ios - Swift JSONDecoder-编码键不适用于下划线

有人能解释一下为什么A部分的代码是有效的，而B部分的代码不是。这让我很困惑。
工作

struct Coded : Codable, Hashable {
  public let avar1: String
  public let avar2: String

  enum CodingKeys: String, CodingKey {
    case avar1 = "avar1"
    case avar2 = "avar2"
  }

  init(from decoder: Decoder) throws {
    let container = try decoder.container(keyedBy: CodingKeys.self)
    print (container.allKeys)
    avar1 = try container.decode(String.self, forKey: .avar1)
    avar2 = try container.decode(String.self, forKey: .avar2)
  }
}

let JSONStr = """
{
  "avar1": "This is a string",
  "avar2": "This is a string2",
}
"""

if let jsdata = JSONStr.data(using: .utf8) {
  let decoder = JSONDecoder()
  decoder.keyDecodingStrategy = .convertFromSnakeCase
  let aobj: Coded? = try? decoder.decode(Coded.self, from: jsdata)
  print (aobj ?? "No object")
}

输出

[CodingKeys(stringValue: "avar1", intValue: nil), CodingKeys(stringValue: "avar2", intValue: nil)]
Coded(avar1: "This is a string", avar2: "This is a string2")

不工作

struct Coded : Codable, Hashable {
  public let avar1: String
  public let avar2: String

  enum CodingKeys: String, CodingKey {
    case avar1 = "avar1"
    case avar2 = "avar_2"
  }

  init(from decoder: Decoder) throws {
    let container = try decoder.container(keyedBy: CodingKeys.self)
    print (container.allKeys)
    avar1 = try container.decode(String.self, forKey: .avar1)
    avar2 = try container.decode(String.self, forKey: .avar2)
  }
}

let JSONStr = """
{
  "avar1": "This is a string",
  "avar_2": "This is a string2",
}
"""

if let jsdata = JSONStr.data(using: .utf8) {
  let decoder = JSONDecoder()
  decoder.keyDecodingStrategy = .convertFromSnakeCase
  let aobj: Coded? = try? decoder.decode(Coded.self, from: jsdata)
  print (aobj ?? "No object")
}

输出

[CodingKeys(stringValue: "avar1", intValue: nil)]
No object

第二个函数只显示不带下划线的编码键。但只要我去掉下划线，它就在所有键中都有编码键。。。
斯威夫特4.2-Xcode 10.2。
有什么想法吗？

最佳答案

.convertFromSnakeCase在访问CodingKeys之前，将snake\u case变量转换为camelCase。
如果要指定CodingKeys，则必须在不工作的示例中使用转换后的值

enum CodingKeys: String, CodingKey {
    case avar1 = "avar1"
    case avar2 = "avar2"
}

但这说明了CodingKeys的无意义。所以换个角度考虑，利用密钥解码策略。
而不是删除.convertFromSnakeCase删除CodingKeys和初始值设定项。
捕捉所有可能的Decoding错误。

struct Coded : Codable {
    public let avar1: String
    public let avar2: String
}

let jsonStr = """
{
"avar1": "This is a string",
"avar_2": "This is a string2",
}
"""

let jsdata = Data(jsonStr.utf8)
let decoder = JSONDecoder()
decoder.keyDecodingStrategy = .convertFromSnakeCase
do {
    let aobj = try decoder.decode(Coded.self, from: jsdata)
    print(aobj)
} catch { print(error) }