使用可编码的值，有时是Int，有时是String

Question

我有一个API，它有时会将JSON中的特定键值（在本例中为＜code＞id＜/code＞）作为Int返回，而其他时候它会将相同的键值作为String返回。我如何使用codible解析JSON？

struct GeneralProduct: Codable {
    var price: Double!
    var id: String?
    var name: String!

    private enum CodingKeys: String, CodingKey {
        case price = "p"
        case id = "i"
        case name = "n"
    }

    init(price: Double? = nil, id: String? = nil, name: String? = nil) {
        self.price = price
        self.id = id
        self.name = name
    }
}

我不断收到此错误消息：预期解码字符串，但找到了一个数字。它返回数字的原因是 id 字段为空，当 id 字段为空时，它默认返回 0 作为可编码标识为数字的 ID。我基本上可以忽略 ID 键，但据我所知，可编码并没有给我忽略它的选项。处理这个问题的最佳方法是什么？

这是JSON。它超级简单

加工

{
  "p":2.12,
  "i":"3k3mkfnk3",
  "n":"Blue Shirt"
}

错误-因为系统中没有id，所以它返回0作为默认值，codable显然将其视为数字而不是字符串。

{
  "p":2.19,
  "i":0,
  "n":"Black Shirt"
}

Answer 1

将Int或String无缝解码为相同的属性需要编写一些代码。

但是，由于语言的（某种程度上）新添加（属性包装器），您可以在需要的任何地方轻松重用此逻辑：

// note this is only `Decodable`
struct GeneralProduct: Decodable {
    var price: Double
    @Flexible var id: Int // note this is an Int
    var name: String
}

属性包装器及其支持代码可以像这样实现：

@propertyWrapper struct Flexible<T: FlexibleDecodable>: Decodable {
    var wrappedValue: T
    
    init(from decoder: Decoder) throws {
        wrappedValue = try T(container: decoder.singleValueContainer())
    }
}

protocol FlexibleDecodable {
    init(container: SingleValueDecodingContainer) throws
}

extension Int: FlexibleDecodable {
    init(container: SingleValueDecodingContainer) throws {
        if let int = try? container.decode(Int.self) {
            self = int
        } else if let string = try? container.decode(String.self), let int = Int(string) {
            self = int
        } else {
            throw DecodingError.dataCorrupted(.init(codingPath: container.codingPath, debugDescription: "Invalid int value"))
        }
    }
}

您可以在知道如何从任何基本JSON数据类型解码的字符串上使用包装器：字符串、数字、布尔值：

struct RelaxedString: Codable {
    let value: String
    
    init(_ value: String) {
        self.value = value
    }
    
    init(from decoder: Decoder) throws {
        let container = try decoder.singleValueContainer()
        // attempt to decode from all JSON primitives
        if let str = try? container.decode(String.self) {
            value = str
        } else if let int = try? container.decode(Int.self) {
            value = int.description
        } else if let double = try? container.decode(Double.self) {
            value = double.description
        } else if let bool = try? container.decode(Bool.self) {
            value = bool.description
        } else {
            throw DecodingError.typeMismatch(String.self, .init(codingPath: decoder.codingPath, debugDescription: ""))
        }
    }
    
    func encode(to encoder: Encoder) throws {
        var container = encoder.singleValueContainer()
        try container.encode(value)
    }
}

然后可以在结构中使用此新类型。一个小缺点是结构的使用者需要进行另一个间接访问来访问包装的字符串。但是，可以通过将解码的＜code＞RelaxedString＜/code＞属性声明为private来避免这种情况，并为公共接口使用一个计算的属性：

struct GeneralProduct: Codable {
    var price: Double!
    var _id: RelaxedString?
    var name: String!
    
    var id: String? {
        get { _id?.value }
        set { _id = newValue.map(RelaxedString.init) }
    }

    private enum CodingKeys: String, CodingKey {
        case price = "p"
        case _id = "i"
        case name = "n"
    }

    init(price: Double? = nil, id: String? = nil, name: String? = nil) {
        self.price = price
        self._id = id.map(RelaxedString.init)
        self.name = name
    }
}

