import Foundation struct Model<Base: Modelable> { var base: Base } protocol Modelable { var model: Model<Self> { get } } extension Modelable { var model: Model<Self> { return Model(base: self) } } struct Hoge: Codable, Modelable { fileprivate var int: Int fileprivate var url: String? var model: Model<Hoge> { get { return Model(base: self) } set { self = newValue.base } } } extension Model where Base == Hoge { var int: Int { get { return base.int } set { base.int = newValue } } var url: URL? { get { return base.url.flatMap(URL.init) } set { base.url = newValue?.absoluteString } } } func x(hoge: Hoge) { hoge.model.int var hoge = hoge hoge.model.url = URL(string: "https://www.google.com") } いけた。
