Add Preferences (#307)

This adds the `PreferenceKey` protocol and related modifiers.

* Initial PreferenceKey implementation

* Don't send default value to match SwiftUI behavior

* Add CustomDebugStringConvertible conformance to Color

* PR fixes

* Fix onAppear and preference modification calls

* Attempt macOS build fix

* Fix <background/overlay>PreferenceValue

* Implement/revise transformPreference

* Fix linter warnings, apply SwiftFormat

Co-authored-by: Max Desiatov <max@desiatov.com>

NativeDemo/TokamakDemo.xcodeproj/project.pbxproj

@@ -41,6 +41,8 @@
 		B5C76E4B24C73ED5003EABB2 /* AppStorageDemo.swift in Sources */ = {isa = PBXBuildFile; fileRef = B5C76E4924C73ED4003EABB2 /* AppStorageDemo.swift */; };
 		B5DBA22B24D509B4003D3347 /* RedactDemo.swift in Sources */ = {isa = PBXBuildFile; fileRef = B5DBA22A24D509B4003D3347 /* RedactDemo.swift */; };
 		B5DBA22C24D509B4003D3347 /* RedactDemo.swift in Sources */ = {isa = PBXBuildFile; fileRef = B5DBA22A24D509B4003D3347 /* RedactDemo.swift */; };
+		B5F2BE032571443D00FB3653 /* PreferenceKeyDemo.swift in Sources */ = {isa = PBXBuildFile; fileRef = B5F2BE022571443D00FB3653 /* PreferenceKeyDemo.swift */; };
+		B5F2BE042571443D00FB3653 /* PreferenceKeyDemo.swift in Sources */ = {isa = PBXBuildFile; fileRef = B5F2BE022571443D00FB3653 /* PreferenceKeyDemo.swift */; };
 		D1B4229024B3B9BB00682F74 /* ListDemo.swift in Sources */ = {isa = PBXBuildFile; fileRef = D1B4228E24B3B9BB00682F74 /* ListDemo.swift */; };
 		D1B4229124B3B9BB00682F74 /* ListDemo.swift in Sources */ = {isa = PBXBuildFile; fileRef = D1B4228E24B3B9BB00682F74 /* ListDemo.swift */; };
 		D1B4229224B3B9BB00682F74 /* OutlineGroupDemo.swift in Sources */ = {isa = PBXBuildFile; fileRef = D1B4228F24B3B9BB00682F74 /* OutlineGroupDemo.swift */; };
@@ -110,6 +112,7 @@
 		B56F22E224BD1C26001738DF /* GridDemo.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = GridDemo.swift; sourceTree = "<group>"; };
 		B5C76E4924C73ED4003EABB2 /* AppStorageDemo.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = AppStorageDemo.swift; sourceTree = "<group>"; };
 		B5DBA22A24D509B4003D3347 /* RedactDemo.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = RedactDemo.swift; sourceTree = "<group>"; };
+		B5F2BE022571443D00FB3653 /* PreferenceKeyDemo.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = PreferenceKeyDemo.swift; sourceTree = "<group>"; };
 		D1B4228E24B3B9BB00682F74 /* ListDemo.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = ListDemo.swift; sourceTree = "<group>"; };
 		D1B4228F24B3B9BB00682F74 /* OutlineGroupDemo.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = OutlineGroupDemo.swift; sourceTree = "<group>"; };
 		D1C726F224CB63C6003B576D /* ButtonStyleDemo.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = ButtonStyleDemo.swift; sourceTree = "<group>"; };
@@ -188,6 +191,7 @@
 				D1B4228F24B3B9BB00682F74 /* OutlineGroupDemo.swift */,
 				B51F214F24B920B400CF2583 /* PathDemo.swift */,
 				D1EE7EA624C0DD2100C0D127 /* PickerDemo.swift */,
+				B5F2BE022571443D00FB3653 /* PreferenceKeyDemo.swift */,
 				B5DBA22A24D509B4003D3347 /* RedactDemo.swift */,
 				3DCDE44324CA6AD400910F17 /* SidebarDemo.swift */,
 				8500293E24D2FF3E001A2E84 /* SliderDemo.swift */,
@@ -350,6 +354,7 @@
 				85ED18A324AD425E0085DFA0 /* SpacerDemo.swift in Sources */,
 				D1B4229024B3B9BB00682F74 /* ListDemo.swift in Sources */,
 				D1EE7EA724C0DD2100C0D127 /* PickerDemo.swift in Sources */,
+				B5F2BE032571443D00FB3653 /* PreferenceKeyDemo.swift in Sources */,
 				8500293F24D2FF3E001A2E84 /* SliderDemo.swift in Sources */,
 				85ED18A924AD425E0085DFA0 /* TokamakDemo.swift in Sources */,
 				B5C76E4A24C73ED5003EABB2 /* AppStorageDemo.swift in Sources */,
@@ -378,6 +383,7 @@
 				85ED18B024AD425E0085DFA0 /* EnvironmentDemo.swift in Sources */,
 				D1B4229124B3B9BB00682F74 /* ListDemo.swift in Sources */,
 				D1EE7EA824C0DD2100C0D127 /* PickerDemo.swift in Sources */,
+				B5F2BE042571443D00FB3653 /* PreferenceKeyDemo.swift in Sources */,
 				8500294024D2FF3E001A2E84 /* SliderDemo.swift in Sources */,
 				85ED18B624AD42D70085DFA0 /* NSAppDelegate.swift in Sources */,
 				B5C76E4B24C73ED5003EABB2 /* AppStorageDemo.swift in Sources */,

Package.swift

@@ -7,7 +7,7 @@ import PackageDescription
 let package = Package(
   name: "Tokamak",
   platforms: [
-    .macOS(.v10_15),
+    .macOS(.v11),
     .iOS(.v13),
   ],
   products: [

Sources/TokamakCore/Environment/EnvironmentKey.swift

@@ -30,9 +30,7 @@ public struct _EnvironmentKeyWritingModifier<Value>: ViewModifier, EnvironmentMo
     self.value = value
   }
 
-  public func body(content: Content) -> some View {
+  public typealias Body = Never
-    content
-  }
 
   func modifyEnvironment(_ values: inout EnvironmentValues) {
     values[keyPath: keyPath] = value

Sources/TokamakCore/Modifiers/Navigation.swift

@@ -13,13 +13,27 @@
 // limitations under the License.
 
