用于从Swift中的多个服务解码JSON的简单方法

Question

前提

我有一个符合struct的Decodable，因此它可以通过init(from:)从各种响应中解码JSON。对于我希望解码的每种JSON响应，我都有一个符合enum的CodingKey。

示例

这是一个简化的示例，可以放在Swift游乐场中：

import Foundation

// MARK: - Services -

struct Service1 {}
struct Service2 {}

// MARK: - Person Model -

struct Person {
    let name: String
}

extension Person: Decodable {
    enum CodingKeys: String, CodingKey {
        case name = "name"
    }

    enum Service2CodingKeys: String, CodingKey {
        case name = "person_name"
    }

    // And so on through service n...

    init(from decoder: Decoder) throws {
        switch decoder.userInfo[.service] {
        case is Service1.Type:
            let container = try decoder.container(keyedBy: CodingKeys.self)
            name = try container.decode(String.self, forKey: .name)
        case is Service2.Type:
            let container = try decoder.container(keyedBy: Service2CodingKeys.self)
            name = try container.decode(String.self, forKey: .name)
        // And so on through service n...
        default:
            fatalError("Missing implementation for service.")
        }
    }
}

// MARK: - CodingUserInfoKey -

extension CodingUserInfoKey {
    static let service = CodingUserInfoKey(rawValue: "service")!
}

// MARK: - Responses -

// The JSON response from service 1.
let service1JSONResponse = """
[
    {
        "name": "Peter",
    }
]
""".data(using: .utf8)!

// The JSON response from service 2.
let service2JSONResponse = """
[
    {
        "person_name": "Paul",
    }
]
""".data(using: .utf8)!

// And so on through service n... where other services have JSON responses with keys of varied names ("full_name", "personName").

// MARK: - Decoding -

let decoder = JSONDecoder()

decoder.userInfo[.service] = Service1.self
let service1Persons = try decoder.decode([Person].self, from: service1JSONResponse)

decoder.userInfo[.service] = Service2.self
let service2Persons = try decoder.decode([Person].self, from: service2JSONResponse)

问题

我遇到的问题是，我需要解码响应所需要的许多不同服务，并且该模型比该简化示例具有更多的属性。随着服务数量的增加，解码这些响应所需的案例数量也随之增加。

问题

如何简化我的init(from:)实现以减少所有这些代码重复？

尝试

我尝试为每个服务存储正确的CodingKey.Type并将其传递给container(keyedBy:)，但出现此错误：

无法使用类型为（（keyedBy：CodingKey.Type）'的参数列表来调用'container'。

init(from decoder: Decoder) throws {
    let codingKeyType: CodingKey.Type

    switch decoder.userInfo[.service] {
    case is Service1.Type: codingKeyType = CodingKeys.self
    case is Service2.Type: codingKeyType = Service2CodingKeys.self
    default: fatalError("Missing implementation for service.")
    }

    let container = try decoder.container(keyedBy: codingKeyType) // ← Error
    name = try container.decode(String.self, forKey: .name)
}

Answer 1

而不是尝试使用CodingKeys和日益复杂的init解决此问题，我建议通过协议进行组合：

protocol PersonLoader: Decodable {
    var name: String { get }
    // additional properties
}

extension Person {
    init(loader: PersonLoader) {
        self.name = loader.name
        // additional properties, but this is one-time
    }
}

或者，特别是如果Person是只读的简单数据对象，则可以使Person成为协议，然后可以避免执行此额外的复制步骤。

然后您可以分别定义每个服务的接口：

struct Service1Person: PersonLoader {
    let name: String
}

struct Service2Person: PersonLoader {
    let person_name: String

    var name: String { person_name }
}

然后在完成后映射到Persons：

let service2Persons = try decoder.decode([Service2Person].self,
                                         from: service2JSONResponse)
    .map(Person.init)

如果使用仅协议的方法，则看起来像这样：

protocol Person: Decodable {
    var name: String { get }
    // additional properties
}

struct Service1Person: Person {
    let name: String
}

struct Service2Person: Person {
    var name: String { person_name }
    let person_name: String
}

let service2Personsx = try decoder.decode([Service2Person].self,
                                         from: service2JSONResponse) as [Person]

Answer 2

Person的init（from :)中没有一堆自定义的每项服务（或每项服务类型）功能，我认为这很难做到。您无法将符合自定义CodingKey的枚举传递给decoder.container(keyedBy:)，因为它在该枚举的类型上是通用的。

您可以通过使用自定义密钥解码策略并通过字典或自定义密钥解码方法/闭包中的函数执行映射的一种方法。

在下面的示例中，我使用枚举来表示服务。映射字典是在枚举情况下键入的，因此反映了服务/服务类型的键映射。希望它可以为您更复杂的实际用例提供有用的路线图。

import Foundation

// MARK: - Custom Key Decoding -

struct MyCodingKey: CodingKey {
    var stringValue: String
    var intValue: Int?

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

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

// MARK: - Services -

enum Services: String {
    case service1
    case service2
}

extension Services {

    var mapping: [String:String] {
        switch self {
        case .service1: return [:]
        case .service2: return ["person_name": "name"]
        }
    }

    func getPersons(jsonData: Data) throws -> [Person] {
        let decoder = JSONDecoder()

        decoder.keyDecodingStrategy = .custom { (keys: [CodingKey]) -> CodingKey in
            let lastKey = keys.last!
            guard lastKey.intValue == nil else {
                return MyCodingKey(intValue: lastKey.intValue!)!
            }
            guard let stringValue = self.mapping[lastKey.stringValue] else {
                return lastKey
            }

            return MyCodingKey(stringValue: stringValue)!
        }

        let persons = try decoder.decode([Person].self, from: jsonData)
        return persons
    }
}

// MARK: - Person Model -

struct Person: Decodable {
    let name: String
}

// MARK: - Responses -

// The JSON response from service 1.
let service1JSONResponse = """
[ {  "name": "Peter", } ]
""".data(using: .utf8)!

// The JSON response from service 2.
let service2JSONResponse = """
[ { "person_name": "Paul", } ]
""".data(using: .utf8)!

// MARK: - Sample Calls -

print((try? Services.service1.getPersons(jsonData: service1JSONResponse))!)
print((try? Services.service2.getPersons(jsonData: service2JSONResponse))!)