrossUI/Scenes/WindowGroupNode.swift

@@ -24,8 +24,13 @@ public final class WindowGroupNode<Content: View>: SceneGraphNode {
         environment: EnvironmentValues
     ) {
         self.scene = scene
-        viewGraph = ViewGraph(for: scene.body, backend: backend, environment: environment)
         let window = backend.createWindow(withDefaultSize: scene.defaultSize)
+
+        viewGraph = ViewGraph(
+            for: scene.body,
+            backend: backend,
+            environment: environment.with(\.window, window)
+        )
         let rootWidget = viewGraph.rootNode.concreteNode(for: Backend.self).widget
 
         let container = backend.createContainer()