How to decode a dictionary into struct with key as a value in Swift

Question

I need to decode a dictionary like:

{
  "Bob": "London",
  "Alice": "Berlin"
  ...
}

Into an array of Person structs:

struct Person {
  let name: String
  let city: String
}

I would like to implement this mapping using Coding protocol, but struggling to use each key as a value for the struct.

Q: What about NSJsonSerialization ?

A: I know it's quite trivial with NSJsonSerialization , but implementing it with Decodable protocol was bugging me a bit :)

Answer 1

Some more thought reminded me I don't need AnyCodingKey in this case because this is such a simple structure. Decode as a [String: String] , and then make the mapping:

struct PersonList: Decodable {
    let persons: [Person]

init(from decoder: Decoder) throws {
    self.persons = try decoder.singleValueContainer()
        .decode([String: String].self)
        .map(Person.init)
    }
}

let list = try JSONDecoder().decode(PersonList.self, from: json).persons

// [Person(name: "Bob", city: "London"), Person(name: "Alice", city: "Berlin")]

---

Old answer, because sometimes this technique is handy.

The key tool for this is the oft-needed CodingKey (that really should be in stdlib), AnyCodingKey:

struct AnyCodingKey: CodingKey {
    var stringValue: String = ""
    init?(stringValue: String) { self.stringValue = stringValue }
    var intValue: Int?
    init?(intValue: Int) { self.intValue = intValue }
}

With that, you just need a container to hang the decoder on. Call it PersonList:

struct PersonList: Decodable {
    let persons: [Person]

    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: AnyCodingKey.self)
        self.persons = try container.allKeys.map { key in
            Person(name: key.stringValue,
                   city: try container.decode(String.self, forKey: key))
        }
    }
}

let list = try JSONDecoder().decode(PersonList.self, from: json).persons

// [Person(name: "Bob", city: "London"), Person(name: "Alice", city: "Berlin")]

This just maps each key/value to a Person.

Keep in mind that JSON objects are not ordered, so the resulting array may be in a different order than the JSON, as they are in this example. This is not fixable with Foundation (JSONSerialization or JSONDecoder); you'd have to use a different JSON parser if you needed that.

Answer 2

In this case I'd prefer traditional JSONSerialization , the returned dictionary can be easily mapped to an array of Person

let json = """
{
  "Bob": "London",
  "Alice": "Berlin"
}
"""

struct Person {
  let name: String
  let city: String
}

do {
    if let dataDictionary = try JSONSerialization.jsonObject(with: Data(json.utf8)) as? [String:String] {
        let people = dataDictionary.map{Person(name: $0.key, city: $0.value)}
        print(people)
    }
} catch {
    print(error)
}