 public extension View {
-  // FIXME: Implement
+  @available(*, deprecated, renamed: "navigationTitle(_:)")
-  func navigationBarTitle<S>(_ title: S) -> some View where S: StringProtocol {
+  func navigationBarTitle(_ title: Text) -> some View {
-    self
+    navigationTitle(title)
   }
 
-  // FIXME: Implement
+  @available(*, deprecated, renamed: "navigationTitle(_:)")
-  func navigationTitle<S>(_ title: S) -> some View where S: StringProtocol {
+  func navigationBarTitle<S: StringProtocol>(_ title: S) -> some View {
-    self
+    navigationTitle(title)
+  }
+
+  func navigationTitle(_ title: Text) -> some View {
+    navigationTitle { title }
+  }
+
+  func navigationTitle<S: StringProtocol>(_ titleKey: S) -> some View {
+    navigationTitle(Text(titleKey))
+  }
+
+  func navigationTitle<V>(@ViewBuilder _ title: () -> V) -> some View
+    where V: View
+  {
+    preference(key: NavigationTitleKey.self, value: AnyView(title()))
   }
 }

Sources/TokamakCore/Modifiers/StyleModifiers.swift

@@ -28,7 +28,11 @@ public struct _BackgroundModifier<Background>: ViewModifier, EnvironmentReader
   }
 
   public func body(content: Content) -> some View {
-    content
+    // FIXME: Clip to bounds of foreground.
+    ZStack(alignment: alignment) {
+      background
+      content
+    }
   }
 
   mutating func setContent(from values: EnvironmentValues) {
@@ -67,6 +71,7 @@ public struct _OverlayModifier<Overlay>: ViewModifier, EnvironmentReader
   }
 
   public func body(content: Content) -> some View {
+    // FIXME: Clip to content shape.
     ZStack(alignment: alignment) {
       content
       overlay

Sources/TokamakCore/Modifiers/ViewModifier.swift

@@ -22,8 +22,13 @@ public struct _ViewModifier_Content<Modifier>: View where Modifier: ViewModifier
   public let modifier: Modifier
   public let view: AnyView
 
-  public var body: Never {
+  public init(modifier: Modifier, view: AnyView) {
-    neverBody("_ViewModifier_Content")
+    self.modifier = modifier
+    self.view = view
+  }
+
+  public var body: AnyView {
+    view
   }
 }
 
@@ -35,6 +40,8 @@ public extension View {
 
 public extension ViewModifier where Body == Never {
   func body(content: Content) -> Body {
-    fatalError("\(self) is a primitive `ViewModifier`, you're not supposed to run `body(content:)`")
+    fatalError(
+      "\(Self.self) is a primitive `ViewModifier`, you're not supposed to run `body(content:)`"
+    )
   }
 }

Sources/TokamakCore/MountedViews/MountedApp.swift

@@ -22,13 +22,18 @@ import Runtime
 // is the computed content of the specified `Scene`, instead of having child
 // `View`s
 final class MountedApp<R: Renderer>: MountedCompositeElement<R> {
-  override func mount(before _: R.TargetType? = nil, with reconciler: StackReconciler<R>) {
+  override func mount(
+    before _: R.TargetType? = nil,
+    on _: MountedElement<R>? = nil,
+    with reconciler: StackReconciler<R>
+  ) {
     // `App` elements have no siblings, hence the `before` argument is discarded.
+    // They also have no parents, so the `parent` argument is discarded as well.
     let childBody = reconciler.render(mountedApp: self)
 
     let child: MountedElement<R> = mountChild(childBody)
     mountedChildren = [child]
-    child.mount(before: nil, with: reconciler)
+    child.mount(before: nil, on: self, with: reconciler)
   }
 
   override func unmount(with reconciler: StackReconciler<R>) {
@@ -36,7 +41,8 @@ final class MountedApp<R: Renderer>: MountedCompositeElement<R> {
   }
 
   private func mountChild(_ childBody: _AnyScene) -> MountedElement<R> {
-    let mountedScene: MountedScene<R> = childBody.makeMountedScene(parentTarget, environmentValues)
+    let mountedScene: MountedScene<R> = childBody
+      .makeMountedScene(parentTarget, environmentValues, self)
     if let title = mountedScene.title {
       // swiftlint:disable force_cast
       (app.type as! _TitledApp.Type)._setTitle(title)

Sources/TokamakCore/MountedViews/MountedCompositeElement.swift

@@ -37,19 +37,34 @@ class MountedCompositeElement<R: Renderer>: MountedElement<R> {
    */
   var persistentSubscriptions = [AnyCancellable]()
 
-  init<A: App>(_ app: A, _ parentTarget: R.TargetType, _ environmentValues: EnvironmentValues) {
+  init<A: App>(
+    _ app: A,
+    _ parentTarget: R.TargetType,
+    _ environmentValues: EnvironmentValues,
+    _ parent: MountedElement<R>?
+  ) {
     self.parentTarget = parentTarget
-    super.init(_AnyApp(app), environmentValues)
+    super.init(_AnyApp(app), environmentValues, parent)
   }
 
-  init(_ scene: _AnyScene, _ parentTarget: R.TargetType, _ environmentValues: EnvironmentValues) {
+  init(
+    _ scene: _AnyScene,
+    _ parentTarget: R.TargetType,
+    _ environmentValues: EnvironmentValues,
+    _ parent: MountedElement<R>?
+  ) {
     self.parentTarget = parentTarget
-    super.init(scene, environmentValues)
+    super.init(scene, environmentValues, parent)
   }
 
-  init(_ view: AnyView, _ parentTarget: R.TargetType, _ environmentValues: EnvironmentValues) {
+  init(
+    _ view: AnyView,
+    _ parentTarget: R.TargetType,
+    _ environmentValues: EnvironmentValues,
+    _ parent: MountedElement<R>?
+  ) {
     self.parentTarget = parentTarget
-    super.init(view, environmentValues)
+    super.init(view, environmentValues, parent)
   }
 }
 

Sources/TokamakCore/MountedViews/MountedCompositeView.swift

@@ -19,12 +19,20 @@ import CombineShim
 import Runtime
 
 final class MountedCompositeView<R: Renderer>: MountedCompositeElement<R> {
-  override func mount(before sibling: R.TargetType? = nil, with reconciler: StackReconciler<R>) {
+  override func mount(
+    before sibling: R.TargetType? = nil,
+    on parent: MountedElement<R>? = nil,
+    with reconciler: StackReconciler<R>
+  ) {
     let childBody = reconciler.render(compositeView: self)
 
