如何扁平化嵌套JSON结构并解决Swift Decodable解码报错问题

需求 #

将指定嵌套JSON结构做扁平化处理，提取嵌套字段转换为目标结构。

待处理原始JSON #

{
    "items": [{
        "track": {
            "name": "Never Gonna Give You Up"
        }
    }]
}

期望输出结构 #

{
    "tracks": [{
        "name": "Never Gonna Give You Up"
    }]
}

原有实现与报错 #

尝试的代码 #

struct TrackResponse: Decodable {
  let tracks: [Track]

  enum CodingKeys: String, CodingKey {
    case tracks = "items"
  }

  enum TrackKeys: String, CodingKey {
    case track
  }

  init(from decoder: Decoder) throws {
    let outerContainer = try decoder.container(keyedBy: CodingKeys.self)
    let trackContainer = try outerContainer.nestedContainer(keyedBy: TrackKeys.self,
                                                            forKey: .tracks)

    self.tracks = try trackContainer.decode([Track].self, forKey: .track)
  }

  struct Track: Decodable {
    var name: String
  }
}

报错信息 #

Expected to decode Dictionary<String, Any> but found an array instead.

解决方案 #

错误根因 #

items字段对应的JSON类型是数组，而nestedContainer方法仅能作用于字典类型的容器，直接对数组调用该方法会触发类型不匹配错误。

正确实现 #

解码时先将items字段解析为数组类型的无键容器，遍历数组内的每个元素，逐个提取嵌套的track字段后拼装为最终的tracks数组即可，实现代码如下：

struct TrackResponse: Decodable {
    let tracks: [Track]

    enum CodingKeys: String, CodingKey {
        case items
    }

    enum ItemCodingKeys: String, CodingKey {
        case track
    }

    init(from decoder: Decoder) throws {
        let outerContainer = try decoder.container(keyedBy: CodingKeys.self)
        // 解析items为数组类型的无键容器
        var itemsContainer = try outerContainer.nestedUnkeyedContainer(forKey: .items)
        var tracksTemp = [Track]()
        // 遍历数组所有元素提取track
        while !itemsContainer.isAtEnd {
            let itemContainer = try itemsContainer.nestedContainer(keyedBy: ItemCodingKeys.self)
            let singleTrack = try itemContainer.decode(Track.self, forKey: .track)
            tracksTemp.append(singleTrack)
        }
        self.tracks = tracksTemp
    }

    struct Track: Decodable {
        var name: String
    }
}

内容的提问来源于stack exchange，提问作者Gabriel D M