ObservableProperty.swift

import SwiftUI

public protocol ObservablePropertySource: ObservableObject {
    static var shared: Self { get }
}

@propertyWrapper
public struct ObservableProperty<S: ObservablePropertySource, T>: DynamicProperty {
    @ObservedObject private var source: S

    private let keyPath: KeyPath<S, T>

    public init(_ keyPath: KeyPath<S, T>) {
        source = S.shared
        self.keyPath = keyPath
    }

    public var wrappedValue: T {
        source[keyPath: keyPath]
    }
}


public final class Features: ObservablePropertySource {
    public static let shared = Features()
    private init() { }

    @Published public var isDebugMenuEnabled = false
}

public final class AppSettings: ObservablePropertySource {
    public static let shared = AppSettings()
    private init() { }


    @AppStorage("appSetting1") public var appSetting1: Int = 42
    // could just be `static` and access directly without going through @wrapper
    @AppStorage("appSetting2") public static var appSetting2: String = "Hello"

}

public final class SceneSettings: ObservableObject {
    public static let shared = SceneSettings()
    private init() { }


    @SceneStorage("sceneValue1") public var sceneSetting1: Int = 54
    @SceneStorage("sceneValue2") public var sceneSetting2: String = "World"

}


struct ContentView: View {
//    @Feature(\.isDebugMenuEnabled) var showDebugMenu
//    @AppSetting(\.appSetting1) var appSetting1
    @ObservableProperty(\Features.isDebugMenuEnabled) var showDebugMenu
    @ObservableProperty(\AppSettings.appSetting1) var appSetting1

    var body: some View {
        VStack {
            Text("Hello, world!")
                .padding()
            if showDebugMenu {
                Text("Debug Menu on")
            } else {
                Text("Debug Menu off")
            }
            Text("appSetting1 = \(appSetting1)")
            // directly access static, better?
            Text("appSetting2 = \(AppSettings.appSetting2)")
        }
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}