ProxyPropertyWrapper.swift

@propertyWrapper
public struct Proxy<EnclosingSelf, Value> {
  private let keyPath: ReferenceWritableKeyPath<EnclosingSelf, Value>
  
  public init(_ keyPath: ReferenceWritableKeyPath<EnclosingSelf, Value>) {
    self.keyPath = keyPath
  }
  
  @available(*, unavailable)
  public var wrappedValue: Value {
    get { fatalError() }
    set { fatalError() }
  }
  
  public static subscript(
    _enclosingInstance observed: EnclosingSelf,
    wrapped wrappedKeyPath: ReferenceWritableKeyPath<EnclosingSelf, Value>,
    storage storageKeyPath: ReferenceWritableKeyPath<EnclosingSelf, Self>
  ) -> Value {
    get {
      let storageValue = observed[keyPath: storageKeyPath]
      let value = observed[keyPath: storageValue.keyPath]
      return value
    }
    set {
      let storageValue = observed[keyPath: storageKeyPath]
      observed[keyPath: storageValue.keyPath] = newValue
    }
  }
}

@propertyWrapper
public struct DefaultProxy<EnclosingSelf, Value> {
  private let keyPath: ReferenceWritableKeyPath<EnclosingSelf, Value?>
  let defaultValue: Value
  
  public init(
    _ keyPath: ReferenceWritableKeyPath<EnclosingSelf, Value?>,
    defaultValue: Value
  ) {
    self.keyPath = keyPath
    self.defaultValue = defaultValue
  }
  
  @available(*, unavailable)
  public var wrappedValue: Value {
    get { fatalError() }
    set { fatalError() }
  }
  
  public static subscript(
    _enclosingInstance observed: EnclosingSelf,
    wrapped wrappedKeyPath: ReferenceWritableKeyPath<EnclosingSelf, Value>,
    storage storageKeyPath: ReferenceWritableKeyPath<EnclosingSelf, Self>
  ) -> Value {
    get {
      let storageValue = observed[keyPath: storageKeyPath]
      return observed[keyPath: storageValue.keyPath] ?? storageValue.defaultValue
    }
    set {
      let storageValue = observed[keyPath: storageKeyPath]
      observed[keyPath: storageValue.keyPath] = newValue
    }
  }
}