How to set/copy Struct property while decoding JSON content - ios

Question

I have one struct to decode the json content:
struct CustomResponse: Decodable {
var network: String
var weight: Int = 0
var pureWeight: Int
enum CodingKeys: String, CodingKey {
case network = "network"
case weight = "weight"
}}
I'd like to set the initial weight value which is parsed from JSON to pureWeight property. Even if weight is changed later, pureWeight should be the one coming from JSON response.
Is there any easy way to set pureWeight one time (after parsing) without using manually decode all the json content?
init(from decoder: Decoder) throws {
let container = try decoder.container(keyedBy: CodingKeys.self)
...parsing all items one by one.
}
I used this code but obviously it always returned the mutated values of weight:
var pureWeight: Int {
get {
return weight
}
}

Answer 1

You can approach like following
Keeping the init:
import Foundation
let json = """
{
"network" : "My Awesome network",
"weight" : 100
}
"""
struct CustomResponse: Decodable {
var network: String
var pureWeight: Int
// Computed
var weight: Int {
get {
return hiddenWeight ?? pureWeight
}
set {
hiddenWeight = newValue
}
}
// Hidden from the outside world
private var hiddenWeight: Int?
enum CodingKeys: String, CodingKey {
case network
case weight
}
init(from decoder: Decoder) throws {
let container = try decoder.container(keyedBy: CodingKeys.self)
network = try container.decode(String.self, forKey: .network)
pureWeight = try container.decode(Int.self, forKey: .weight)
}
}
let jsonDecoder = JSONDecoder()
if let data = json.data(using: .utf8),
var response = try? jsonDecoder.decode(CustomResponse.self, from: data) {
print(response)
response.weight = 200
print(response)
}
So what is done here is, maintaining a private property, hidden from the outside world. Initially it's been set to nil. While parsed from JSON, it's nil.
While getting weight property we get hidden property hiddenWeight or weight in case hiddenWeight is nil.
When weight is set to something, then hiddenWeight is set.
So from next time, whenever weight is asked for then hiddenWeight is returned.
Without using init:
If you want to omit the init implementation, then you have to use any other property name except weight. I'm using myWeight.
import Foundation
let json = """
{
"network" : "My Awesome network",
"weight" : 100
}
"""
struct CustomResponse: Decodable {
var network: String
var weight: Int
// Computed
var myWeight: Int {
get {
return hiddenWeight ?? weight
}
set {
hiddenWeight = newValue
}
}
// Hidden from the outside world
private var hiddenWeight: Int?
}
let jsonDecoder = JSONDecoder()
if let data = json.data(using: .utf8),
var response = try? jsonDecoder.decode(CustomResponse.self, from: data) {
print(response)
response.myWeight = 200
print(response)
}

Answer 2

You still need a custom init but the easiest way to do it then is to let declare the initial weight property since it should not change and set it in the init
struct CustomResponse: Decodable {
var network: String
var weight: Int
let pureWeight: Int
enum CodingKeys: String, CodingKey {
case network = "network"
case weight = "weight"
}
init(from decoder: Decoder) throws {
let container = try decoder.container(keyedBy: CodingKeys.self)
network = try container.decode(String.self, forKey: .network)
weight = try container.decode(Int.self, forKey: .weight)
pureWeight = weight
}
}