一个简单的解决方案是制作一个名为Extra的结构，该结构对于每种情况都有四个可选属性。

struct Extra: Codable {
    let contract: Contract?
    let message: Message?
    let feedback: Feedback?
    let campaign: Campaign?
}

只要消息，活动，合同和反馈中的每一个都有固定的响应，那么您就应该能够为它们构造符合Codable的结构。

这里的意思不是Any。如您所说，extra可以是“消息，广告系列，合同或反馈”。它不能是UIViewController或CBPeripheral。不是“任何东西”。这是四件事之一。

“ X件事之一”是一个枚举：

enum Extra {
    case contract(Contract)
    case campaign(Campaign)
    case message(Message)
    case feedback(Feedback)
}

要使其可解码，我们只需要寻找正确的键即可：

extension Extra: Decodable {
    enum CodingKeys: CodingKey {
        case contract,campaign,message,feedback
    }

    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)

        // Try to decode each thing it could be; throw if nothing matches.
        if let contract = try? container.decode(Contract.self,forKey: .contract) {
            self = .contract(contract)
        } else if let campaign = try? container.decode(Campaign.self,forKey: .campaign) {
            self = .campaign(campaign)
        } else if let message = try? container.decode(Message.self,forKey: .message) {
            self = .message(message)
        } else if let feedback = try? container.decode(Feedback.self,forKey: .feedback) {
            self = .feedback(feedback)
        } else {
            throw DecodingError.valueNotFound(Self.self,DecodingError.Context(codingPath: container.codingPath,debugDescription: "Could not find extra"))
        }
    }
}

在本文中，您可以找到一种支持带有编码的任何类型的方法，因此JSON返回String或Int无关紧要：

anycodableValue

它不是Any类型，您有四种不同但可预测的类型。

参考Rob的答案，如果您将if - else if解码为枚举并根据其值解码子类型，则可以摆脱notificationType链中的解码尝试

enum NotificationType : String,Decodable {
    case contract = "Contract",message = "Message",campaign = "Campaign",feedback = "Feedback"
}

enum Extra {
    case contract(Contract)
    case campaign(Campaign)
    case message(Message)
    case feedback(Feedback)
}

struct Alert: Decodable {
    let id: Int
    let isUnread: Bool
    let userId: Int
    let message: String
    let notificationType: NotificationType
    let extra: Extra
    
    private enum CodingKeys : String,CodingKey { case id,isUnread,userId,notificationType,extra}
    
    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        id = try container.decode(Int.self,forKey: .id)
        isUnread = try container.decode(Bool.self,forKey: .isUnread)
        userId = try container.decode(Int.self,forKey: .userId)
        message = try container.decode(String.self,forKey: .message)
        notificationType = try container.decode(NotificationType.self,forKey: .notificationType)
        switch notificationType {
            case .contract:
                let contractData = try container.decode([String:Contract].self,forKey: .extra)
                extra = .contract(contractData[notificationType.rawValue.lowercased()]!)
            case .campaign:
                let campaignData = try container.decode([String:Campaign].self,forKey: .extra)
                extra = .campaign(campaignData[notificationType.rawValue.lowercased()]!)
            case .message:
                let messageData = try container.decode([String:Message].self,forKey: .extra)
                extra = .message(messageData[notificationType.rawValue.lowercased()]!)
            case .feedback:
                let feedbackData = try container.decode([String:Feedback].self,forKey: .extra)
                extra = .feedback(feedbackData[notificationType.rawValue.lowercased()]!)
        }
    }
}