Add ViewModifier implementation

Package.resolved

@@ -1,13 +1,13 @@
 {
-  "originHash" : "c92107a159713b44e293a2fe944a02f419f98dc401c81653bfee56d07c8a2b70",
+  "originHash" : "807d4a48456544b2827b8d4f69dc5e39b01103e9105f11c138529cc3159d18d4",
   "pins" : [
     {
       "identity" : "opengraph",
       "kind" : "remoteSourceControl",
       "location" : "https://github.com/OpenSwiftUIProject/OpenGraph",
       "state" : {
         "branch" : "main",
-        "revision" : "acbee3c7c30cac49d64a8f619f3e3856a4e943f8"
+        "revision" : "47a81fde4bfa4092577abd29122206c19ad0cf98"
       }
     },
     {

Sources/OpenSwiftUI/Core/BodyAccessor/BodyAccessor.swift

@@ -10,7 +10,7 @@ internal import OpenGraphShims
 protocol BodyAccessor<Container, Body> {
     associatedtype Container
     associatedtype Body
-    func updateBody(of: Container, changed: Bool)
+    func updateBody(of container: Container, changed: Bool)
 }
 
 extension BodyAccessor {

Sources/OpenSwiftUI/Core/Graph/GraphInputsModifier.swift

@@ -5,6 +5,6 @@
 //  Audited for RELEASE_2021
 //  Status: Complete
 
-protocol _GraphInputsModifier {
+public protocol _GraphInputsModifier {
     static func _makeInputs(modifier: _GraphValue<Self>, inputs: inout _GraphInputs)
 }

Sources/OpenSwiftUI/Core/Modifier/EmptyModifier.swift

@@ -0,0 +1,69 @@
+//
+//  EmptyModifier.swift
+//  OpenSwiftUI
+//
+//  Audited for RELEASE_2021
+//  Status: Blocked by PrimitiveSceneModifier
+
+/// An empty, or identity, modifier, used during development to switch
+/// modifiers at compile time.
+///
+/// Use the empty modifier to switch modifiers at compile time during
+/// development. In the example below, in a debug build the ``Text``
+/// view inside `ContentView` has a yellow background and a red border.
+/// A non-debug build reflects the default system, or container supplied
+/// appearance.
+///
+///     struct EmphasizedLayout: ViewModifier {
+///         func body(content: Content) -> some View {
+///             content
+///                 .background(Color.yellow)
+///                 .border(Color.red)
+///         }
+///     }
+///
+///     struct ContentView: View {
+///         var body: some View {
+///             Text("Hello, World!")
+///                 .modifier(modifier)
+///         }
+///
+///         var modifier: some ViewModifier {
+///             #if DEBUG
+///                 return EmphasizedLayout()
+///             #else
+///                 return EmptyModifier()
+///             #endif
+///         }
+///     }
+///
+@frozen
+public struct EmptyModifier: PrimitiveViewModifier/*, PrimitiveSceneModifier*/ {
+    public static let identity = EmptyModifier()
+
+    @inlinable
+    public init() {}
+
+    public static func _makeView(
+        modifier: _GraphValue<Self>,
+        inputs: _ViewInputs,
+        body: @escaping (_Graph, _ViewInputs) -> _ViewOutputs
+    ) -> _ViewOutputs {
+        body(_Graph(), inputs)
+    }
+
+    public static func _makeViewList(
+        modifier: _GraphValue<Self>,
+        inputs: _ViewListInputs,
+        body: @escaping (_Graph, _ViewListInputs) -> _ViewListOutputs
+    ) -> _ViewListOutputs {
+        body(_Graph(), inputs)
+    }
+
+    public static func _viewListCount(
+        inputs: _ViewListCountInputs,
+        body: (_ViewListCountInputs) -> Int?
+    ) -> Int? {
+        body(inputs)
+    }
+}

