json - 带有嵌套 json 作为字符串的 Swift Codable

Question

我用于后端调用的服务返回所有这些 json 结构:

{
    "status" : "OK",
    "payload" : **something**
}

something 可以是一个简单的字符串:

{
    "status" : "OK",
    "payload" : "nothing changed"
}

或嵌套的 json(具有任何属性的任何 json)，例如:

{
    "status" : "OK",
    "payload" : {
                    "someInt" : 2,
                    "someString" : "hi",
                    ...
                }
}

这是我的结构:

struct GenericResponseModel: Codable {

    let status:String?
    let payload:String?
}

我想始终将“payload”解码为字符串。所以在第二种情况下，我希望我的“GenericResponseModel”的有效负载属性包含该字段的 json 字符串，但是如果我尝试解码该响应，我会收到错误消息:

Type 'String' mismatch: Expected to decode String but found a dictionary instead

可以存档我想要的吗？

非常感谢

Answer 1

这个怎么样……

声明一个PayloadType协议(protocol)...

protocol PayloadType: Decodable { }

并使String和struct Payload符合它……

extension String: PayloadType { }

struct Payload: Decodable, PayloadType {
    let someInt: Int
    let someString: String
}

然后使 GenericResponseModel 通用...

struct GenericResponseModel<T: PayloadType>: Decodable {

    let status: String
    let payload: T

    enum CodingKeys: CodingKey {
        case status, payload
    }

    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        status = try container.decode(String.self, forKey: .status)            
        payload = try container.decode(T.self, forKey: .payload)
    }
}

然后你可以解码如下...

let data = """
{
"status" : "OK",
"payload" : "nothing changed"
}
""".data(using: .utf8)!

print(try JSONDecoder().decode(GenericResponseModel<String>.self, from: data))

// GenericResponseModel<String>(status: "OK", payload: "nothing changed")

和

let data2 = """
{
"status" : "OK",
"payload" : {
"someInt" : 2,
"someString" : "hi"
}
}
""".data(using: .utf8)!

print(try JSONDecoder().decode(GenericResponseModel<Payload>.self, from: data2))

// GenericResponseModel<Payload>(status: "OK", payload: Payload(someInt: 2, someString: "hi"))

当然，这依赖于你提前知道payload类型。如果负载类型错误，您可以通过抛出特定错误来解决这个问题……

enum GenericResponseModelError: Error {
    case wrongPayloadType
}

然后……

init(from decoder: Decoder) throws {
    let container = try decoder.container(keyedBy: CodingKeys.self)
    status = try container.decode(String.self, forKey: .status)

    do {
        payload = try container.decode(T.self, forKey: .payload)
    } catch {
        throw GenericResponseModelError.wrongPayloadType
    }
}

然后在你解码的时候处理这​​个错误...

let data = """
{
"status" : "OK",
"payload" : {
"someInt" : 2,
"someString" : "hi"
}
}
""".data(using: .utf8)!

do {
    let response = try JSONDecoder().decode(GenericResponseModel<String>.self, from: data) // Throws
    print(response) 
} catch let error as GenericResponseModelError where error == .wrongPayloadType {
    let response = try JSONDecoder().decode(GenericResponseModel<Payload>.self, from: data2) // Success!
    print(response)
}