为什么 Int 枚举作为字典键，产生与 Int 作为字典键不同的 json 字符串？

Question

我尝试使用 Int 枚举转换字典

enum TypeE: Int, Codable
{
    case note = 1
    case tab
}

let encoder = JSONEncoder()

let dictionary0 = [TypeE.note:"VALUE0", TypeE.tab:"VALUE1"]
var data = try encoder.encode(dictionary0)
var string = String(data: data, encoding: .utf8)!
// [1,"VALUE0",2,"VALUE1"]
print(string)

生成的json字符串输出是

[1,"VALUE0",2,"VALUE1"]

对我来说看起来很奇怪。因为，生成的 json 字符串表示一个数组。

---

如果我测试

let encoder = JSONEncoder()

let dictionary1 = [1:"VALUE0", 2:"VALUE1"]
var data = try encoder.encode(dictionary1)
var string = String(data: data, encoding: .utf8)!
// {"1":"VALUE0","2":"VALUE1"}
print(string)

生成的json字符串输出是

{"1":"VALUE0","2":"VALUE1"}

似乎如果我使用 Int 枚举作为字典键，生成的 json 字符串将成为数组的表示？

我的代码中是否有任何错误，或者我的期望不正确？

Answer 1

Codable的源代码对此行为提供了解释。如果键不是 aString或 an Int，则结果类型是 an Array：

  public func encode(to encoder: Encoder) throws {
    if Key.self == String.self {
      // Since the keys are already Strings, we can use them as keys directly.
      ...
    } else if Key.self == Int.self {
      // Since the keys are already Ints, we can use them as keys directly.
      ...
    } else {
      // Keys are Encodable but not Strings or Ints, so we cannot arbitrarily
      // convert to keys. We can encode as an array of alternating key-value
      // pairs, though.
      var container = encoder.unkeyedContainer()
      for (key, value) in self {
        try container.encode(key)
        try container.encode(value)
      }
    }
  }