Is it possible to decode additional parameters using JSONDecoder? - json

Question

We have some response returned by backend:
{
"name": "Some name",
"number": 42,
............
"param0": value0,
"param1": value1,
"param2": value2
}
Model structure for response:
struct Model: Codable {
let name: String
let number: Int
let params: [String: Any]
}
How to make JSONDecoder combine all unknown key-value pairs into params property?

Answer 1

Decodable is incredibly powerful. It can decode completely arbitrary JSON, so this is just a sub-set of that problem. For a fully worked-out JSON Decodable, see this JSON.
I'll pull the concept of Key from example, but for simplicity I'll assume that values must be either Int or String. You could make parameters be [String: JSON] and use my JSON decoder instead.
struct Model: Decodable {
let name: String
let number: Int
let params: [String: Any]
// An arbitrary-string Key, with a few "well known and required" keys
struct Key: CodingKey, Equatable {
static let name = Key("name")
static let number = Key("number")
static let knownKeys = [Key.name, .number]
static func ==(lhs: Key, rhs: Key) -> Bool {
return lhs.stringValue == rhs.stringValue
}
let stringValue: String
init(_ string: String) { self.stringValue = string }
init?(stringValue: String) { self.init(stringValue) }
var intValue: Int? { return nil }
init?(intValue: Int) { return nil }
}
init(from decoder: Decoder) throws {
let container = try decoder.container(keyedBy: Key.self)
// First decode what we know
name = try container.decode(String.self, forKey: .name)
number = try container.decode(Int.self, forKey:. number)
// Find all the "other" keys
let optionalKeys = container.allKeys
.filter { !Key.knownKeys.contains($0) }
// Walk through the keys and try to decode them in every legal way
// Throw an error if none of the decodes work. For this simple example
// I'm assuming it is a String or Int, but this is also solvable for
// arbitarily complex data (it's just more complicated)
// This code is uglier than it should be because of the `Any` result.
// It could be a lot nicer if parameters were a more restricted type
var p: [String: Any] = [:]
for key in optionalKeys {
if let stringValue = try? container.decode(String.self, forKey: key) {
p[key.stringValue] = stringValue
} else {
p[key.stringValue] = try container.decode(Int.self, forKey: key)
}
}
params = p
}
}
let json = Data("""
{
"name": "Some name",
"number": 42,
"param0": 1,
"param1": "2",
"param2": 3
}
""".utf8)
try JSONDecoder().decode(Model.self, from: json)
// Model(name: "Some name", number: 42, params: ["param0": 1, "param1": "2", "param2": 3])
ADDITIONAL THOUGHTS
I think the comments below are really important and future readers should look them over. I wanted to show how little code duplication is required, and how much of this can be easily extracted and reused, such that no magic or dynamic features are required.
First, extract the pieces that are common and reusable:
func additionalParameters<Key>(from container: KeyedDecodingContainer<Key>,
excludingKeys: [Key]) throws -> [String: Any]
where Key: CodingKey {
// Find all the "other" keys and convert them to Keys
let excludingKeyStrings = excludingKeys.map { $0.stringValue }
let optionalKeys = container.allKeys
.filter { !excludingKeyStrings.contains($0.stringValue)}
var p: [String: Any] = [:]
for key in optionalKeys {
if let stringValue = try? container.decode(String.self, forKey: key) {
p[key.stringValue] = stringValue
} else {
p[key.stringValue] = try container.decode(Int.self, forKey: key)
}
}
return p
}
struct StringKey: CodingKey {
let stringValue: String
init(_ string: String) { self.stringValue = string }
init?(stringValue: String) { self.init(stringValue) }
var intValue: Int? { return nil }
init?(intValue: Int) { return nil }
}
Now, the decoder for Model is reduced to this
struct Model: Decodable {
let name: String
let number: Int
let params: [String: Any]
init(from decoder: Decoder) throws {
let container = try decoder.container(keyedBy: StringKey.self)
name = try container.decode(String.self, forKey: StringKey("name"))
number = try container.decode(Int.self, forKey: StringKey("number"))
params = try additionalParameters(from: container,
excludingKeys: ["name", "number"].map(StringKey.init))
}
}
It would be nice if there were some magic way to say "please take care of these properties in the default way," but I don't quite know what that would look like frankly. The amount of code here is about the same as for implementing NSCoding, and much less than for implementing against NSJSONSerialization, and is easily handed to swiftgen if it were too tedious (it's basically the code you have to write for init). In exchange, we get full compile-time type checking, so we know it won't crash when we get something unexpected.
There are a few ways to make even the above a bit shorter (and I'm currently thinking about ideas involving KeyPaths to make it even more convenient). The point is that the current tools are very powerful and worth exploring.