Implement StateObject property wrapper

Sources/TokamakCore/MountedViews/MountedCompositeElement.swift

@@ -20,10 +20,10 @@ import CombineShim
 class MountedCompositeElement<R: Renderer>: MountedElement<R> {
   let parentTarget: R.TargetType
 
-  /** An array that stores type-erased values captured with the `@State` property wrappers used
-   in declarations of this element.
+  /** An array that stores type-erased values captured with the `@State` and `@StateObject` property
+    wrappers used in declarations of this element.
    */
-  var state = [Any]()
+  var storage = [Any]()
 
   /** An array that stores subscriptions to updates on `@ObservableObject` property wrappers used
     in declarations of this element. These subscriptions are transient and may be cleaned up on

Sources/TokamakCore/StackReconciler.swift

@@ -99,12 +99,12 @@ public final class StackReconciler<R: Renderer> {
     }
   }
 
-  private func queueStateUpdate(
+  private func queueStorageUpdate(
     for mountedElement: MountedCompositeElement<R>,
     id: Int,
     updater: (inout Any) -> ()
   ) {
-    updater(&mountedElement.state[id])
+    updater(&mountedElement.storage[id])
     queueUpdate(for: mountedElement)
   }
 
@@ -125,7 +125,7 @@ public final class StackReconciler<R: Renderer> {
     queuedRerenders.removeAll()
   }
 
-  private func setupState(
+  private func setupStorage(
     id: Int,
     for property: PropertyInfo,
     of compositeElement: MountedCompositeElement<R>,
@@ -134,23 +134,28 @@ public final class StackReconciler<R: Renderer> {
     // swiftlint:disable force_try
     // `ValueStorage` property already filtered out, so safe to assume the value's type
     // swiftlint:disable:next force_cast
-    var state = try! property.get(from: compositeElement[keyPath: bodyKeypath]) as! ValueStorage
+    var storage = try! property.get(from: compositeElement[keyPath: bodyKeypath]) as! ValueStorage
 
-    if compositeElement.state.count == id {
-      compositeElement.state.append(state.anyInitialValue)
+    if compositeElement.storage.count == id {
+      compositeElement.storage.append(storage.anyInitialValue)
     }
 
-    if state.getter == nil || state.setter == nil {
-      state.getter = { compositeElement.state[id] }
+    if storage.getter == nil {
+      storage.getter = { compositeElement.storage[id] }
+
+      guard var writableStorage = storage as? WritableValueStorage else {
+        return try! property.set(value: storage, on: &compositeElement[keyPath: bodyKeypath])
+      }
 
       // Avoiding an indirect reference cycle here: this closure can be owned by callbacks
       // owned by view's target, which is strongly referenced by the reconciler.
-      state.setter = { [weak self, weak compositeElement] newValue in
+      writableStorage.setter = { [weak self, weak compositeElement] newValue in
         guard let element = compositeElement else { return }
-        self?.queueStateUpdate(for: element, id: id) { $0 = newValue }
+        self?.queueStorageUpdate(for: element, id: id) { $0 = newValue }
       }
+
+      try! property.set(value: writableStorage, on: &compositeElement[keyPath: bodyKeypath])
     }
-    try! property.set(value: state, on: &compositeElement[keyPath: bodyKeypath])
   }
 
   private func setupTransientSubscription(
@@ -207,9 +212,10 @@ public final class StackReconciler<R: Renderer> {
     for property in dynamicProps {
       // Setup state/subscriptions
       if property.type is ValueStorage.Type {
-        setupState(id: stateIdx, for: property, of: compositeElement, body: bodyKeypath)
+        setupStorage(id: stateIdx, for: property, of: compositeElement, body: bodyKeypath)
         stateIdx += 1
-      } else if property.type is ObservedProperty.Type {
+      }
+      if property.type is ObservedProperty.Type {
         setupTransientSubscription(for: property, of: compositeElement, body: bodyKeypath)
       }
     }

Sources/TokamakCore/State/ObservedObject.swift

@@ -47,10 +47,10 @@ public struct ObservedObject<ObjectType>: DynamicProperty where ObjectType: Obse
   }
 
   public let projectedValue: Wrapper
+}
 
+extension ObservedObject: ObservedProperty {
   var objectWillChange: AnyPublisher<(), Never> {
     wrappedValue.objectWillChange.map { _ in }.eraseToAnyPublisher()
   }
 }
-
-extension ObservedObject: ObservedProperty {}

Sources/TokamakCore/State/State.swift

@@ -16,10 +16,13 @@
 //
 protocol ValueStorage {
   var getter: (() -> Any)? { get set }
-  var setter: ((Any) -> ())? { get set }
   var anyInitialValue: Any { get }
 }
 
+protocol WritableValueStorage: ValueStorage {
+  var setter: ((Any) -> ())? { get set }
+}
+
 @propertyWrapper public struct State<Value>: DynamicProperty {
   private let initialValue: Value
 
@@ -46,7 +49,7 @@ protocol ValueStorage {
   }
 }
 
-extension State: ValueStorage {}
+extension State: WritableValueStorage {}
 
 extension State where Value: ExpressibleByNilLiteral {
   @inlinable

Sources/TokamakCore/State/StateObject.swift

@@ -12,4 +12,32 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-public typealias StateObject = ObservedObject
+import CombineShim
+
+@propertyWrapper
+public struct StateObject<ObjectType: ObservableObject>: DynamicProperty {
+  public var wrappedValue: ObjectType { (getter?() as? ObservedObject.Wrapper)?.root ?? initial() }
+
+  let initial: () -> ObjectType
+  var getter: (() -> Any)?
+
+  public init(wrappedValue initial: @autoclosure @escaping () -> ObjectType) {
+    self.initial = initial
+  }
+
+  public var projectedValue: ObservedObject<ObjectType>.Wrapper {
+    getter?() as? ObservedObject.Wrapper ?? ObservedObject.Wrapper(root: initial())
+  }
+}
+
+extension StateObject: ObservedProperty {
+  var objectWillChange: AnyPublisher<(), Never> {
+    wrappedValue.objectWillChange.map { _ in }.eraseToAnyPublisher()
+  }
+}
+
+extension StateObject: ValueStorage {
+  var anyInitialValue: Any {
+    ObservedObject.Wrapper(root: initial())
+  }
+}

Sources/TokamakCore/Views/Navigation/NavigationView.swift

@@ -22,7 +22,7 @@ final class NavigationContext: ObservableObject {
 public struct NavigationView<Content>: View where Content: View {
   let content: Content
 
-  @ObservedObject var context = NavigationContext()
+  @StateObject var context = NavigationContext()
 
   public init(@ViewBuilder content: () -> Content) {
     self.content = content()

Sources/TokamakDOM/Core.swift

@@ -27,6 +27,7 @@ public typealias ObservableObject = TokamakCore.ObservableObject
 public typealias ObservedObject = TokamakCore.ObservedObject
 public typealias Published = TokamakCore.Published
 public typealias State = TokamakCore.State
+public typealias StateObject = TokamakCore.StateObject
 
 // MARK: Modifiers & Styles