ios - Swift: Codable - 提取单个编码 key

Question

我有以下代码来提取编码 key 中包含的 JSON:

let value = try! decoder.decode([String:Applmusic].self, from: $0["applmusic"])

这成功地处理了以下 JSON:

{
  "applmusic":{
    "code":"AAPL",
    "quality":"good",
    "line":"She told me don't worry",
}

但是，无法从以下代码中提取编码 key 为 applmusic 的 JSON:

{
  "applmusic":{
    "code":"AAPL",
    "quality":"good",
    "line":"She told me don't worry",
  },
  "spotify":{
    "differentcode":"SPOT",
    "music_quality":"good",
    "spotify_specific_code":"absent in apple"
  },
  "amazon":{
    "amzncode":"SPOT",
    "music_quality":"good",
    "stanley":"absent in apple"
  }
}

applmusic、spotify 和amazon 的数据模型不同。但是，我只需要提取 applmusic 并省略其他编码键。

我的 Swift 数据模型如下:

public struct Applmusic: Codable {
    public let code: String
    public let quality: String
    public let line: String
}

API 以完整的 JSON 响应，我不能要求它只给我所需的字段。

如何只解码json的特定部分？看来，Decodable 需要我先反序列化整个 json，所以我必须知道它的完整数据模型。

显然，其中一个解决方案是创建一个单独的 Response 模型来包含 applmusic 参数，但它看起来像一个 hack:

public struct Response: Codable {
    public struct Applmusic: Codable {
        public let code: String
        public let quality: String
        public let line: String
    }
    // The only parameter is `applmusic`, ignoring the other parts - works fine
    public let applmusic: Applmusic
}

您能提出一种更好的方法来处理此类 JSON 结构吗？

多一点洞察力

我在通用扩展中使用了以下技术，它会自动为我解码 API 响应。因此，我更愿意概括一种处理此类情况的方法，而无需创建 Root 结构。如果我需要的键是 JSON 结构中的 3 层深度怎么办？

这是为我解码的扩展:

extension Endpoint where Response: Swift.Decodable {
  convenience init(method: Method = .get,
                   path: Path,
                   codingKey: String? = nil,
                   parameters: Parameters? = nil) {
    self.init(method: method, path: path, parameters: parameters, codingKey: codingKey) {
      if let key = codingKey {
        guard let value = try decoder.decode([String:Response].self, from: $0)[key] else {
          throw RestClientError.valueNotFound(codingKey: key)
        }
        return value
      }

      return try decoder.decode(Response.self, from: $0)
    }
  }
}

API 定义如下:

extension API {
  static func getMusic() -> Endpoint<[Applmusic]> {
    return Endpoint(method: .get,
                    path: "/api/music",
                    codingKey: "applmusic")
  }
}

Answer 1

更新:我根据这个答案对 JSONDecoder 进行了扩展，您可以在这里查看:https://github.com/aunnnn/NestedDecodable ，它允许您使用关键路径解码任何深度的嵌套模型。

你可以这样使用它:

let post = try decoder.decode(Post.self, from: data, keyPath: "nested.post")

---

您可以制作一个Decodable 包装器(例如，此处的ModelResponse)，然后将所有逻辑放入其中，以提取带有键的嵌套模型:

struct DecodingHelper {

    /// Dynamic key
    private struct Key: CodingKey {
        let stringValue: String
        init?(stringValue: String) {
            self.stringValue = stringValue
            self.intValue = nil
        }

        let intValue: Int?
        init?(intValue: Int) {
            return nil
        }
    }

    /// Dummy model that handles model extracting logic from a key
    private struct ModelResponse<NestedModel: Decodable>: Decodable {
        let nested: NestedModel

        public init(from decoder: Decoder) throws {
            let key = Key(stringValue: decoder.userInfo[CodingUserInfoKey(rawValue: "my_model_key")!]! as! String)!
            let values = try decoder.container(keyedBy: Key.self)
            nested = try values.decode(NestedModel.self, forKey: key)
        }
    }

    static func decode<T: Decodable>(modelType: T.Type, fromKey key: String) throws -> T {
        // mock data, replace with network response
        let path = Bundle.main.path(forResource: "test", ofType: "json")!
        let data = try Data(contentsOf: URL(fileURLWithPath: path), options: .mappedIfSafe)

        let decoder = JSONDecoder()

        // ***Pass in our key through `userInfo`
        decoder.userInfo[CodingUserInfoKey(rawValue: "my_model_key")!] = key
        let model = try decoder.decode(ModelResponse<T>.self, from: data).nested
        return model
    }
}

您可以通过JSONDecoder ("my_model_key") 的userInfo 传递您想要的 key 。然后将其转换为 ModelResponse 中的动态 Key 以实际提取模型。

然后你可以像这样使用它:

let appl = try DecodingHelper.decode(modelType: Applmusic.self, fromKey: "applmusic")
let amazon = try DecodingHelper.decode(modelType: Amazon.self, fromKey: "amazon")
let spotify = try DecodingHelper.decode(modelType: Spotify.self, fromKey: "spotify")
print(appl, amazon, spotify)

完整代码: https://gist.github.com/aunnnn/2d6bb20b9dfab41189a2411247d04904

---

奖励:深度嵌套的键

经过更多尝试后，我发现您可以使用这个修改后的 ModelResponse 轻松解码任意深度的 key :

private struct ModelResponse<NestedModel: Decodable>: Decodable {
    let nested: NestedModel

    public init(from decoder: Decoder) throws {
        // Split nested paths with '.'
        var keyPaths = (decoder.userInfo[CodingUserInfoKey(rawValue: "my_model_key")!]! as! String).split(separator: ".")

        // Get last key to extract in the end
        let lastKey = String(keyPaths.popLast()!)

        // Loop getting container until reach final one
        var targetContainer = try decoder.container(keyedBy: Key.self)
        for k in keyPaths {
            let key = Key(stringValue: String(k))!
            targetContainer = try targetContainer.nestedContainer(keyedBy: Key.self, forKey: key)
        }
        nested = try targetContainer.decode(NestedModel.self, forKey: Key(stringValue: lastKey)!)
    }

然后你可以像这样使用它:

let deeplyNestedModel = try DecodingHelper.decode(modelType: Amazon.self, fromKey: "nest1.nest2.nest3")

来自这个 json:

{
    "apple": { ... },
    "amazon": {
        "amzncode": "SPOT",
        "music_quality": "good",
        "stanley": "absent in apple"
    },
    "nest1": {
        "nest2": {
            "amzncode": "Nest works",
            "music_quality": "Great",
            "stanley": "Oh yes",

            "nest3": {
                "amzncode": "Nest works, again!!!",
                "music_quality": "Great",
                "stanley": "Oh yes"
            }
        }
    }
}

完整代码:https://gist.github.com/aunnnn/9a6b4608ae49fe1594dbcabd9e607834