Sources/OpenSwiftUI/Core/Modifier/ModifiedContent.swift

@@ -0,0 +1,119 @@
+//
+//  ModifiedContent.swift
+//  OpenSwiftUI
+//
+//  Audited for RELEASE_2021
+//  Status: WIP
+
+extension View {
+    @inlinable
+    public func modifier<T>(_ modifier: T) -> ModifiedContent<Self, T> {
+        .init(content: self, modifier: modifier)
+    }
+}
+
+/// A value with a modifier applied to it.
+@frozen
+public struct ModifiedContent<Content, Modifier> {
+    public typealias Body = Never
+
+    /// The content that the modifier transforms into a new view or new
+    /// view modifier.
+    public var content: Content
+
+    /// The view modifier.
+    public var modifier: Modifier
+
+    /// A structure that the defines the content and modifier needed to produce
+    /// a new view or view modifier.
+    ///
+    /// If `content` is a ``View`` and `modifier` is a ``ViewModifier``, the
+    /// result is a ``View``. If `content` and `modifier` are both view
+    /// modifiers, then the result is a new ``ViewModifier`` combining them.
+    ///
+    /// - Parameters:
+    ///     - content: The content that the modifier changes.
+    ///     - modifier: The modifier to apply to the content.
+    @inlinable
+    public init(content: Content, modifier: Modifier) {
+        self.content = content
+        self.modifier = modifier
+    }
+}
+
+extension ModifiedContent: Equatable where Content: Equatable, Modifier: Equatable {
+    public static func == (a: ModifiedContent<Content, Modifier>, b: ModifiedContent<Content, Modifier>) -> Bool {
+        a.content == b.content && a.modifier == b.modifier
+    }
+}
+
+extension ModifiedContent: View where Content: View, Modifier: ViewModifier {
+    public static func _makeView(
+        view: _GraphValue<Self>,
+        inputs: _ViewInputs
+    ) -> _ViewOutputs {
+        _ViewDebug.makeView(
+            view: view[offset: { .of(&$0.modifier) }],
+            inputs: inputs
+        ) { modifier, inputs in
+            Modifier._makeView(
+                modifier: modifier,
+                inputs: inputs
+            ) { _, inputs in
+                _ViewDebug.makeView(
+                    view: view[offset: { .of(&$0.content) }],
+                    inputs: inputs
+                ) { view, inputs in
+                    Content._makeView(view: view, inputs: inputs)
+                }
+            }
+        }
+    }
+
+    public static func _makeViewList(
+        view: _GraphValue<Self>,
+        inputs: _ViewListInputs
+    ) -> _ViewListOutputs {
+        Modifier.makeDebuggableViewList(
+            modifier: view[offset: { .of(&$0.modifier) }],
+            inputs: inputs
+        ) { _, inputs in
+            Content.makeDebuggableViewList(
+                view: view[offset: { .of(&$0.content) }],
+                inputs: inputs
+            )
+        }
+    }
+
+    public static func _viewListCount(
+        inputs: _ViewListCountInputs
+    ) -> Int? {
+        Modifier._viewListCount(inputs: inputs) { inputs in
+            Content._viewListCount(inputs: inputs)
+        }
+    }
+
+    public var body: ModifiedContent<Content, Modifier>.Body {
+        bodyError()
+    }
+}
+
+extension ModifiedContent: ViewModifier where Content: ViewModifier, Modifier: ViewModifier {
+//    public static func _makeView(modifier: _GraphValue<ModifiedContent<Content, Modifier>>, inputs: _ViewInputs, body: @escaping (_Graph, _ViewInputs) -> _ViewOutputs) -> _ViewOutputs {
+//
+//    }
+//    public static func _makeViewList(modifier: _GraphValue<ModifiedContent<Content, Modifier>>, inputs: _ViewListInputs, body: @escaping (_Graph, _ViewListInputs) -> _ViewListOutputs) -> _ViewListOutputs {
+//
+//    }
+//    public static func _viewListCount(inputs: _ViewListCountInputs, body: (_ViewListCountInputs) -> Int?) -> Int? {
+//
+//    }
+}
+
+extension ViewModifier {
+    @inlinable
+    @inline(__always)
+    public func concat<T>(_ modifier: T) -> ModifiedContent<Self, T> {
+        .init(content: self, modifier: modifier)
+    }
+}