Add SceneModifier support

Sources/OpenSwiftUI/App/App/AppGraph.swift

@@ -145,8 +145,10 @@ protocol AppGraphObserver: AnyObject {
 
 // MARK: - AppBodyAccessor
 
-private struct AppBodyAccessor<Container: App>: BodyAccessor {
-    typealias Body = Container.Body
+private struct AppBodyAccessor<Application>: BodyAccessor where Application: App {
+    typealias Container = Application
+
+    typealias Body = Application.Body
 
     func updateBody(of container: Container, changed: Bool) {
         guard changed else {

Sources/OpenSwiftUI/Modifier/SceneModifier/SceneModifier.swift

@@ -0,0 +1,242 @@
+//
+//  SceneModifier.swift
+//  OpenSwiftUI
+//
+//  Audited for 6.5.4
+//  Status: Complete
+//  ID: 88477CCB0AEDAD452D0818B33FCEDC5F (SwiftUI)
+
+import OpenAttributeGraphShims
+import OpenSwiftUICore
+
+// MARK: - _SceneModifier
+
+/// A modifier that can be applied to a scene or other scene modifier,
+/// producing a different version of the original value.
+@available(OpenSwiftUI_v2_0, *)
+public protocol _SceneModifier {
+
+    /// The type of scene representing the body of `Self`.
+    associatedtype Body: Scene
+
+    /// Returns the current body of `self`. `content` is a proxy for
+    /// the scene that will have the modifier represented by `Self`
+    /// applied to it.
+    @SceneBuilder
+    func body(content: SceneContent) -> Body
+
+    /// The content scene type passed to `body()`.
+    typealias SceneContent = _SceneModifier_Content<Self>
+
+    static func _makeScene(
+        modifier: _GraphValue<Self>,
+        inputs: _SceneInputs,
+        body: @escaping (_Graph, _SceneInputs) -> _SceneOutputs
+    ) -> _SceneOutputs
+}
+
+// MARK: - PrimitiveSceneModifier
+
+protocol PrimitiveSceneModifier: _SceneModifier where Body == Never {}
+
+extension _SceneModifier {
+    func sceneBodyError() -> Never {
+        preconditionFailure("body() should not be called on \(self).")
+    }
+}
+
+@available(OpenSwiftUI_v2_0, *)
+extension _SceneModifier where Body == Never {
+    public func body(content: SceneContent) -> Body {
+        sceneBodyError()
+    }
+}
+
+// MARK: - _GraphInputsModifier + _SceneModifier
+
+@available(OpenSwiftUI_v2_0, *)
+extension _SceneModifier where Self: _GraphInputsModifier, Body == Never {
+    public static func _makeScene(
+        modifier: _GraphValue<Self>,
+        inputs: _SceneInputs,
+        body: @escaping (_Graph, _SceneInputs) -> _SceneOutputs
+    ) -> _SceneOutputs {
+        var inputs = inputs
+        _makeInputs(modifier: modifier, inputs: &inputs.base)
+        return body(.init(), inputs)
+    }
+}
+
+// MARK: - EmptyModifier + _SceneModifier
+
+@available(OpenSwiftUI_v2_0, *)
+extension EmptyModifier: _SceneModifier {
+    @MainActor
+    @preconcurrency
+    public static func _makeScene(
+        modifier: _GraphValue<EmptyModifier>,
+        inputs: _SceneInputs,
+        body: @escaping (_Graph, _SceneInputs) -> _SceneOutputs
+    ) -> _SceneOutputs {
+        body(.init(), inputs)
+    }
+}
+
+// MARK: - Scene + modifier
+
+@available(OpenSwiftUI_v2_0, *)
+extension Scene {
+    /// Returns a new scene representing `self` with `modifier` applied
+    /// to it.
+    @inlinable
+    @MainActor
+    @preconcurrency
+    internal func modifier<T>(_ modifier: T) -> ModifiedContent<Self, T> {
+        return .init(content: self, modifier: modifier)
+    }
+}
+
+@_spi(Private)
+@available(OpenSwiftUI_v4_0, *)
+extension Scene {
+    @_spi(Private)
+    nonisolated public func sceneModifier<T>(_ modifier: T) -> ModifiedContent<Self, T> {
+        self.modifier(modifier)
+    }
+}
+
+// MARK: - ModifiedContent + Scene & _SceneModifier
+
+@available(OpenSwiftUI_v2_0, *)
+extension ModifiedContent: Scene where Content: Scene, Modifier: _SceneModifier {
+    nonisolated public static func _makeScene(
+        scene: _GraphValue<Self>,
+        inputs: _SceneInputs
+    ) -> _SceneOutputs {
+        Modifier._makeScene(
+            modifier: scene[offset: { .of(&$0.modifier) }],
+            inputs: inputs
+        ) {
+            Content._makeScene(
+                scene: scene[offset: { .of(&$0.content) }],
+                inputs: $1
+            )
+        }
+    }
+
+    @MainActor
+    @preconcurrency
+    public var body: Body {
+        sceneBodyError()