-    let child: MountedElement<R> = childBody.makeMountedView(parentTarget, environmentValues)
+    let child: MountedElement<R> = childBody.makeMountedView(
+      parentTarget,
+      environmentValues,
+      self
+    )
     mountedChildren = [child]
-    child.mount(before: sibling, with: reconciler)
+    child.mount(before: sibling, on: self, with: reconciler)
 
     // `_TargetRef` is a composite view, so it's enough to check for it only here
     if var targetRef = view.view as? TargetRefType {
@@ -41,12 +49,27 @@ final class MountedCompositeView<R: Renderer>: MountedCompositeElement<R> {
       view.view = targetRef
     }
 
-    // FIXME: this has to be implemented in a render-specific way, otherwise it's equivalent to
+    reconciler.afterCurrentRender(perform: { [weak self] in
-    // `_onMount` and `_onUnmount` at the moment,
+      guard let self = self else { return }
-    // see https://github.com/swiftwasm/Tokamak/issues/175 for more details
+
-    if let appearanceAction = view.view as? AppearanceActionType {
+      // FIXME: this has to be implemented in a render-specific way, otherwise it's equivalent to
-      appearanceAction.appear?()
+      // `_onMount` and `_onUnmount` at the moment,
-    }
+      // see https://github.com/swiftwasm/Tokamak/issues/175 for more details
+      if let appearanceAction = self.view.view as? AppearanceActionType {
+        appearanceAction.appear?()
+      }
+
+      if let preferenceModifier = self.view.view as? _PreferenceWritingViewProtocol {
+        self.view = preferenceModifier.modifyPreferenceStore(&self.preferenceStore)
+        if let parent = parent {
+          parent.preferenceStore.merge(with: self.preferenceStore)
+        }
+      }
+
+      if let preferenceReader = self.view.view as? _PreferenceReadingViewProtocol {
+        preferenceReader.preferenceStore(self.preferenceStore)
+      }
+    })
   }
 
   override func unmount(with reconciler: StackReconciler<R>) {
@@ -67,7 +90,7 @@ final class MountedCompositeView<R: Renderer>: MountedCompositeElement<R> {
         $0.environmentValues = environmentValues
         $0.view = AnyView(element)
       },
-      mountChild: { $0.makeMountedView(parentTarget, environmentValues) }
+      mountChild: { $0.makeMountedView(parentTarget, environmentValues, self) }
     )
   }
 }

Sources/TokamakCore/MountedViews/MountedElement.swift

@@ -88,20 +88,31 @@ public class MountedElement<R: Renderer> {
   var mountedChildren = [MountedElement<R>]()
   var environmentValues: EnvironmentValues
 
-  init(_ app: _AnyApp, _ environmentValues: EnvironmentValues) {
+  unowned var parent: MountedElement<R>?
+  /// `didSet` on this field propagates the preference changes up the view tree.
+  var preferenceStore: _PreferenceStore = .init() {
+    didSet {
+      parent?.preferenceStore.merge(with: preferenceStore)
+    }
+  }
+
+  init(_ app: _AnyApp, _ environmentValues: EnvironmentValues, _ parent: MountedElement<R>?) {
     element = .app(app)
+    self.parent = parent
     self.environmentValues = environmentValues
     updateEnvironment()
   }
 
-  init(_ scene: _AnyScene, _ environmentValues: EnvironmentValues) {
+  init(_ scene: _AnyScene, _ environmentValues: EnvironmentValues, _ parent: MountedElement<R>?) {
     element = .scene(scene)
+    self.parent = parent
     self.environmentValues = environmentValues
     updateEnvironment()
   }
 
-  init(_ view: AnyView, _ environmentValues: EnvironmentValues) {
+  init(_ view: AnyView, _ environmentValues: EnvironmentValues, _ parent: MountedElement<R>?) {
     element = .view(view)
+    self.parent = parent
     self.environmentValues = environmentValues
     updateEnvironment()
   }
@@ -122,7 +133,11 @@ public class MountedElement<R: Renderer> {
     return info
   }
 
-  func mount(before sibling: R.TargetType? = nil, with reconciler: StackReconciler<R>) {
+  func mount(
+    before sibling: R.TargetType? = nil,
+    on parent: MountedElement<R>? = nil,
+    with reconciler: StackReconciler<R>
+  ) {
     fatalError("implement \(#function) in subclass")
   }
 
@@ -217,14 +232,15 @@ extension TypeInfo {
 extension AnyView {
   func makeMountedView<R: Renderer>(
     _ parentTarget: R.TargetType,
-    _ environmentValues: EnvironmentValues
+    _ environmentValues: EnvironmentValues,
+    _ parent: MountedElement<R>?
   ) -> MountedElement<R> {
     if type == EmptyView.self {
-      return MountedEmptyView(self, environmentValues)
+      return MountedEmptyView(self, environmentValues, parent)
     } else if bodyType == Never.self && !(type is ViewDeferredToRenderer.Type) {
-      return MountedHostView(self, parentTarget, environmentValues)
+      return MountedHostView(self, parentTarget, environmentValues, parent)
     } else {
-      return MountedCompositeView(self, parentTarget, environmentValues)
+      return MountedCompositeView(self, parentTarget, environmentValues, parent)
     }
   }
 }

Sources/TokamakCore/MountedViews/MountedEmptyView.swift

@@ -16,7 +16,11 @@
 //
 
 final class MountedEmptyView<R: Renderer>: MountedElement<R> {
-  override func mount(before sibling: R.TargetType? = nil, with reconciler: StackReconciler<R>) {}
+  override func mount(
+    before sibling: R.TargetType? = nil,
+    on parent: MountedElement<R>? = nil,
+    with reconciler: StackReconciler<R>
+  ) {}
 
   override func unmount(with reconciler: StackReconciler<R>) {}
 

Sources/TokamakCore/MountedViews/MountedHostView.swift

@@ -31,13 +31,22 @@ public final class MountedHostView<R: Renderer>: MountedElement<R> {
   /// Target of this host view supplied by a renderer after mounting has completed.
   private(set) var target: R.TargetType?
 
