Swift Codable：解码具有相同根对象的不同项目数组

Question

我目前正在尝试解码如下所示的 JSON：

{
  "result": {
    "success": true,
    "items": [
      {
        "timeEntryID": "1",
        "start": "1519558200",
        "end": "1519563600",
        "customerName": "Test-Customer",
        "projectName": "Test-Project",
        "description": "Entry 1",
      },
      {
        "timeEntryID": "2",
        "start": "1519558200",
        "end": "1519563600",
        "customerName": "Test-Customer",
        "projectName": "Test-Project",
        "description": "Entry 2",
      }
    ],
    "total": "2"
  },
  "id": "1"
}

这种特定类型的 JSON 的解码过程非常简单。我只需要这样的东西：

struct ResponseKeys: Decodable {
    let result: ResultKeys

    struct ResultKeys: Decodable {
        let success: Bool
        let items: [Item]
    }
}

现在我面临的问题是服务器的每个响应都具有与上述 JSON 相同的结构，但具有不同的项目类型。所以有时它是let items: [Item]，但如果我调用用户端点，它也可能是let items: [User]。

因为如果我只需修改 items 数组就为每个端点编写上述 swift 代码将是不必要的代码重复，因此我创建了一个自定义解码器：

enum KimaiAPIResponseKeys: String, CodingKey {
    case result

    enum KimaiResultKeys: String, CodingKey {
        case success
        case items
    }
}

struct Activity: Codable {
    let id: Int
    let description: String?
    let customerName: String
    let projectName: String
    let startDateTime: Date
    let endDateTime: Date

    enum CodingKeys: String, CodingKey {
        case id = "timeEntryID"
        case description
        case customerName
        case projectName
        case startDateTime = "start"
        case endDateTime = "end"
    }
}

extension Activity {

    init(from decoder: Decoder) throws {
        let resultContainer = try decoder.container(keyedBy: KimaiAPIResponseKeys.self)
        let itemsContainer = try resultContainer.nestedContainer(keyedBy: KimaiAPIResponseKeys.KimaiResultKeys.self, forKey: .result)
        let activityContainer = try itemsContainer.nestedContainer(keyedBy: Activity.CodingKeys.self, forKey: .items)

        id = Int(try activityContainer.decode(String.self, forKey: .id))!
        description = try activityContainer.decodeIfPresent(String.self, forKey: .description)
        customerName = try activityContainer.decode(String.self, forKey: .customerName)
        projectName = try activityContainer.decode(String.self, forKey: .projectName)
        startDateTime = Date(timeIntervalSince1970: Double(try activityContainer.decode(String.self, forKey: .startDateTime))!)
        endDateTime = Date(timeIntervalSince1970: Double(try activityContainer.decode(String.self, forKey: .endDateTime))!)
    }

}

如果"items" 只包含单个对象而不包含数组，则解码器可以完美运行：

{
  "result": {
    "success": true,
    "items":
      {
        "timeEntryID": "2",
        "start": "1519558200",
        "end": "1519563600",
        "customerName": "Test-Customer",
        "projectName": "Test-Project",
        "description": "Entry 2",
      },
    "total": "2"
  },
  "id": "1"
}

如果items 是一个数组，我会收到以下错误：

typeMismatch(Swift.Dictionary, Swift.DecodingError.Context(codingPath: [__lldb_expr_151.KimaiAPIResponseKeys.result], debugDescription: "期望解码字典，但找到了一个数组。",底层错误：nil))

我只是不知道如何修改我的解码器以处理一组项目。我创建了一个包含工作和不工作版本的 JSON 的 Playground 文件。请看一下并尝试一下：Decodable.playground

感谢您的帮助！

Answer 1

我的建议是分别为items 解码字典

struct Item : Decodable {

    enum CodingKeys: String, CodingKey {
        case id = "timeEntryID"
        case description, customerName, projectName
        case startDateTime = "start"
        case endDateTime = "end"
    }

    let id: Int
    let startDateTime: Date
    let endDateTime: Date
    let customerName: String
    let projectName: String
    let description: String?

    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        id = Int(try container.decode(String.self, forKey: .id))!
        description = try container.decodeIfPresent(String.self, forKey: .description)
        customerName = try container.decode(String.self, forKey: .customerName)
        projectName = try container.decode(String.self, forKey: .projectName)
        startDateTime = Date(timeIntervalSince1970: Double(try container.decode(String.self, forKey: .startDateTime))!)
        endDateTime = Date(timeIntervalSince1970: Double(try container.decode(String.self, forKey: .endDateTime))!)
    }
}

在Activity 中使用条件初始化器，它提供了自己的do catch 块。首先，它尝试解码单个项目并将单个项目作为数组分配给属性。如果失败，它会解码一个数组。

enum KimaiAPIResponseKeys: String, CodingKey {
    case result, id

    enum KimaiResultKeys: String, CodingKey {
        case success
        case items
    }
}

struct Activity: Decodable {
    let id: String
    let items: [Item]
}

extension Activity {

    init(from decoder: Decoder) throws {
        let rootContainer = try decoder.container(keyedBy: KimaiAPIResponseKeys.self)
        id = try rootContainer.decode(String.self, forKey: .id)
        let resultContainer = try rootContainer.nestedContainer(keyedBy: KimaiAPIResponseKeys.KimaiResultKeys.self, forKey: .result)
        do {
            let item = try resultContainer.decode(Item.self, forKey: .items)
            items = [item]
        } catch {
            items = try resultContainer.decode([Item].self, forKey: .items)
        }
    }
} 

Answer 2

您可以使用泛型，这是处理这种情况的一种巧妙方法。

  struct MainClass<T: Codable>: Codable  {
     let result: Result<T>
     let id: String
  }

  struct Result <T: Codable>: Codable {
     let success: Bool
     let items: [T]
     let total: String
  }

在这里你会得到物品

   let data = Data()
   let decoder = JSONDecoder()
   let modelObjet = try! decoder.decode(MainClass<User>.self, from: data)
   let users = modelObjet.result.items

在我看来，泛型是处理这种情况下重复代码的最佳方式。