https://twitter.com/krzyzanowskim/status/1548765900626661376

AppSettings.swift

// Usage

@MainActor
final class AppSettings: ObservableObject {

    static let standard = AppSettings()

    @Published(preferenceKey: AppSettingsKey.editorHighlightSelectedLine)
    var editorHighlightSelectedLine: Bool = true
}

enum AppSettingsKey: String {
    case editorHighlightSelectedLine = "editor.HighlightSelectedLine"
}

Published+UserDefaults.swift

import Foundation
import Combine

private var cancellables: Set<AnyCancellable> = []

extension Published {

    init<Key>(wrappedValue defaultValue: Value, preferenceKey: Key, store: UserDefaults = .standard) where Key: RawRepresentable, Key.RawValue == String {
        self.init(wrappedValue: defaultValue, preferenceKey: preferenceKey.rawValue, store: store)
    }

    init(wrappedValue defaultValue: Value, preferenceKey: String, store: UserDefaults = .standard) {
        let initialValue = store.value(forKey: preferenceKey) as? Value ?? defaultValue

        self.init(initialValue: initialValue)

        projectedValue.sink { value in
            if let value = value as? AnyOptional, !value.isNil {
                store.setValue(value, forKey: preferenceKey)
            } else if let value = value as? AnyOptional, value.isNil {
                store.removeObject(forKey: preferenceKey)
            } else {
                store.setValue(value, forKey: preferenceKey)
            }
        }
        .store(in: &cancellables)
    }
}

// Since our property wrapper's Value type isn't optional, but
// can still contain nil values, we'll have to introduce this
// protocol to enable us to cast any assigned value into a type
// that we can compare against nil:
private protocol AnyOptional {
    var isNil: Bool { get }
}

extension Optional: AnyOptional {
    var isNil: Bool { self == nil }
}