Swift 4 Decodable - 以枚举为键的字典

Question

我的数据结构有一个枚举作为键，我希望下面的内容能够自动解码。这是一个错误还是一些配置问题？

import Foundation

enum AnEnum: String, Codable {
  case enumValue
}

struct AStruct: Codable {
  let dictionary: [AnEnum: String]
}

let jsonDict = ["dictionary": ["enumValue": "someString"]]
let data = try! JSONSerialization.data(withJSONObject: jsonDict,     options: .prettyPrinted)
let decoder = JSONDecoder()
do {
  try decoder.decode(AStruct.self, from: data)
} catch {
  print(error)
}

我得到的错误是这样的，似乎将字典与数组混淆了。

typeMismatch(Swift.Array, Swift.DecodingError.Context(codingPath: [可选(__lldb_expr_85.AStruct.(CodingKeys in _0E2FD0A9B523101D0DCD67578F72D1DD).dictionary)], debugDescription: "期望解码 Array 但找到了字典。"))

Answer 1

问题是Dictionary's Codable conformance 目前只能正确处理String 和Int 键。对于具有任何其他 Key 类型的字典（其中 Key 是 Encodable/Decodable），它使用具有交替键值的 unkeyed 容器（JSON 数组）进行编码和解码.

因此在尝试解码 JSON 时：

{"dictionary": {"enumValue": "someString"}}

进入AStruct，"dictionary" 键的值应该是一个数组。

所以，

let jsonDict = ["dictionary": ["enumValue", "someString"]]

会工作，产生 JSON：

{"dictionary": ["enumValue", "someString"]}

然后将其解码为：

AStruct(dictionary: [AnEnum.enumValue: "someString"])

但是，我真的认为Dictionary 的Codable 一致性应该能够正确处理任何符合CodingKey 的类型作为其Key（其中AnEnum 可以) – 因为它可以使用该密钥对密钥容器进行编码和解码（请随时 file a bug 请求）。

在实现之前（如果有的话），我们总是可以构建一个包装器类型来执行此操作：

struct CodableDictionary<Key : Hashable, Value : Codable> : Codable where Key : CodingKey {

    let decoded: [Key: Value]

    init(_ decoded: [Key: Value]) {
        self.decoded = decoded
    }

    init(from decoder: Decoder) throws {

        let container = try decoder.container(keyedBy: Key.self)

        decoded = Dictionary(uniqueKeysWithValues:
            try container.allKeys.lazy.map {
                (key: $0, value: try container.decode(Value.self, forKey: $0))
            }
        )
    }

    func encode(to encoder: Encoder) throws {

        var container = encoder.container(keyedBy: Key.self)

        for (key, value) in decoded {
            try container.encode(value, forKey: key)
        }
    }
}

然后像这样实现：

enum AnEnum : String, CodingKey {
    case enumValue
}

struct AStruct: Codable {

    let dictionary: [AnEnum: String]

    private enum CodingKeys : CodingKey {
        case dictionary
    }

    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        dictionary = try container.decode(CodableDictionary.self, forKey: .dictionary).decoded
    }

    func encode(to encoder: Encoder) throws {
        var container = encoder.container(keyedBy: CodingKeys.self)
        try container.encode(CodableDictionary(dictionary), forKey: .dictionary)
    }
}

（或者只拥有CodableDictionary<AnEnum, String> 类型的dictionary 属性并使用自动生成的Codable 一致性——那么就用dictionary.decoded 来说吧）

现在我们可以按预期解码嵌套的 JSON 对象了：

let data = """
{"dictionary": {"enumValue": "someString"}}
""".data(using: .utf8)!

let decoder = JSONDecoder()
do {
    let result = try decoder.decode(AStruct.self, from: data)
    print(result)
} catch {
    print(error)
}

// AStruct(dictionary: [AnEnum.enumValue: "someString"])

---

尽管说了这么多，但可以说，您使用带有 enum 作为键的字典所实现的只是带有可选属性的 struct（如果您希望给定的值始终是那里；使它成为非可选的）。

因此，您可能只希望您的模型看起来像：

struct BStruct : Codable {
    var enumValue: String?
}

struct AStruct: Codable {

    private enum CodingKeys : String, CodingKey {
        case bStruct = "dictionary"
    }

    let bStruct: BStruct
}

这对你当前的 JSON 工作得很好：

let data = """
{"dictionary": {"enumValue": "someString"}}
""".data(using: .utf8)!

