Swift Codable with Different Array Types - swift

Question

I'm writing a program where I'm parsing JSON data that includes array of arrays, where the nested arrays have different object types (specifically, [[String, String, Int]]). For example,
{
"number": 5295,
"bets": [
[
"16",
"83",
9
],
[
"75",
"99",
4
],
[
"46",
"27",
5
]
]
}
I'm trying to use codable to help me parse the data, but when I try something like
struct OrderBook: Codable {
let number: Int
let bets: [Bet]
}
struct Bet: Codable {
let price: String
let sale: String
let quantity: Int
}
it gives me errors saying that
Expected to decode Dictionary<String, Any> but found an array instead
How do I get around this? I can't declare an array of empty type.

Answer 1

One solution (assuming you can't change the JSON) is to implement custom decoding logic for Bet. You can use an unkeyed container (which reads from a JSON array) in order to decode each of the properties in turn (the order in which you call decode(_:) is the order they're expected to appear in the array).
import Foundation
struct OrderBook : Codable {
let number: Int
let bets: [Bet]
}
struct Bet : Codable {
let price: String
let sale: String
let quantity: Int
init(from decoder: Decoder) throws {
var container = try decoder.unkeyedContainer()
self.price = try container.decode(String.self)
self.sale = try container.decode(String.self)
self.quantity = try container.decode(Int.self)
}
// if you need encoding (if not, make Bet Decodable
// and remove this method)
func encode(to encoder: Encoder) throws {
var container = encoder.unkeyedContainer()
try container.encode(price)
try container.encode(sale)
try container.encode(quantity)
}
}
Example decoding:
let jsonString = """
{ "number": 5295, "bets": [["16","83",9], ["75","99",4], ["46","27",5]] }
"""
let jsonData = Data(jsonString.utf8)
do {
let decoded = try JSONDecoder().decode(OrderBook.self, from: jsonData)
print(decoded)
} catch {
print(error)
}
// OrderBook(number: 5295, bets: [
// Bet(price: "16", sale: "83", quantity: 9),
// Bet(price: "75", sale: "99", quantity: 4),
// Bet(price: "46", sale: "27", quantity: 5)
// ])