问：

使用 iOS 13+ 的 Swift，我需要解码 JSON 响应。它包含一个嵌套对象，该对象需要父对象的值（）才能解码。type

JSON 结构：

{
    "name":"My email",
    "type":"Email",
    "content":{
        "issued":"2023-08-25T12:58:39Z",
        "attributes":{
            "email":"[email protected]"
        }
    }
}

或

{
    "name":"My telephone",
    "type":"Telephone",
    "content":{
        "issued":"2023-08-25T12:58:39Z",
        "attributes":{
            "telephone":"+33123456789"
        }
    }
}

attributes内容取决于 。所以嵌套对象需要知道才能解码。typecontenttypeattributes

结构：

struct Response: Decodable {
    let name: String
    let type: String
    let content: ContentResponse?

    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        name = try container.decode(String.self, forKey: .name)
        type = try container.decode(String.self, forKey: .type)
        // !!! Here I need to pass the "type" value to content decoding
        content = try container.decodeIfPresent(ContentResponse.self, forKey: .content)
    }
}

struct ContentResponse: Decodable {
    let issued: Date
    let attributes: AttributesResponse?

    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        issued = try container.decode(Date.self, forKey: .issued)
        if container.contains(.attributes) {
            // !!! Here I can't access the "type" value from parent object
            switch Type.fromString(type: type) {
            case .telephone:
                attributes = try container.decode(AttributesTelephoneResponse.self, forKey: .attributes)
            case .email:
                attributes = try container.decode(AttributesEmailResponse.self, forKey: .attributes)
            default:
                // Unsupported type 
                throw DecodingError.dataCorruptedError(forKey: .attributes, in: contentContainer, debugDescription: "Type \"\(type)\" not supported for attributes")
            }
        } else {
            attributes = nil
        }
    }
}

class DocumentResponse: Decodable {}

class AttributesEmailResponse: DocumentResponse {
    let email: String
}

class AttributesTelephoneResponse: DocumentResponse {
    let telephone: String
}

正如你所看到的，需要知道 才能知道使用哪个类进行解码。init(from decoder: Decoder)ContentResponsetypeattributes

如何将解码传递给嵌套对象进行解码？typeResponseContentResponse

不工作 #1

我在这里找到了一个 https://www.andyibanez.com/posts/the-mysterious-codablewithconfiguration-protocol/ 使用的解决方案，但它针对的是 iOS 15+，而我的目标是 iOS 13+。CodableWithConfigurationdecodeIfPresent(_:forKey:configuration:)

不工作 #2

我本可以使用 in 传递 ，但它是只读的：userInfodecoderinit(from decoder: Decoder)type

var userInfo: [CodingUserInfoKey : Any] { get }

溶液

感谢 @Joakim Danielson 的回答。

解决方案是在以下方面进行整个解码：Response

struct Response: Decodable {
    let name: String
    let type: String
    let content: ContentResponse?

    enum CodingKeys: String, CodingKey {
        case name
        case type
        case content
    }

    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        name = try container.decode(String.self, forKey: .name)
        type = try container.decode(String.self, forKey: .type)
        // Content
        if container.contains(.content) {
            let issued = try contentContainer.decode(Date.self, forKey: .issued)
            let attributes: DocumentResponse?
            switch DocumentType.fromString(type: type) {
            case .telephone:
                let value = try contentContainer.decode(DocumentTelephoneResponse.self, forKey: .attributes)
                attributes = .telephone(value)
            case .email:
                let value = try contentContainer.decode(DocumentEmailResponse.self, forKey: .attributes)
                attributes = .email(value)
            default:
                throw DecodingError.dataCorruptedError(forKey: .attributes, in: contentContainer, debugDescription: "Type \"\(type)\" not supported for attributes")
            }
            content = ContentResponse(issued: issued, expires: expires, attributes: attributes, controls: controls)
        } else {
            content = nil
        }
    }
}

跟：

public enum DocumentType: String, Codable {
    case telephone = "Telephone"
    case email = "Email"

    static public func fromString(type: String) -> Type? {
        Type(rawValue: type) ?? nil
    }
}

struct ContentResponse: Decodable {
    let issued: Date
    let attributes: DocumentResponse?

    enum CodingKeys: String, CodingKey {
        case issued
        case attributes
    }
}

enum DocumentResponse: Decodable {
    case email(DocumentEmailResponse)
    case telephone(DocumentTelephoneResponse)
}