How do I use custom keys with Swift 4's Decodable protocol?

Question

Swift 4 introduced support for native JSON encoding and decoding via the Decodable protocol. How do I use custom keys for this?

E.g., say I have a struct

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Answer 1

With Swift 4.2, according to your needs, you may use one of the 3 following strategies in order to make your model objects custom property names match your JSON keys.

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#1. Using custom coding keys

When you declare a struct that conforms to Codable (Decodable and Encodable protocols) with the following implementation...

struct Address: Codable {
    var street: String
    var zip: String
    var city: String
    var state: String        
}

... the compiler automatically generates a nested enum that conforms to CodingKey protocol for you.

struct Address: Codable {
    var street: String
    var zip: String
    var city: String
    var state: String

    // compiler generated
    private enum CodingKeys: String, CodingKey {
        case street
        case zip
        case city
        case state
    }
}

Therefore, if the keys used in your serialized data format don't match the property names from your data type, you can manually implement this enum and set the appropriate rawValue for the required cases.

The following example shows how to do:

import Foundation

struct Address: Codable {
    var street: String
    var zip: String
    var city: String
    var state: String

    private enum CodingKeys: String, CodingKey {
        case street
        case zip = "zip_code"
        case city
        case state
    }
}

Encode (replacing zip property with "zip_code" JSON key):

let address = Address(street: "Apple Bay Street", zip: "94608", city: "Emeryville", state: "California")

let encoder = JSONEncoder()
if let jsonData = try? encoder.encode(address), let jsonString = String(data: jsonData, encoding: .utf8) {
    print(jsonString)
}

/*
 prints:
 {"state":"California","street":"Apple Bay Street","zip_code":"94608","city":"Emeryville"}
 */

Decode (replacing "zip_code" JSON key with zip property):

let jsonString = """
{"state":"California","street":"Apple Bay Street","zip_code":"94608","city":"Emeryville"}
"""

let decoder = JSONDecoder()
if let jsonData = jsonString.data(using: .utf8), let address = try? decoder.decode(Address.self, from: jsonData) {
    print(address)
}

/*
 prints:
 Address(street: "Apple Bay Street", zip: "94608", city: "Emeryville", state: "California")
 */

---

#2. Using snake case to camel case key coding strategies

If your JSON has snake-cased keys and you want to convert them to camel-cased properties for your model object, you can set your JSONEncoder's keyEncodingStrategy and JSONDecoder's keyDecodingStrategy properties to .convertToSnakeCase.

The following example shows how to do:

import Foundation

struct Address: Codable {
    var street: String
    var zipCode: String
    var cityName: String
    var state: String
}

Encode (converting camel cased properties into snake cased JSON keys):

let address = Address(street: "Apple Bay Street", zipCode: "94608", cityName: "Emeryville", state: "California")

let encoder = JSONEncoder()
encoder.keyEncodingStrategy = .convertToSnakeCase
if let jsonData = try? encoder.encode(address), let jsonString = String(data: jsonData, encoding: .utf8) {
    print(jsonString)
}

/*
 prints:
 {"state":"California","street":"Apple Bay Street","zip_code":"94608","city_name":"Emeryville"}
 */

Decode (converting snake cased JSON keys into camel cased properties):

let jsonString = """
{"state":"California","street":"Apple Bay Street","zip_code":"94608","city_name":"Emeryville"}
"""

let decoder = JSONDecoder()
decoder.keyDecodingStrategy = .convertFromSnakeCase
if let jsonData = jsonString.data(using: .utf8), let address = try? decoder.decode(Address.self, from: jsonData) {
    print(address)
}

/*
 prints:
 Address(street: "Apple Bay Street", zipCode: "94608", cityName: "Emeryville", state: "California")
 */

---

#3. Using custom key coding strategies

If necessary, JSONEncoder and JSONDecoder allow you to set a custom strategy to map coding keys using JSONEncoder.KeyEncodingStrategy.custom(_:) and JSONDecoder.KeyDecodingStrategy.custom(_:).

The following example shows how to implement them:

import Foundation

struct Address: Codable {
    var street: String
    var zip: String
    var city: String
    var state: String
}

struct AnyKey: CodingKey {
    var stringValue: String
    var intValue: Int?

    init?(stringValue: String) {
        self.stringValue = stringValue
    }

    init?(intValue: Int) {
        self.stringValue = String(intValue)
        self.intValue = intValue
    }
}

Encode (converting lowercased first letter properties into uppercased first letter JSON keys):

let address = Address(street: "Apple Bay Street", zip: "94608", city: "Emeryville", state: "California")

let encoder = JSONEncoder()
encoder.keyEncodingStrategy = .custom({ (keys) -> CodingKey in
    let lastKey = keys.last!
    guard lastKey.intValue == nil else { return lastKey }
    let stringValue = lastKey.stringValue.prefix(1).uppercased() + lastKey.stringValue.dropFirst()
    return AnyKey(stringValue: stringValue)!
})

if let jsonData = try? encoder.encode(address), let jsonString = String(data: jsonData, encoding: .utf8) {
    print(jsonString)
}

/*
 prints:
 {"Zip":"94608","Street":"Apple Bay Street","City":"Emeryville","State":"California"}
 */

Decode (converting uppercased first letter JSON keys into lowercased first letter properties):

let jsonString = """
{"State":"California","Street":"Apple Bay Street","Zip":"94608","City":"Emeryville"}
"""

let decoder = JSONDecoder()
decoder.keyDecodingStrategy = .custom({ (keys) -> CodingKey in
    let lastKey = keys.last!
    guard lastKey.intValue == nil else { return lastKey }
    let stringValue = lastKey.stringValue.prefix(1).lowercased() + lastKey.stringValue.dropFirst()
    return AnyKey(stringValue: stringValue)!
})

if let jsonData = jsonString.data(using: .utf8), let address = try? decoder.decode(Address.self, from: jsonData) {
    print(address)
}

/*
 prints:
 Address(street: "Apple Bay Street", zip: "94608", city: "Emeryville", state: "California")
 */

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Sources:

• Apple developer documentation: "Encoding and Decoding Custom Types"
• WWDC 2017 session 212: "What's new in Foundation"
• MartianCraft: "Implementing a custom key strategy for coding types"