-  init(_ view: AnyView, _ parentTarget: R.TargetType, _ environmentValues: EnvironmentValues) {
+  init(
+    _ view: AnyView,
+    _ parentTarget: R.TargetType,
+    _ environmentValues: EnvironmentValues,
+    _ parent: MountedElement<R>?
+  ) {
     self.parentTarget = parentTarget
 
-    super.init(view, environmentValues)
+    super.init(view, environmentValues, parent)
   }
 
-  override func mount(before sibling: R.TargetType? = nil, with reconciler: StackReconciler<R>) {
+  override func mount(
+    before sibling: R.TargetType? = nil,
+    on parent: MountedElement<R>? = nil,
+    with reconciler: StackReconciler<R>
+  ) {
     guard let target = reconciler.renderer?.mountTarget(
       before: sibling,
       to: parentTarget,
@@ -50,7 +59,7 @@ public final class MountedHostView<R: Renderer>: MountedElement<R> {
     guard !view.children.isEmpty else { return }
 
     mountedChildren = view.children.map {
-      $0.makeMountedView(target, environmentValues)
+      $0.makeMountedView(target, environmentValues, self)
     }
 
     /* Remember that `GroupView`s are always "flattened", their `target` instances are targets of
@@ -59,7 +68,9 @@ public final class MountedHostView<R: Renderer>: MountedElement<R> {
      `GroupView`.
      */
     let isGroupView = view.type is GroupView.Type
-    mountedChildren.forEach { $0.mount(before: isGroupView ? sibling : nil, with: reconciler) }
+    mountedChildren.forEach {
+      $0.mount(before: isGroupView ? sibling : nil, on: self, with: reconciler)
+    }
   }
 
   override func unmount(with reconciler: StackReconciler<R>) {
@@ -92,8 +103,8 @@ public final class MountedHostView<R: Renderer>: MountedElement<R> {
 
     // if no existing children then mount all new children
     case (true, false):
-      mountedChildren = childrenViews.map { $0.makeMountedView(target, environmentValues) }
+      mountedChildren = childrenViews.map { $0.makeMountedView(target, environmentValues, self) }
-      mountedChildren.forEach { $0.mount(with: reconciler) }
+      mountedChildren.forEach { $0.mount(on: self, with: reconciler) }
 
     // if both arrays have items then reconcile by types and keys
     case (false, false):
@@ -115,8 +126,8 @@ public final class MountedHostView<R: Renderer>: MountedElement<R> {
             as a "cursor" sibling when mounting. Only then we can dispose of the old mounted child
             by unmounting it.
            */
-          newChild = childView.makeMountedView(target, environmentValues)
+          newChild = childView.makeMountedView(target, environmentValues, self)
-          newChild.mount(before: mountedChild.firstDescendantTarget, with: reconciler)
+          newChild.mount(before: mountedChild.firstDescendantTarget, on: self, with: reconciler)
           mountedChild.unmount(with: reconciler)
         }
         newChildren.append(newChild)
@@ -135,8 +146,8 @@ public final class MountedHostView<R: Renderer>: MountedElement<R> {
         // mount remaining views
         for firstChild in childrenViews {
           let newChild: MountedElement<R> =
-            firstChild.makeMountedView(target, environmentValues)
+            firstChild.makeMountedView(target, environmentValues, self)
-          newChild.mount(with: reconciler)
+          newChild.mount(on: self, with: reconciler)
           newChildren.append(newChild)
         }
       }

Sources/TokamakCore/MountedViews/MountedScene.swift

@@ -22,19 +22,25 @@ final class MountedScene<R: Renderer>: MountedCompositeElement<R> {
     _ title: String?,
     _ children: [MountedElement<R>],
     _ parentTarget: R.TargetType,
-    _ environmentValues: EnvironmentValues
+    _ environmentValues: EnvironmentValues,
+    _ parent: MountedElement<R>?
   ) {
     self.title = title
-    super.init(scene, parentTarget, environmentValues)
+    super.init(scene, parentTarget, environmentValues, parent)
     mountedChildren = children
   }
 
-  override func mount(before sibling: R.TargetType? = nil, with reconciler: StackReconciler<R>) {
+  override func mount(
+    before sibling: R.TargetType? = nil,
+    on parent: MountedElement<R>? = nil,
+    with reconciler: StackReconciler<R>
+  ) {
     let childBody = reconciler.render(mountedScene: self)
 
-    let child: MountedElement<R> = childBody.makeMountedElement(parentTarget, environmentValues)
+    let child: MountedElement<R> = childBody
+      .makeMountedElement(parentTarget, environmentValues, self)
     mountedChildren = [child]
-    child.mount(before: sibling, with: reconciler)
+    child.mount(before: sibling, on: self, with: reconciler)
   }
 
   override func unmount(with reconciler: StackReconciler<R>) {
@@ -56,7 +62,7 @@ final class MountedScene<R: Renderer>: MountedCompositeElement<R> {
           $0.view = AnyView(view)
         }
       },
-      mountChild: { $0.makeMountedElement(parentTarget, environmentValues) }
+      mountChild: { $0.makeMountedElement(parentTarget, environmentValues, self) }
     )
   }
 }
@@ -73,13 +79,14 @@ extension _AnyScene.BodyResult {
 
   func makeMountedElement<R: Renderer>(
     _ parentTarget: R.TargetType,
-    _ environmentValues: EnvironmentValues
+    _ environmentValues: EnvironmentValues,
+    _ parent: MountedElement<R>?
   ) -> MountedElement<R> {
     switch self {
     case let .scene(scene):
-      return scene.makeMountedScene(parentTarget, environmentValues)
+      return scene.makeMountedScene(parentTarget, environmentValues, parent)
     case let .view(view):
-      return view.makeMountedView(parentTarget, environmentValues)
+      return view.makeMountedView(parentTarget, environmentValues, parent)
     }
   }
 }
@@ -87,7 +94,8 @@ extension _AnyScene.BodyResult {
 extension _AnyScene {
   func makeMountedScene<R: Renderer>(
     _ parentTarget: R.TargetType,
-    _ environmentValues: EnvironmentValues
+    _ environmentValues: EnvironmentValues,
+    _ parent: MountedElement<R>?
   ) -> MountedScene<R> {
     var title: String?
     if let titledSelf = scene as? TitledScene,
@@ -97,12 +105,16 @@ extension _AnyScene {
     }
     let children: [MountedElement<R>]
     if let deferredScene = scene as? SceneDeferredToRenderer {
-      children = [deferredScene.deferredBody.makeMountedView(parentTarget, environmentValues)]
+      children = [
+        deferredScene.deferredBody.makeMountedView(parentTarget, environmentValues, parent),
+      ]
     } else if let groupScene = scene as? GroupScene {
-      children = groupScene.children.map { $0.makeMountedScene(parentTarget, environmentValues) }
+      children = groupScene.children.map {
+        $0.makeMountedScene(parentTarget, environmentValues, parent)
+      }
     } else {
       children = []
     }
-    return .init(self, title, children, parentTarget, environmentValues)
+    return .init(self, title, children, parentTarget, environmentValues, parent)
   }
 }

