Swift - How to have multiple decoder method for a single model

Question

struct Classroom: Codable {
  let teacher: String
  let id: Int
  let status: Bool

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

Now I need a way to create Classroom instance with a simple String

{ "classroom": "Test" }//received from API

let classRoom = ClassRoom(teacher: "Test", id: 0, status: true)

Now I need to add a secondary decoder method which can create this classroom instance using the "classroom": "Test" data. The "Test" value should be used as value for "teacher" and other properties should contain default values.

I know I can decode the String value and create a new initializer. Is there a way to directly decode String to this model object?

Answer 1

if i understand well, i assume you have a bad json format like below

[
  {
     "teacher":"test",
     "id":5,
     "status":true
  },
  {
     "classroom":"Test"
  }
]

And you want to decode both objects, you can do the following

let data = """
[
  {
    "teacher": "test",
    "id": 5,
    "status": true
  },
  {
    "classroom": "Test"
  }
]
""".data(using: .utf8)!

struct Classroom: Codable {
    
  let teacher: String
  let id: Int
  let status: Bool
    
    private enum CodingKeys: CodingKey {
        case teacher, id, status
    }
    
    private enum SecCodingKeys: CodingKey {
        case classroom
    }
    
    init(from decoder: Decoder) throws {
        let value = try decoder.container(keyedBy: CodingKeys.self)
        let secValue = try decoder.container(keyedBy: SecCodingKeys.self)
        let teacher_1 = try value.decodeIfPresent(String.self, forKey: .teacher)
        let teacher_2 = try secValue.decodeIfPresent(String.self, forKey: .classroom)
        teacher = teacher_1 ?? teacher_2 ?? ""
        id = try value.decodeIfPresent(Int.self, forKey: .id) ?? 0
        status = try value.decodeIfPresent(Bool.self, forKey: .status) ?? false
    }
}
do {
    let rooms = try JSONDecoder().decode([Classroom].self, from: data)
    print(rooms.map(\.teacher))
} catch {
    print(error)
}

and the result,

["test", "Test"]