Swift 4 可使用直到解码时间才知道的密钥进行解码

Question

Swift 4 Decodable 协议如何处理包含直到运行时才知道名称的键的字典？例如：

  [
    {
      "categoryName": "Trending",
      "Trending": [
        {
          "category": "Trending",
          "trailerPrice": "",
          "isFavourit": null,
          "isWatchlist": null
        }
      ]
    },
    {
      "categoryName": "Comedy",
      "Comedy": [
        {
          "category": "Comedy",
          "trailerPrice": "",
          "isFavourit": null,
          "isWatchlist": null
        }
      ]
    }
  ]

这里有一组字典；第一个有键 categoryName 和 Trending，而第二个有键 categoryName 和 Comedy。 categoryName 键的值告诉我第二个键的名称。我如何使用 Decodable 表达？

Answer 1

关键在于你如何定义CodingKeys属性。虽然它最常见的是 enum，但它可以是任何符合 CodingKey 协议的东西。要制作动态键，您可以调用静态函数：

struct Category: Decodable {
    struct Detail: Decodable {
        var category: String
        var trailerPrice: String
        var isFavorite: Bool?
        var isWatchlist: Bool?
    }

    var name: String
    var detail: Detail

    private struct CodingKeys: CodingKey {
        var intValue: Int?
        var stringValue: String

        init?(intValue: Int) { self.intValue = intValue; self.stringValue = "\(intValue)" }
        init?(stringValue: String) { self.stringValue = stringValue }

        static let name = CodingKeys.make(key: "categoryName")
        static func make(key: String) -> CodingKeys {
            return CodingKeys(stringValue: key)!
        }
    }

    init(from coder: Decoder) throws {
        let container = try coder.container(keyedBy: CodingKeys.self)
        self.name = try container.decode(String.self, forKey: .name)
        self.detail = try container.decode([Detail].self, forKey: .make(key: name)).first!
    }
}

用法：

let jsonData = """
  [
    {
      "categoryName": "Trending",
      "Trending": [
        {
          "category": "Trending",
          "trailerPrice": "",
          "isFavourite": null,
          "isWatchlist": null
        }
      ]
    },
    {
      "categoryName": "Comedy",
      "Comedy": [
        {
          "category": "Comedy",
          "trailerPrice": "",
          "isFavourite": null,
          "isWatchlist": null
        }
      ]
    }
  ]
""".data(using: .utf8)!

let categories = try! JSONDecoder().decode([Category].self, from: jsonData)

（我将 JSON 中的 isFavourit 更改为 isFavourite，因为我认为这是一个拼写错误。如果不是这种情况，修改代码很容易）

Answer 2

您可以编写一个用作 CodingKeys 对象的自定义结构，并使用字符串对其进行初始化，以便它提取您指定的键：

private struct CK : CodingKey {
    var stringValue: String
    init?(stringValue: String) {
        self.stringValue = stringValue
    }
    var intValue: Int?
    init?(intValue: Int) {
        return nil
    }
}

因此，一旦您知道所需的键是什么，您就可以说（在 init(from:) 覆盖中：

let key = // whatever the key name turns out to be
let con2 = try! decoder.container(keyedBy: CK.self)
self.unknown = try! con2.decode([Inner].self, forKey: CK(stringValue:key)!)

所以我最后做的是从解码器中制作两个容器——一个使用标准 CodingKeys 枚举来提取 "categoryName" 键的值，另一个使用 CK 结构提取我们刚刚知道其名称的键的值：

init(from decoder: Decoder) throws {
    let con = try! decoder.container(keyedBy: CodingKeys.self)
    self.categoryName = try! con.decode(String.self, forKey:.categoryName)
    let key = self.categoryName
    let con2 = try! decoder.container(keyedBy: CK.self)
    self.unknown = try! con2.decode([Inner].self, forKey: CK(stringValue:key)!)
}