Sources/TokamakCore/Preferences/PreferenceKey.swift

@@ -0,0 +1,120 @@
+// Copyright 2020 Tokamak contributors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+//  Created by Carson Katri on 11/26/20.
+//
+
+public protocol PreferenceKey {
+  associatedtype Value
+  static var defaultValue: Value { get }
+  static func reduce(value: inout Value, nextValue: () -> Value)
+}
+
+public extension PreferenceKey where Self.Value: ExpressibleByNilLiteral {
+  static var defaultValue: Value { nil }
+}
+
+public struct _PreferenceValue<Key> where Key: PreferenceKey {
+  /// Every value the `Key` has had.
+  var valueList: [Key.Value]
+  /// The latest value.
+  public var value: Key.Value {
+    reduce(valueList)
+  }
+
+  func reduce(_ values: [Key.Value]) -> Key.Value {
+    values.reduce(into: Key.defaultValue) { prev, next in
+      Key.reduce(value: &prev) { next }
+    }
+  }
+}
+
+public struct _PreferenceStore {
+  /// The backing values of the `_PreferenceStore`.
+  private var values: [String: Any]
+
+  public init(values: [String: Any] = [:]) {
+    self.values = values
+  }
+
+  public func value<Key>(forKey key: Key.Type = Key.self) -> _PreferenceValue<Key>
+    where Key: PreferenceKey
+  {
+    values[String(describing: key)] as? _PreferenceValue<Key>
+      ?? _PreferenceValue(valueList: [Key.defaultValue])
+  }
+
+  public mutating func insert<Key>(_ value: Key.Value, forKey key: Key.Type = Key.self)
+    where Key: PreferenceKey
+  {
+    let previousValues = self.value(forKey: key).valueList
+    values[String(describing: key)] = _PreferenceValue<Key>(valueList: previousValues + [value])
+  }
+
+  public mutating func merge(with other: Self) {
+    self = merging(with: other)
+  }
+
+  public func merging(with other: Self) -> Self {
+    var result = values
+    for (key, value) in other.values {
+      result[key] = value
+    }
+    return .init(values: result)
+  }
+}
+
+/// A protocol that allows a `View` to read values from the current `_PreferenceStore`.
+/// The key difference between `_PreferenceReadingViewProtocol` and
+/// `_PreferenceWritingViewProtocol` is that `_PreferenceReadingViewProtocol`
+/// calls `preferenceStore` during the current render, and `_PreferenceWritingViewProtocol`
+/// waits until the current render finishes.
+public protocol _PreferenceReadingViewProtocol {
+  func preferenceStore(_ preferenceStore: _PreferenceStore)
+}
+
+/// A protocol that allows a `View` to modify values from the current `_PreferenceStore`.
+public protocol _PreferenceWritingViewProtocol {
+  func modifyPreferenceStore(_ preferenceStore: inout _PreferenceStore) -> AnyView
+}
+
+/// A protocol that allows a `ViewModifier` to modify values from the current `_PreferenceStore`.
+public protocol _PreferenceWritingModifierProtocol: ViewModifier
+  where Body == AnyView
+{
+  func body(_ content: Self.Content, with preferenceStore: inout _PreferenceStore) -> AnyView
+}
+
+public extension _PreferenceWritingViewProtocol where Self: View {
+  var body: Never {
+    neverBody(String(describing: Self.self))
+  }
+}
+
+public extension _PreferenceWritingModifierProtocol {
+  func body(content: Content) -> AnyView {
+    content.view
+  }
+}
+
+extension ModifiedContent: _PreferenceWritingViewProtocol
+  where Content: View, Modifier: _PreferenceWritingModifierProtocol
+{
+  public func modifyPreferenceStore(_ preferenceStore: inout _PreferenceStore) -> AnyView {
+    AnyView(
+      modifier
+        .body(.init(modifier: modifier, view: AnyView(content)), with: &preferenceStore)
+    )
+  }
+}

Sources/TokamakCore/Preferences/_PreferenceActionModifier.swift

@@ -0,0 +1,45 @@
+// Copyright 2020 Tokamak contributors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+//  Created by Carson Katri on 11/26/20.
+//
+
+public struct _PreferenceActionModifier<Key>: _PreferenceWritingModifierProtocol
+  where Key: PreferenceKey, Key.Value: Equatable
+{
+  public let action: (Key.Value) -> ()
+  public init(action: @escaping (Key.Value) -> Swift.Void) {
+    self.action = action
+  }
+
+  public func body(_ content: Content, with preferenceStore: inout _PreferenceStore) -> AnyView {
+    let value = preferenceStore.value(forKey: Key.self)
+    let previousValue = value.reduce(value.valueList.dropLast())
+    if previousValue != value.value {
+      action(value.value)
+    }
+    return content.view
+  }
+}
+
+public extension View {
+  func onPreferenceChange<K>(
+    _ key: K.Type = K.self,
+    perform action: @escaping (K.Value) -> ()
+  ) -> some View
+    where K: PreferenceKey, K.Value: Equatable
+  {
+    modifier(_PreferenceActionModifier<K>(action: action))
+  }
+}