上述方法的优点：

1. 无需编写自定义 init（来自解码器：解码器）代码，如果要解码的属性数量增加，代码可能会变得乏味
2. 可重用性 - RelaxedString 可以无缝地用于其他结构
3. id可以从字符串或int解码的事实仍然是一个实现细节，GeneralProduct 的消费者不知道/关心 id 可以来自字符串或 int
4. 公共接口公开字符串值，这使使用者代码保持简单，因为它不必处理多种类型的数据

Answer 2

这是< code >元数据类型的一个可能的解决方案，好的一面是它不仅是< code >一般产品的通用解决方案，也是所有具有相同歧义的< code >结构的通用解决方案:

struct GeneralProduct: Codable {
  var price:Double?
  var id:MetadataType?
  var name:String?

  private enum CodingKeys: String, CodingKey {
    case price = "p"
    case id = "i"
    case name = "n"
  }

  init(price:Double? = nil, id: MetadataType? = nil, name: String? = nil) {
    self.price = price
    self.id = id
    self.name = name
  }
}

enum MetadataType: Codable {
  case int(Int)
  case string(String)

  init(from decoder: Decoder) throws {
    let container = try decoder.singleValueContainer()
    do {
      self = try .int(container.decode(Int.self))
    } catch DecodingError.typeMismatch {
      do {
        self = try .string(container.decode(String.self))
      } catch DecodingError.typeMismatch {
        throw DecodingError.typeMismatch(MetadataType.self, DecodingError.Context(codingPath: decoder.codingPath, debugDescription: "Encoded payload not of an expected type"))
      }
    }
  }

  func encode(to encoder: Encoder) throws {
    var container = encoder.singleValueContainer()
    switch self {
    case .int(let int):
      try container.encode(int)
    case .string(let string):
      try container.encode(string)
    }
  }
}

这是测试：

let decoder = JSONDecoder()
var json =  "{\"p\":2.19,\"i\":0,\"n\":\"Black Shirt\"}"
var product = try! decoder.decode(GeneralProduct.self, from: json.data(using: .utf8)!)
if let id = product.id {
  print(id) // 0
}

json =  "{\"p\":2.19,\"i\":\"hello world\",\"n\":\"Black Shirt\"}"
product = try! decoder.decode(GeneralProduct.self, from: json.data(using: .utf8)!)
if let id = product.id {
  print(id) // hello world
}

Answer 3

struct GeneralProduct: Codable {
    var price: Double?
    var id: String?
    var name: String?
    private enum CodingKeys: String, CodingKey {
        case price = "p", id = "i", name = "n"
    }
    init(price: Double? = nil, id: String? = nil, name: String? = nil) {
        self.price = price
        self.id = id
        self.name = name
    }
    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        price = try container.decode(Double.self, forKey: .price)
        name = try container.decode(String.self, forKey: .name)
        do {
            id = try String(container.decode(Int.self, forKey: .id))
        } catch DecodingError.typeMismatch {
            id = try container.decode(String.self, forKey: .id)
        }
    }
}

let json1 = """
{
"p":2.12,
"i":"3k3mkfnk3",
"n":"Blue Shirt"
}
"""

let json2 = """
{
"p":2.12,
"i":0,
"n":"Blue Shirt"
}
"""

do {
    let product = try JSONDecoder().decode(GeneralProduct.self, from: Data(json2.utf8))
    print(product.price ?? "nil")
    print(product.id ?? "nil")
    print(product.name ?? "nil")
} catch {
    print(error)
}

编辑/更新：

当api返回< code>0时，您也可以简单地将< code>nil赋给您的< code>id:

do {
    let value = try container.decode(Int.self, forKey: .id)
    id = value == 0 ? nil : String(value)
} catch DecodingError.typeMismatch {
    id = try container.decode(String.self, forKey: .id)
}