+    }
+}
+
+@available(OpenSwiftUI_v2_0, *)
+extension ModifiedContent: _SceneModifier where Content: _SceneModifier, Modifier: _SceneModifier {
+    public static func _makeScene(
+        modifier: _GraphValue<Self>,
+        inputs: _SceneInputs,
+        body: @escaping (_Graph, _SceneInputs) -> _SceneOutputs
+    ) -> _SceneOutputs {
+        Modifier._makeScene(
+            modifier: modifier[offset: { .of(&$0.modifier) }],
+            inputs: inputs
+        ) {
+            Content._makeScene(
+                modifier: modifier[offset: { .of(&$0.content) }],
+                inputs: $1,
+                body: body
+            )
+        }
+    }
+}
+
+// MARK: - _SceneModifier + concat
+
+@available(OpenSwiftUI_v2_0, *)
+extension _SceneModifier {
+    /// Returns a new modifier that is the result of concatenating
+    /// `self` with `modifier`.
+    @inlinable
+    internal func concat<T>(_ modifier: T) -> ModifiedContent<Self, T> {
+        return .init(content: self, modifier: modifier)
+    }
+}
+
+// MARK: - _SceneModifier_Content
+
+@available(OpenSwiftUI_v2_0, *)
+@MainActor
+@preconcurrency
+public struct _SceneModifier_Content<Modifier>: PrimitiveScene where Modifier: _SceneModifier {
+    nonisolated public static func _makeScene(
+        scene: _GraphValue<Self>,
+        inputs: _SceneInputs
+    ) -> _SceneOutputs {
+        var inputs = inputs
+        guard let body = inputs.popLast(BodyInput.self) else {
+            return .init()
+        }
+        return body(.init(), inputs)
+    }
+
+    fileprivate struct BodyInput: SceneInput {
+        typealias BodyInputElement = (_Graph, _SceneInputs) -> _SceneOutputs
+
+        static var defaultValue: Stack<BodyInputElement> { .init() }
+    }
+}
+
+@available(*, unavailable)
+extension _SceneModifier_Content: Sendable {}
+
+// MARK: - _SceneModifier default implementation
+
+@available(OpenSwiftUI_v2_0, *)
+extension _SceneModifier {
+    public static func _makeScene(
+        modifier: _GraphValue<Self>,
+        inputs: _SceneInputs,
+        body: @escaping (_Graph, _SceneInputs) -> _SceneOutputs
+    ) -> _SceneOutputs {
+        let fields = DynamicPropertyCache.fields(of: Self.self)
+        var inputs = inputs
+        let (scene, buffer) = makeBody(modifier: modifier, inputs: &inputs.base, fields: fields)
+        inputs.append(body, to: _SceneModifier_Content<Self>.BodyInput.self)
+        let outputs = Body._makeScene(scene: scene, inputs: inputs)
+        if let buffer {
+            buffer.traceMountedProperties(to: modifier, fields: fields)
+        }
+        return outputs
+    }
+
+    private static func makeBody(
+        modifier: _GraphValue<Self>,
+        inputs: inout _GraphInputs,
+        fields: DynamicPropertyCache.Fields
+    ) -> (_GraphValue<Body>, _DynamicPropertyBuffer?) {
+        precondition(
+            Metadata(Self.self).isValueType,
+            "view modifiers must be value types: \(Self.self)"
+        )
+        let accessor = AppModifierBodyAccessor<Self>()
+        return accessor.makeBody(container: modifier, inputs: &inputs, fields: fields)
+    }
+}
+
+// MARK: - AppModifierBodyAccessor
+
+private struct AppModifierBodyAccessor<Modifier>: BodyAccessor where Modifier: _SceneModifier {
+    typealias Container = Modifier
+
+    typealias Body = Modifier.Body
+
+    func updateBody(of container: Container, changed: Bool) {
+        guard changed else {
+            return
+        }
+        setBody {
+            container.body(content: .init())
+        }
+    }
+}

Sources/OpenSwiftUICore/Modifier/CustomViewModifier.swift

@@ -329,15 +329,17 @@ private struct BodyCountInput<V>: ViewInput {
 
 // MARK: - ModifierBodyAccessor
 
-private struct ModifierBodyAccessor<Container>: BodyAccessor where Container: ViewModifier {
-    typealias Body = Container.Body
+private struct ModifierBodyAccessor<Modifier>: BodyAccessor where Modifier: ViewModifier {
+    typealias Container = Modifier
+
+    typealias Body = Modifier.Body
 
     func updateBody(of container: Container, changed: Bool) {
         guard changed else {
             return
         }
         setBody {
-            container.body(content: Container.Content())
+            container.body(content: .init())
         }
     }
 }