NSSecureCoding custom strategy kay or how to add custom mapping on top of an underlying persistence strategy

[KaiOelfke]

Sometimes I want to use an underlying persistence strategy and add some extra mapping. But I never managed to make this work well. The latest use case would be NSSecureCoding on top of a (file or app storage) key. This should basically use the base key similar to the default key to load and subscribe and then transform the value with encoding / decoding via NSCoding. It should resurface underlying load / save errors in addition to errors from NSCoding, which can fail.

I would expect this to be simple, but I'm not sure how to hook into the base in order to transform things at the right time.

The underlying key's Value should be Data? as it can be nil and NSCoding works with Data.

public struct NSSecureCodingKey<Base: SharedReaderKey<Data?>, Anchor: NSObject & NSSecureCoding>: SharedReaderKey {
  public typealias Value = Optional<Anchor>
  let base: Base
  let type: Anchor.Type
  
  public init(_ key: Base, type: Anchor.Type) {
    base = key
    self.type = type
  }
  
  public struct ID: Hashable {
    let baseID: Base.ID
    let type: ObjectIdentifier
  }

  public var id: ID {
    ID(baseID: base.id, type: ObjectIdentifier(type))
  }
  
  public func load(context: LoadContext<Value>, continuation: LoadContinuation<Value>) {
    fatalError()
  }

  public func subscribe(context: LoadContext<Value>, subscriber: SharedSubscriber<Value>) -> SharedSubscription {
    fatalError()
  }
}

extension NSSecureCodingKey: SharedKey where Base: SharedKey {
  public func save(_ value: Value, context: SaveContext, continuation: SaveContinuation) {
    do {
      let data = try value.flatMap {
        try NSKeyedArchiver.archivedData(withRootObject: $0, requiringSecureCoding: true)
      }
      base.save(data, context: context, continuation: continuation)
    } catch {
      continuation.resume(throwing: error)
    }
  }
}

extension NSSecureCodingKey {
  public static func secureCoding(
    _ base: Base,
    type: Anchor.Type
  ) -> Self
  where Anchor: NSObject, Anchor: NSSecureCoding, Base: SharedReaderKey<Data?>, Self == NSSecureCodingKey<Base, Anchor> {
    NSSecureCodingKey<Base, Anchor>(base, type: type)
  }
}

The load and subscribe implementations should transform Data? <-> Anchor?, forward underlying errors, and set errors, if loading fails due to a thrown error in the archiving / unarchiving calls.