ios - 具有原始值的枚举，可编码

Question

以下代码无法编译:

enum Occupation: String {
  case designer = "Designer"
  case engineer = "Engineer"
}

public struct SteveJobs: Codable {
  let name: String
  let occupation: Occupation
}

另一方面，它应该编译，因为 Occupation 表示为 String，它是 Codable。

为什么我不能在 Codable 结构中使用带有原始值的 enum？

特别是为什么自动一致性在这种情况下不起作用。

Answer 1

自动 Codable 合成是“选择加入”的，即您必须声明 明确的一致性:

enum Occupation: String, Codable { // <--- HERE
    case designer = "Designer"
    case engineer = "Engineer"
}

public struct SteveJobs: Codable {
    let name: String
    let occupation: Occupation
}

参见 SE-0166 Swift Archival & Serialization

By adopting these protocols, user types opt in to this system.

自动Hashable和Equatable合成也是如此， 比较 Requesting synthesis is opt-in在 SE-0185 中，其中 列出了一些原因:

• The syntax for opting in is natural; there is no clear analogue in Swift today for having a type opt out of a feature.

• It requires users to make a conscious decision about the public API surfaced by their types. Types cannot accidentally "fall into" conformances that the user does not wish them to; a type that does not initially support Equatable can be made to at a later date, but the reverse is a breaking change.

• The conformances supported by a type can be clearly seen by examining its source code; nothing is hidden from the user.

• We reduce the work done by the compiler and the amount of code generated by not synthesizing conformances that are not desired and not used.