Sources/TokamakCore/Preferences/_PreferenceReadingView.swift

@@ -0,0 +1,69 @@
+// Copyright 2020 Tokamak contributors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+//  Created by Carson Katri on 11/26/20.
+//
+
+/// Delays the retrieval of a `PreferenceKey.Value` by passing the `_PreferenceValue` to a build
+/// function.
+public struct _DelayedPreferenceView<Key, Content>: View, _PreferenceReadingViewProtocol
+  where Key: PreferenceKey, Content: View
+{
+  @State private var resolvedValue: _PreferenceValue<Key> = _PreferenceValue(
+    valueList: [Key.defaultValue]
+  )
+  public let transform: (_PreferenceValue<Key>) -> Content
+
+  public init(transform: @escaping (_PreferenceValue<Key>) -> Content) {
+    self.transform = transform
+  }
+
+  public func preferenceStore(_ preferenceStore: _PreferenceStore) {
+    resolvedValue = preferenceStore.value(forKey: Key.self)
+  }
+
+  public var body: some View {
+    transform(resolvedValue)
+  }
+}
+
+public extension PreferenceKey {
+  static func _delay<T>(
+    _ transform: @escaping (_PreferenceValue<Self>) -> T
+  ) -> some View
+    where T: View
+  {
+    _DelayedPreferenceView(transform: transform)
+  }
+}
+
+public extension View {
+  func overlayPreferenceValue<Key, T>(
+    _ key: Key.Type = Key.self,
+    @ViewBuilder _ transform: @escaping (Key.Value) -> T
+  ) -> some View
+    where Key: PreferenceKey, T: View
+  {
+    Key._delay { self.overlay(transform($0.value)) }
+  }
+
+  func backgroundPreferenceValue<Key, T>(
+    _ key: Key.Type = Key.self,
+    @ViewBuilder _ transform: @escaping (Key.Value) -> T
+  ) -> some View
+    where Key: PreferenceKey, T: View
+  {
+    Key._delay { self.background(transform($0.value)) }
+  }
+}

Sources/TokamakCore/Preferences/_PreferenceTransformModifier.swift

@@ -0,0 +1,48 @@
+// Copyright 2020 Tokamak contributors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+//  Created by Carson Katri on 11/26/20.
+//
+
+/// Transforms a `PreferenceKey.Value`.
+public struct _PreferenceTransformModifier<Key>: _PreferenceWritingModifierProtocol
+  where Key: PreferenceKey
+{
+  public let transform: (inout Key.Value) -> ()
+
+  public init(
+    key _: Key.Type = Key.self,
+    transform: @escaping (inout Key.Value) -> ()
+  ) {
+    self.transform = transform
+  }
+
+  public func body(_ content: Content, with preferenceStore: inout _PreferenceStore) -> AnyView {
+    var newValue = preferenceStore.value(forKey: Key.self).value
+    transform(&newValue)
+    preferenceStore.insert(newValue, forKey: Key.self)
+    return content.view
+  }
+}
+
+public extension View {
+  func transformPreference<K>(
+    _ key: K.Type = K.self,
+    _ callback: @escaping (inout K.Value) -> ()
+  ) -> some View
+    where K: PreferenceKey
+  {
+    modifier(_PreferenceTransformModifier<K>(transform: callback))
+  }
+}

Sources/TokamakCore/Preferences/_PreferenceWritingModifier.swift

@@ -0,0 +1,44 @@
+// Copyright 2020 Tokamak contributors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+//  Created by Carson Katri on 11/26/20.
+//
+
+public struct _PreferenceWritingModifier<Key>: _PreferenceWritingModifierProtocol
+  where Key: PreferenceKey
+{
+  public let value: Key.Value
+  public init(key: Key.Type = Key.self, value: Key.Value) {
+    self.value = value
+  }
+
+  public func body(_ content: Content, with preferenceStore: inout _PreferenceStore) -> AnyView {
+    preferenceStore.insert(value, forKey: Key.self)
+    return content.view
+  }
+}
+
+extension _PreferenceWritingModifier: Equatable where Key.Value: Equatable {
+  public static func == (a: Self, b: Self) -> Bool {
+    a.value == b.value
+  }
+}
+
+public extension View {
+  func preference<K>(key: K.Type = K.self, value: K.Value) -> some View
+    where K: PreferenceKey
+  {
+    modifier(_PreferenceWritingModifier<K>(value: value))
+  }
+}

Sources/TokamakCore/StackReconciler.swift

@@ -74,9 +74,9 @@ public final class StackReconciler<R: Renderer> {
     self.scheduler = scheduler
     rootTarget = target
 
-    rootElement = AnyView(view).makeMountedView(target, environment)
+    rootElement = AnyView(view).makeMountedView(target, environment, nil)
 
-    rootElement.mount(with: self)
+    performInitialMount()
   }
 
   public init<A: App>(
@@ -90,15 +90,20 @@ public final class StackReconciler<R: Renderer> {
     self.scheduler = scheduler
     rootTarget = target
 
-    rootElement = MountedApp(app, target, environment)
+    rootElement = MountedApp(app, target, environment, nil)
 
-    rootElement.mount(with: self)
+    performInitialMount()
     if let mountedApp = rootElement as? MountedApp<R> {
       setupPersistentSubscription(for: app._phasePublisher, to: \.scenePhase, of: mountedApp)
       setupPersistentSubscription(for: app._colorSchemePublisher, to: \.colorScheme, of: mountedApp)
     }
   }
 
+  private func performInitialMount() {
+    rootElement.mount(with: self)
+    performPostrenderCallbacks()
+  }
+
   private func queueStorageUpdate(
     for mountedElement: MountedCompositeElement<R>,
     id: Int,
@@ -108,7 +113,7 @@ public final class StackReconciler<R: Renderer> {
     queueUpdate(for: mountedElement)
   }
 
-  private func queueUpdate(for mountedElement: MountedCompositeElement<R>) {
+  internal func queueUpdate(for mountedElement: MountedCompositeElement<R>) {
     let shouldSchedule = queuedRerenders.isEmpty
     queuedRerenders.insert(mountedElement)
 
@@ -123,6 +128,7 @@ public final class StackReconciler<R: Renderer> {
     }
 
     queuedRerenders.removeAll()
+    performPostrenderCallbacks()
   }
 
   private func setupStorage(
@@ -278,4 +284,14 @@ public final class StackReconciler<R: Renderer> {
       }
     }
   }
+
+  private var queuedPostrenderCallbacks = [() -> ()]()
+  func afterCurrentRender(perform callback: @escaping () -> ()) {
+    queuedPostrenderCallbacks.append(callback)
+  }
+
+  private func performPostrenderCallbacks() {
+    queuedPostrenderCallbacks.forEach { $0() }
+    queuedPostrenderCallbacks.removeAll()
+  }
 }

Sources/TokamakCore/Tokens/Color.swift

@@ -39,7 +39,7 @@ public protocol AnyColorBoxDeferredToRenderer: AnyColorBox {
   func deferredResolve(in environment: EnvironmentValues) -> AnyColorBox.ResolvedValue
 }
 
-public class AnyColorBox: AnyTokenBox {
+public class AnyColorBox: AnyTokenBox, Equatable {
   public struct _RGBA: Hashable, Equatable {
     public let red: Double
     public let green: Double
@@ -61,7 +61,15 @@ public class AnyColorBox: AnyTokenBox {
     }
   }
 