那么，这是我的整个 Decodable 结构：

struct ResponseData : Codable {
    let categoryName : String
    let unknown : [Inner]
    struct Inner : Codable {
        let category : String
        let trailerPrice : String
        let isFavourit : String?
        let isWatchList : String?
    }
    private enum CodingKeys : String, CodingKey {
        case categoryName
    }
    private struct CK : CodingKey {
        var stringValue: String
        init?(stringValue: String) {
            self.stringValue = stringValue
        }
        var intValue: Int?
        init?(intValue: Int) {
            return nil
        }
    }
    init(from decoder: Decoder) throws {
        let con = try! decoder.container(keyedBy: CodingKeys.self)
        self.categoryName = try! con.decode(String.self, forKey:.categoryName)
        let key = self.categoryName
        let con2 = try! decoder.container(keyedBy: CK.self)
        self.unknown = try! con2.decode([Inner].self, forKey: CK(stringValue:key)!)
    }
}

这是测试台：

    let json = """
      [
        {
          "categoryName": "Trending",
          "Trending": [
            {
              "category": "Trending",
              "trailerPrice": "",
              "isFavourit": null,
              "isWatchlist": null
            }
          ]
        },
        {
          "categoryName": "Comedy",
          "Comedy": [
            {
              "category": "Comedy",
              "trailerPrice": "",
              "isFavourit": null,
              "isWatchlist": null
            }
          ]
        }
      ]
    """
    let myjson = try! JSONDecoder().decode(
        [ResponseData].self, 
        from: json.data(using: .utf8)!)
    print(myjson)

这里是 print 语句的输出，证明我们已经正确地填充了我们的结构：

[JustPlaying.ResponseData(
    categoryName: "Trending", 
    unknown: [JustPlaying.ResponseData.Inner(
        category: "Trending", 
        trailerPrice: "", 
        isFavourit: nil, 
        isWatchList: nil)]), 
 JustPlaying.ResponseData(
    categoryName: "Comedy", 
    unknown: [JustPlaying.ResponseData.Inner(
        category: "Comedy", 
        trailerPrice: "", 
        isFavourit: nil, 
        isWatchList: nil)])
]

当然在现实生活中我们会有一些错误处理，毫无疑问！

EDIT 后来我意识到（部分感谢 CodeDifferent 的回答）我不需要两个容器；我可以消除 CodingKeys 枚举，而我的 CK 结构可以完成所有工作！它是一个通用密钥制造商：

init(from decoder: Decoder) throws {
    let con = try! decoder.container(keyedBy: CK.self)
    self.categoryName = try! con.decode(String.self, forKey:CK(stringValue:"categoryName")!)
    let key = self.categoryName
    self.unknown = try! con.decode([Inner].self, forKey: CK(stringValue:key)!)
}

Answer 3

这是我最终想到的 json：

let json = """
{
    "BTC_BCN":{
        "last":"0.00000057",
        "percentChange":"0.03636363",
        "baseVolume":"47.08463318"
    },
    "BTC_BELA":{
        "last":"0.00001281",
        "percentChange":"0.07376362",
        "baseVolume":"5.46595029"
    }
}
""".data(using: .utf8)!

我们做这样的结构：

struct Pair {
    let name: String
    let details: Details

    struct Details: Codable {
        let last, percentChange, baseVolume: String
    }
}

然后解码：

if let pairsDictionary = try? JSONDecoder().decode([String: Pair.Details].self, from: json) {
    
    var pairs: [Pair] = []
    for (name, details) in pairsDictionary {
        let pair = Pair(name: name, details: details)
        pairs.append(pair)
    }
    
    print(pairs)
}

也可以不调用 pair.details.baseVolume，而是调用 pair.baseVolume:

struct Pair {
    ......
    var baseVolume: String { return details.baseVolume }
    ......

或编写自定义初始化：

struct Pair {
    .....
    let baseVolume: String
    init(name: String, details: Details) {
         self.baseVolume = details.baseVolume
    ......

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