An `Observable` property wrapper that only creates the observable value once

StateObservable.swift

/**
 * An Observable property wrapper that only creates the observable
 * value once.
 *
 * Example:
 * ```swift
 * @Observable class Item {
 * }
 *
 * struct MyView: View {
 *   @StateObservable var item = Item()
 * }
 * ```
 */
@propertyWrapper 
struct StateObservable<ObjectType: Observable>: DynamicProperty {
  final class ObservationBridge: ObservableObject {
    let observable : ObjectType
    init(_ observable: ObjectType) { self.observable = observable }
  }
  
  @StateObject private var bridge : ObservationBridge

  init(wrappedValue thunk: @autoclosure @escaping () -> ObjectType) {
    _bridge = .init(wrappedValue: ObservationBridge(thunk()))
  }

  @MainActor var wrappedValue: ObjectType { bridge.observable }
}

P.S.: Don't use it, but keep your state outside of views! 😜

Related discussion about iOS 17 Observable objects and @State:
https://mastodon.social/@helge/111074594371761479

Here is another version that overloads @StateObject, this doesn't work w/ function calls though (would have to qualify), and is also slower in property access:

extension StateObject {

  /**
   * Use Observable's with `@StateObject`.
   *
   * Example:
   * ```swift
   * @Observable class Item {
   * }
   *
   * struct MyView: View {
   *   @StateObject var item = Item()
   * }
   * ```
   */
  init<T>(wrappedValue thunk: @autoclosure @escaping () -> T)
    where T: Observable, ObjectType == ObservationBridge<T>
  {
    self.init(wrappedValue: ObservationBridge(thunk()))
  }
}
@dynamicMemberLookup
final class ObservationBridge<ObjectType>: ObservableObject {
  let observable : ObjectType
  init(_ observable: ObjectType) { self.observable = observable }
  public subscript<V>(dynamicMember keyPath: KeyPath<ObjectType, V>) -> V {
    observable[keyPath: keyPath]
  }
}