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，即 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”，即您必须声明明确符合：

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

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

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 SE-0185 中的 Requesting synthesis is opt-in，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.

具有原始值的枚举，可编码

Enum with Raw value, Codable

enums

ios

swift

swift-protocols

codable