TitleDecodable.swift

// From https://stackoverflow.com/questions/54129682/use-swift-codable-to-decode-json-with-values-as-keys
import Foundation

let json = Data("""
{
  "7E7-M001" : {
    "Drawer1" : {
      "101" : {
        "Partnumber" : "F101"
      },
      "102" : {
        "Partnumber" : "F121"
      }
    },
    "Drawer2" : {
      "101" : {
        "Partnumber" : "F101"
      },
      "102" : {
        "Partnumber" : "F121"
      }
    }
  },
  "7E7-M002": {
    "Drawer1": {
      "201": {
        "Partnumber": "F201"
      },
      "202": {
        "Partnumber": "F221"
      }
    }
  }
}
""".utf8)

protocol TitleDecodable: Decodable {
    associatedtype Element: Decodable
    init(title: String, elements: [Element])
}

extension TitleDecodable {
    init(from decoder: Decoder) throws {
        self.init(title: try decoder.currentTitle(),
                  elements: try decoder.decodeTitledElements(Element.self))
    }
}

struct Tool: Decodable {
    let title: String
    let partNumber: String

    enum CodingKeys: String, CodingKey {
        case partNumber = "Partnumber"
    }

    init(from decoder: Decoder) throws {
        self.title = try decoder.currentTitle()
        let container = try decoder.container(keyedBy: CodingKeys.self)
        self.partNumber = try container.decode(String.self, forKey: .partNumber)
    }
}

struct Drawer: TitleDecodable {
    let title: String
    let tools: [Tool]
    init(title: String, elements: [Tool]) {
        self.title = title
        self.tools = elements
    }
}

struct Container: TitleDecodable {
    let title: String
    let drawers: [Drawer]

    init(title: String, elements: [Drawer]) {
        self.title = title
        self.drawers = elements
    }
}

struct TitleKey: CodingKey {
    let stringValue: String
    init?(stringValue: String) { self.stringValue = stringValue }
    var intValue: Int? { return nil }
    init?(intValue: Int) { return nil }
}

extension Decoder {
    func currentTitle() throws -> String {
        guard let titleKey = codingPath.last as? TitleKey else {
            throw DecodingError.dataCorrupted(.init(codingPath: codingPath,
                                                    debugDescription: "Not in titled container"))
        }
        return titleKey.stringValue
    }

    func decodeTitledElements<Element: Decodable>(_ type: Element.Type) throws -> [Element] {
        let titles = try container(keyedBy: TitleKey.self)
        return try titles.allKeys.map { title in
            return try titles.decode(Element.self, forKey: title)
        }
    }
}

struct Containers: Decodable {
    let containers: [Container]
    init(from decoder: Decoder) throws {
        self.containers = try decoder.decodeTitledElements(Container.self)
    }
}

let containers = try JSONDecoder().decode(Containers.self, from: json)
print(containers.containers)