-  public static func == (lhs: AnyColorBox, rhs: AnyColorBox) -> Bool { false }
+  public static func == (lhs: AnyColorBox, rhs: AnyColorBox) -> Bool {
+    lhs.equals(rhs)
+  }
+
+  /// We use a function separate from `==` so that subclasses can override the equality checks.
+  public func equals(_ other: AnyColorBox) -> Bool {
+    fatalError("implement \(#function) in subclass")
+  }
+
   public func hash(into hasher: inout Hasher) {
     fatalError("implement \(#function) in subclass")
   }
@@ -74,8 +82,10 @@ public class AnyColorBox: AnyTokenBox {
 public class _ConcreteColorBox: AnyColorBox {
   public let rgba: AnyColorBox._RGBA
 
-  public static func == (lhs: _ConcreteColorBox, rhs: _ConcreteColorBox) -> Bool {
+  override public func equals(_ other: AnyColorBox) -> Bool {
-    lhs.rgba == rhs.rgba
+    guard let other = other as? _ConcreteColorBox
+    else { return false }
+    return rgba == other.rgba
   }
 
   override public func hash(into hasher: inout Hasher) {
@@ -94,10 +104,10 @@ public class _ConcreteColorBox: AnyColorBox {
 public class _EnvironmentDependentColorBox: AnyColorBox {
   public let resolver: (EnvironmentValues) -> Color
 
-  public static func == (lhs: _EnvironmentDependentColorBox,
+  override public func equals(_ other: AnyColorBox) -> Bool {
-                         rhs: _EnvironmentDependentColorBox) -> Bool
+    guard let other = other as? _EnvironmentDependentColorBox
-  {
+    else { return false }
-    lhs.resolver(EnvironmentValues()) == rhs.resolver(EnvironmentValues())
+    return resolver(EnvironmentValues()) == other.resolver(EnvironmentValues())
   }
 
   override public func hash(into hasher: inout Hasher) {
@@ -113,8 +123,8 @@ public class _EnvironmentDependentColorBox: AnyColorBox {
   }
 }
 
-public class _SystemColorBox: AnyColorBox {
+public class _SystemColorBox: AnyColorBox, CustomStringConvertible {
-  public enum SystemColor: Equatable, Hashable {
+  public enum SystemColor: String, Equatable, Hashable {
     case clear
     case black
     case white
@@ -130,10 +140,16 @@ public class _SystemColorBox: AnyColorBox {
     case secondary
   }
 
+  public var description: String {
+    value.rawValue
+  }
+
   public let value: SystemColor
 
-  public static func == (lhs: _SystemColorBox, rhs: _SystemColorBox) -> Bool {
+  override public func equals(_ other: AnyColorBox) -> Bool {
-    lhs.value == rhs.value
+    guard let other = other as? _SystemColorBox
+    else { return false }
+    return value == other.value
   }
 
   override public func hash(into hasher: inout Hasher) {
@@ -252,6 +268,16 @@ public extension Color {
   }
 }
 
+extension Color: CustomStringConvertible {
+  public var description: String {
+    if let providerDescription = provider as? CustomStringConvertible {
+      return providerDescription.description
+    } else {
+      return String(describing: self)
+    }
+  }
+}
+
 public extension Color {
   private init(systemColor: _SystemColorBox.SystemColor) {
     self.init(_SystemColorBox(systemColor))

Sources/TokamakCore/Tokens/ColorScheme.swift

@@ -12,7 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-public enum ColorScheme: CaseIterable {
+public enum ColorScheme: CaseIterable, Equatable {
   case dark
   case light
 }
@@ -35,3 +35,16 @@ public extension View {
     environment(\.colorScheme, colorScheme)
   }
 }
+
+public struct PreferredColorSchemeKey: PreferenceKey {
+  public typealias Value = ColorScheme?
+  public static func reduce(value: inout Value, nextValue: () -> Value) {
+    value = nextValue()
+  }
+}
+
+public extension View {
+  func preferredColorScheme(_ colorScheme: ColorScheme?) -> some View {
+    preference(key: PreferredColorSchemeKey.self, value: colorScheme)
+  }
+}

Sources/TokamakCore/Views/Navigation/NavigationLink.swift

@@ -89,7 +89,7 @@ public struct _NavigationLinkProxy<Label, Destination> where Label: View, Destin
 
   public var style: _AnyNavigationLinkStyle { subject.style }
   public var isSelected: Bool {
-    ObjectIdentifier(subject.destination) == ObjectIdentifier(subject.navigationContext.destination)
+    subject.destination === subject.navigationContext.destination
   }
 
   public func activate() {

Sources/TokamakCore/Views/Navigation/NavigationView.swift

@@ -65,4 +65,9 @@ extension EnvironmentValues {
   }
 }
 
-public let _navigationDestinationKey = \EnvironmentValues.navigationDestination
+struct NavigationTitleKey: PreferenceKey {
+  typealias Value = AnyView?
+  static func reduce(value: inout AnyView?, nextValue: () -> AnyView?) {
+    value = nextValue()
+  }
+}

Sources/TokamakCore/Views/Text/Text.swift

@@ -116,6 +116,10 @@ public extension Text {
     .init(storage: storage, modifiers: modifiers + [.font(font)])
   }
 
+  func foregroundColor(_ color: Color?) -> Text {
+    .init(storage: storage, modifiers: modifiers + [.color(color)])
+  }
+
   func fontWeight(_ weight: Font.Weight?) -> Text {
     .init(storage: storage, modifiers: modifiers + [.weight(weight)])
   }

Sources/TokamakDOM/Core.swift

@@ -22,6 +22,8 @@ import TokamakCore
 public typealias Environment = TokamakCore.Environment
 public typealias EnvironmentObject = TokamakCore.EnvironmentObject
 
+public typealias PreferenceKey = TokamakCore.PreferenceKey
+
 public typealias Binding = TokamakCore.Binding
 public typealias ObservableObject = TokamakCore.ObservableObject
 public typealias ObservedObject = TokamakCore.ObservedObject

Sources/TokamakDOM/DOMRenderer.swift

@@ -157,7 +157,7 @@ final class DOMRenderer: Renderer {
   ) {
     defer { completion() }
 
-    guard let html = mapAnyView(host.view, transform: { (html: AnyHTML) in html })
+    guard mapAnyView(host.view, transform: { (html: AnyHTML) in html }) != nil
     else { return }
 
     _ = parent.ref.removeChild!(target.ref)

