Swift Codable support for objects represented as array - swift

Question

I'm trying to encode and decode data from an API that represent an object as an array of strings, for instance:
[
["username", "message", "date"],
["username", "message", "date"],
["username", "message", "date"]
]
This is the corresponding Codable struct:
struct Message: Codable {
let user: String
let content: String
let date: String
private enum CodingKeys: Int, CodingKey {
case user = 0
case content = 1
case date = 2
}
}
Neither encoding or decoding work; encoding shows that a JSON object is created instead of an array:
let msg = Message(user: "foo", content: "content", date: "2019-06-04")
let jsonData = try! JSONEncoder().encode(msg)
let jsonString = String(data: jsonData, encoding: .utf8)!
The final string is:
{"content":"content","user":"foo","date":"2019-06-04"}
My goal is to obtain the following string
["foo", "content", "2019-06-04"]
Using a custom encode/decode method in the struct solves this, but forces to create a lot of boilerplate for each struct/class.
init(from decoder: Decoder) throws {
let container = try decoder.singleValueContainer()
let values = try container.decode([String].self)
user = values[CodingKeys.user.rawValue]
content = values[CodingKeys.content.rawValue]
date = values[CodingKeys.date.rawValue]
}
How would one proceed to support this for any object?
And yes, this is a weird API, but this is not the first time I encounter one of those and user a different API format is not what I'm looking for here.

Answer 1

Rather than singleValueContainer use unkeyedContainer, it's more robust. If you want to assign the array items to struct members you have to write a custom initializer anyway
struct Message: Codable {
let user: String
let content: String
let date: String
init(from decoder: Decoder) throws {
var arrayContainer = try decoder.unkeyedContainer()
guard arrayContainer.count == 3 else { throw DecodingError.dataCorruptedError(in: arrayContainer, debugDescription: "The array must contain three items") }
user = try arrayContainer.decode(String.self)
content = try arrayContainer.decode(String.self)
date = try arrayContainer.decode(String.self)
}
func encode(to encoder: Encoder) throws {
var arrayContainer = encoder.unkeyedContainer()
try arrayContainer.encode(contentsOf: [user, content, date])
}
}