在 Swift 中动态解码任意 json 字段

Question

TL;DR

有没有一种方法可以让我使用JSONDecoder 并编写一个函数，该函数将从给定的 json 给定的指定可解码类型的字段值中读出？

---

成像我有以下json：

{
   "product":{
      "name":"PR1",
      "price":20
   },
   "employee":{
      "lastName":"Smith",
      "department":"IT",
      "manager":"Anderson"
   }
}

我有 2 个Decodable 结构：

struct Product: Decodable {
    var name: String
    var price: Int
}

struct Employee: Decodable {
    var lastName: String
    var department: String
    var manager: String
}

我想写一个函数

func getValue<T:Decodable>(from json: Data, field: String) -> T { ... }

所以我可以这样称呼它：

let product: Product = getValue(from: myJson, field: "product")
let employee: Employee = getValue(from: myJson, field: "employee")

这可能与JSONDecoder 或我应该与JSONSerialization 混淆，首先读出给定json 的“子树”，然后将其传递给解码器？ swift 中似乎不允许在泛型函数中定义结构。

Answer 1

Decodable 假定您在设计时知道启用静态类型所需的一切。你想要的越有活力，你就必须变得越有创造力。在这种情况下，定义通用编码键结构非常方便：

/// A structure that holds no fixed key but can generate dynamic keys at run time
struct GenericCodingKeys: CodingKey {
    var stringValue: String
    var intValue: Int?

    init?(stringValue: String) { self.stringValue = stringValue }
    init?(intValue: Int) { self.intValue = intValue; self.stringValue = "\(intValue)" }
    static func makeKey(_ stringValue: String) -> GenericCodingKeys { return self.init(stringValue: stringValue)! }
    static func makeKey(_ intValue: Int) -> GenericCodingKeys { return self.init(intValue: intValue)! }
}

/// A structure that retains just the decoder object so we can decode dynamically later
fileprivate struct JSONHelper: Decodable {
    let decoder: Decoder

    init(from decoder: Decoder) throws {
        self.decoder = decoder
    }
}

func getValue<T: Decodable>(from json: Data, field: String) throws -> T {
    let helper = try JSONDecoder().decode(JSONHelper.self, from: json)
    let container = try helper.decoder.container(keyedBy: GenericCodingKeys.self)
    return try container.decode(T.self, forKey: .makeKey(field))
}

let product: Product = try getValue(from: json, field: "product")
let employee: Employee = try getValue(from: json, field: "employee")

Answer 2

我首先要说Code Different's answer 是一个可行且很好的答案，但是如果您寻求一种不同的方式来做这件事，尽管在表面下工作方式大致相同，我有一个替代解决方案，使用主要组件代码不同的答案，导致下面的代码。主要区别之一是，一个JSONDecoder 在同一个JSON 上被重用，对于你提取的每个struct，使用这个。

我也会推荐这些：

• How to use Any in Codable Type

• Swift 4 Codable; How to decode object with single root-level key

---

/// Conforming to this protocol, makes the type decodable using the JSONContainer class
/// You can use `Decodable` instead.
protocol JSONContainerCodable: Codable {

    /// Returns the name that the type is recognized with, in the JSON.
    /// This is overridable in types conforming to the protocol.
    static var containerIdentifier: String { get }

    /// Defines whether or not the type's container identifier is lowercased.
    /// Defaults to `true`
    static var isLowerCased: Bool { get }
}

extension JSONContainerCodable {

    static var containerIdentifier: String {
        let identifier = String(describing: self)
        return !isLowerCased ? identifier : identifier.lowercased()
    }

    static var isLowerCased: Bool {
        return true
    }
}

struct Product: JSONContainerCodable {

    var name:  String
    var price: Int
}

struct Employee: JSONContainerCodable {

    var lastName:   String
    var department: String
    var manager:    String
}

/// This class is simply a wrapper around JSONDecoder
class JSONContainerDecoder: Decodable {

    private struct AnyCodingKeys: CodingKey {

        var stringValue: String
        var intValue: Int?

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

        init?(stringValue: String) {
            self.stringValue = stringValue
        }

        init(_ string: String) {
            stringValue = string
        }
    }

    private let decoder: JSONDecoder
    private let container: KeyedDecodingContainer<AnyCodingKeys>

    /// Overrides the initializer as specified in `Decodable`.
    required init(from decoder: Decoder) throws {
        self.decoder = JSONDecoder()
        self.container = try decoder.container(keyedBy: AnyCodingKeys.self)
    }

    /// Factory initializer. Swift (4.2) currently doesn't support overriding the parentheses operator.
    static func decoding(_ data: Data, with decoder: JSONDecoder = JSONDecoder()) throws -> JSONContainerDecoder {
        return try decoder.decode(JSONContainerDecoder.self, from: myJSON)
    }

    /// Gets the given type from the JSON, based on its field/container identifier, and decodes it. Assumes there exists only one type with the given field/container identifier, in the JSON.
    func get<T: JSONContainerCodable>(_ type: T.Type, field: String? = nil) throws -> T {
        return try container.decode(T.self, forKey: AnyCodingKeys(field ?? T.containerIdentifier))
    }

    /// Short version of the decode getter above; assumes the variable written to already has its type defined.
    func get<T: JSONContainerCodable>(field: String? = nil) throws -> T {
        return try get(T.self, field: field)
    }
}

---

let myJSON = """
{
    "product": {
        "name": "PR1",
        "price": 20
    },
    "employee": {
        "lastName": "Smith",
        "department": "IT",
        "manager": "Anderson"
    }
}
""".data(using: .utf8)!

let container = try! JSONContainer.decoding(myJSON)

print(try! container.get( Product.self))
print(try! container.get(Employee.self))

---

Product(name: "PR1", price: 20)
Employee(lastName: "Smith", department: "IT", manager: "Anderson")