let decoder = JSONDecoder()
do {
    let result = try decoder.decode(AStruct.self, from: data)
    print(result)
} catch {
    print(error)
}

// AStruct(bStruct: BStruct(enumValue: Optional("someString")))

Answer 2

为了解决您的问题，您可以使用以下两个 Playground 代码 sn-ps 之一。

---

#1。使用 Decodable 的 init(from:) 初始化器

import Foundation

enum AnEnum: String, Codable {
    case enumValue
}

struct AStruct {
    enum CodingKeys: String, CodingKey {
        case dictionary
    }
    enum EnumKeys: String, CodingKey {
        case enumValue
    }

    let dictionary: [AnEnum: String]
}

extension AStruct: Decodable {

    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        let dictContainer = try container.nestedContainer(keyedBy: EnumKeys.self, forKey: .dictionary)

        var dictionary = [AnEnum: String]()
        for enumKey in dictContainer.allKeys {
            guard let anEnum = AnEnum(rawValue: enumKey.rawValue) else {
                let context = DecodingError.Context(codingPath: [], debugDescription: "Could not parse json key to an AnEnum object")
                throw DecodingError.dataCorrupted(context)
            }
            let value = try dictContainer.decode(String.self, forKey: enumKey)
            dictionary[anEnum] = value
        }
        self.dictionary = dictionary
    }

}

用法：

let jsonString = """
{
  "dictionary" : {
    "enumValue" : "someString"
  }
}
"""

let data = jsonString.data(using: String.Encoding.utf8)!
let decoder = JSONDecoder()
let aStruct = try! decoder.decode(AStruct.self, from: data)
dump(aStruct)

/*
 prints:
 ▿ __lldb_expr_148.AStruct
   ▿ dictionary: 1 key/value pair
     ▿ (2 elements)
       - key: __lldb_expr_148.AnEnum.enumValue
       - value: "someString"
 */

---

#2。使用KeyedDecodingContainerProtocol的decode(_:forKey:)方法

import Foundation

public enum AnEnum: String, Codable {
    case enumValue
}

struct AStruct: Decodable {
    enum CodingKeys: String, CodingKey {
        case dictionary
    }

    let dictionary: [AnEnum: String]
}

public extension KeyedDecodingContainer  {

    public func decode(_ type: [AnEnum: String].Type, forKey key: Key) throws -> [AnEnum: String] {
        let stringDictionary = try self.decode([String: String].self, forKey: key)
        var dictionary = [AnEnum: String]()

        for (key, value) in stringDictionary {
            guard let anEnum = AnEnum(rawValue: key) else {
                let context = DecodingError.Context(codingPath: codingPath, debugDescription: "Could not parse json key to an AnEnum object")
                throw DecodingError.dataCorrupted(context)
            }
            dictionary[anEnum] = value
        }

        return dictionary
    }

}

用法：

let jsonString = """
{
  "dictionary" : {
    "enumValue" : "someString"
  }
}
"""

let data = jsonString.data(using: String.Encoding.utf8)!
let decoder = JSONDecoder()
let aStruct = try! decoder.decode(AStruct.self, from: data)
dump(aStruct)

/*
 prints:
 ▿ __lldb_expr_148.AStruct
   ▿ dictionary: 1 key/value pair
     ▿ (2 elements)
       - key: __lldb_expr_148.AnEnum.enumValue
       - value: "someString"
 */

Answer 3

根据 Imanou 的回答，变得超级通用。这将转换任何 RawRepresentable 枚举键控字典。可解码项目中不需要更多代码。

public extension KeyedDecodingContainer
{
    func decode<K, V, R>(_ type: [K:V].Type, forKey key: Key) throws -> [K:V]
        where K: RawRepresentable, K: Decodable, K.RawValue == R,
              V: Decodable,
              R: Decodable, R: Hashable
    {
        let rawDictionary = try self.decode([R: V].self, forKey: key)
        var dictionary = [K: V]()

        for (key, value) in rawDictionary {
            guard let enumKey = K(rawValue: key) else {
                throw DecodingError.dataCorrupted(DecodingError.Context(codingPath: codingPath,
                     debugDescription: "Could not parse json key \(key) to a \(K.self) enum"))
            }
            
            dictionary[enumKey] = value
        }

        return dictionary
    }
}