SwiftyUserDefaults Property wrapper that works with SwiftUI

PublishedUserDefault.swift

@available(iOS 13.0, *)
@propertyWrapper
public final class PublishedUserDefault<T: DefaultsSerializable> where T.T == T {
    public let key: DefaultsKey<T>

    private var _value: T.T?
    private var observation: DefaultsDisposable?

    public init(keyPath: KeyPath<DefaultsKeys, DefaultsKey<T>>) {
        self.key = Defaults.keyStore[keyPath: keyPath]
    }

    public static subscript<EnclosingType: ObservableObject>(
        _enclosingInstance instance: EnclosingType,
        wrapped wrappedKeyPath: ReferenceWritableKeyPath<EnclosingType, T>,
        storage storageKeyPath: ReferenceWritableKeyPath<EnclosingType, PublishedUserDefault>
    ) -> T {
        get {
            defer {
                instance[keyPath: storageKeyPath].observation = Defaults.observe(instance[keyPath: storageKeyPath].key) { [weak instance] update in
                    guard let instance else { return }

                    let publisher = instance.objectWillChange
                    // This assumption is definitely not safe to make in
                    // production code, but it's fine for this demo purpose:
                    (publisher as! ObservableObjectPublisher).send()

                    instance[keyPath: storageKeyPath]._value = update.newValue
                }
            }

            return instance[keyPath: storageKeyPath]._value ?? Defaults[key: instance[keyPath: storageKeyPath].key]
        }
        set {
            let publisher = instance.objectWillChange
            // This assumption is definitely not safe to make in
            // production code, but it's fine for this demo purpose:
            (publisher as! ObservableObjectPublisher).send()

            instance[keyPath: storageKeyPath]._value = newValue
            Defaults[key: instance[keyPath: storageKeyPath].key] = newValue
        }
    }

    @available(*, unavailable,
        message: "@Proxy can only be applied to classes"
    )
    public var wrappedValue: T {
        get { fatalError() }
        set { fatalError() }
    }

    deinit {
        observation?.dispose()
    }
}