Sources/TokamakDemo/PreferenceKeyDemo.swift

@@ -0,0 +1,152 @@
+// Copyright 2020 Tokamak contributors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+//  Created by Carson Katri on 11/26/20.
+//
+
+import TokamakShim
+
+struct TestPreferenceKey: PreferenceKey {
+  static let defaultValue = Color.red
+  static func reduce(value: inout Color, nextValue: () -> Color) {
+    value = nextValue()
+  }
+}
+
+@available(macOS 11, iOS 14, *)
+struct PreferenceKeyDemo: View {
+  @State private var testKeyValue: Color = .yellow
+  @Environment(\.colorScheme) var colorScheme
+
+  var body: some View {
+    VStack {
+      Group {
+        Text("Preferences are like reverse-environment values.")
+        Text(
+          """
+          In this demo, the background color of each item \
+          is set to the value of the PreferenceKey.
+          """
+        )
+        Text("Default color: red (this won't show on the innermost because it never 'changed').")
+        Text("Innermost child sets the color to blue.")
+        Text("One level up sets the color to green, and so on.")
+
+        VStack {
+          Text("Root")
+            .padding(.bottom, 8)
+          SetColor(3, .purple) {
+            SetColor(2, .green) {
+              SetColor(1, .blue)
+            }
+          }
+        }
+        .padding()
+        .background(testKeyValue)
+        .onPreferenceChange(TestPreferenceKey.self) {
+          print("Value changed to \($0)")
+          testKeyValue = $0
+        }
+      }
+
+      Group {
+        Text("Preferences can also be accessed and used immediately via a background or overlay:")
+        Circle()
+          .frame(width: 25, height: 25)
+          .foregroundColor(testKeyValue)
+          .backgroundPreferenceValue(TestPreferenceKey.self) {
+            Circle()
+              .frame(width: 50, height: 50)
+              .foregroundColor($0)
+          }
+        Circle()
+          .frame(width: 50, height: 50)
+          .foregroundColor(testKeyValue)
+          .overlayPreferenceValue(TestPreferenceKey.self) {
+            Circle()
+              .frame(width: 25, height: 25)
+              .foregroundColor($0)
+          }
+      }
+
+      Group {
+        Text(
+          """
+          We can also transform the key. Here we perform several transformations and use an \
+          `overlayPreferenceValue`
+          """
+        )
+        Text("1. Set the color to yellow")
+        Text("2. Transform if the color is yellow -> green")
+        Text("3. Transform if the color is green -> blue")
+        Text("4. Use the final color in an overlay.")
+        Circle()
+          .frame(width: 50, height: 50)
+          .foregroundColor(testKeyValue)
+          .preference(key: TestPreferenceKey.self, value: .yellow)
+          .transformPreference(TestPreferenceKey.self) {
+            print("Transforming \($0) ->? green")
+            if $0 == .yellow {
+              $0 = .green
+            }
+          }
+          .transformPreference(TestPreferenceKey.self) {
+            print("Transforming \($0) ->? blue")
+            if $0 == .green {
+              $0 = .blue
+            }
+          }
+          .overlayPreferenceValue(TestPreferenceKey.self) { newColor in
+            Circle()
+              .frame(width: 25, height: 25)
+              .foregroundColor(newColor)
+          }
+      }
+    }
+  }
+
+  struct SetColor<Content: View>: View {
+    let level: Int
+    let color: Color
+    let content: Content
+    @State private var testKeyValue: Color = .yellow
+
+    init(_ level: Int, _ color: Color, @ViewBuilder _ content: () -> Content) {
+      self.level = level
+      self.color = color
+      self.content = content()
+    }
+
+    var body: some View {
+      VStack {
+        Text("Level \(level)")
+          .padding(.bottom, level == 1 ? 0 : 8)
+        content
+      }
+      .padding()
+      .background(testKeyValue)
+      .onPreferenceChange(TestPreferenceKey.self) {
+        testKeyValue = $0
+      }
+      .preference(key: TestPreferenceKey.self, value: color)
+    }
+  }
+}
+
+@available(macOS 11, iOS 14, *)
+extension PreferenceKeyDemo.SetColor where Content == EmptyView {
+  init(_ level: Int, _ color: Color) {
+    self.init(level, color) { EmptyView() }
+  }
+}

Sources/TokamakDemo/TokamakDemo.swift

@@ -129,6 +129,9 @@ struct TokamakDemoView: View {
           Section(header: Text("Misc")) {
             NavItem("Path", destination: PathDemo())
             NavItem("Environment", destination: EnvironmentDemo().font(.system(size: 8)))
+            if #available(macOS 11.0, iOS 14.0, *) {
+              NavItem("Preferences", destination: PreferenceKeyDemo())
+            }
             NavItem("Color", destination: ColorDemo())
             if #available(OSX 11.0, iOS 14.0, *) {
               NavItem("AppStorage", destination: AppStorageDemo())

Sources/TokamakStaticHTML/Modifiers/ModifiedContent.swift

@@ -30,7 +30,7 @@ extension ModifiedContent: AnyModifiedContent where Modifier: DOMViewModifier, C
   }
 }
 
-extension ModifiedContent: ViewDeferredToRenderer where Content: View {
+extension ModifiedContent: ViewDeferredToRenderer where Content: View, Modifier: ViewModifier {
   public var deferredBody: AnyView {
     if let domModifier = modifier as? DOMViewModifier {
       if let adjacentModifier = content as? AnyModifiedContent,
@@ -51,8 +51,10 @@ extension ModifiedContent: ViewDeferredToRenderer where Content: View {
           content
         })
       }
-    } else {
+    } else if Modifier.Body.self == Never.self {
       return AnyView(content)
+    } else {
+      return AnyView(modifier.body(content: .init(modifier: modifier, view: AnyView(content))))
     }
   }
 }

Sources/TokamakStaticHTML/Views/Navigation/NavigationView.swift

@@ -16,14 +16,15 @@ import TokamakCore
 
 extension NavigationView: ViewDeferredToRenderer {
   public var deferredBody: AnyView {
-    AnyView(HTML("div", [
+    let proxy = _NavigationViewProxy(self)
+    return AnyView(HTML("div", [
       "class": "_tokamak-navigationview",
     ]) {
-      _NavigationViewProxy(self).content
+      proxy.content
       HTML("div", [
         "class": "_tokamak-navigationview-content",
       ]) {
-        _NavigationViewProxy(self).destination
+        proxy.destination
       }
     })
   }