嵌套的Swift 4 Codable Struct不规则地使用CodingKeys

Question

我有一些JSON，我正在尝试使用符合Codable协议的Swift结构进行解码。主结构似乎不想识别thread_type的codingKey别名，但很乐意使用明确命名的thread_id属性。有问题的结构如下：

struct Message: Codable {
var id: String?
var type: String?
var thread_id: String?
var threadType: String?
var sender: User?
var body: String?
var media: String?
var sentAt: Double?
var unread: Bool?
var status: String?
var url: String?

enum codingKeys: String, CodingKey {
    case id
    case type
    case thread_id
    case threadType = "thread_type"
    case sender
    case body
    case media
    case sentAt = "sent_at"
    case unread
    case status
    case url
}
}

我正在尝试解析的JSON：

let json =
"""
{
"id": "Jvbl6LY",
"type": "sms",
"thread_id": "60LrVL7",
"thread_type": "578a",
"delay_until": null,
"sender": {
"id": "EVkdNBx",
"type": "user",
"first_name": "Jerry",
"last_name": "Ward",
"mobile_number": "123-456-7890",
"profile_image_url": "",
"is_online": false,
"email": "jerryw+demo@jypsee.com"
},
"body": "Here is a picture of our Coquille Suite.  Let me know if you would like a reservation?",
"media": "",
"sent_at": 1509133604000.167,
"unread": false,
"status": "",
"url": "https://connect-staging.jypsee.com/api/sms/threads/60LrVL7/history/Jvbl6LY/"
}
"""

最后解码器调用自己：

let decoder = JSONDecoder()
let data = json.data(using: .utf8)!
do {
    let message = try decoder.decode(Message.self, from: data)
    print(message.thread_id)
    print(message.threadType)
    print(message.sender?.firstName)
    print(message.sender?.lastName)
} catch {
    print(error)
}

message.thread_id打印出预期的Optional("60LrVL7")\n"。 message.threadType打印nil\n，这是不期望的。更奇怪的是，message.sender？.firstName和message.sender？.lastName分别打印"Optional("Jerry")\n"和"Optional("Ward")\n"。这意味着嵌套的User Codable Struct CodingKey IS正在工作。我真的不知道为什么解码会出现这种不一致的原因。

Xcode Playground Gist is available here

Answer 1

Apple doc在下面的段落的第一句中明确指出。 必须使用名为“CodingKeys”的特殊枚举。 （我遇到了类似的问题，花了很长时间才发现）。

https://developer.apple.com/documentation/foundation/archives_and_serialization/encoding_and_decoding_custom_types