Encodable

Encodable is a protocol that enables a type’s data to be encoded for use with Application Programming Interfaces (APIs).

Note: If data also needs to be decoded, it’s convenient to use the Codable protocol because it includes both the Encodable and Decodable protocols.

Syntax

In a basic case, for a type to conform to Encodable, all of its properties must also conform to Encodable:

struct myStruct: Encodable {
  // Properties that conform to Encodable
}

For the syntax above, all properties will be encoded.

Syntax using CodingKeys

When only specific properties need to be included, or some of the properties don’t conform to Encodable, then a custom specification for data can be used. The following syntax implements the protocol with a CodingKeys enum and encode(to:) function to specify which properties to encode:

struct myStruct: Encodable {
  // Properties

  private enum CodingKeys: String, CodingKey {
    // Enumeration cases for keys for the API data.
  }

  func encode(to encoder: Encoder) throws {
    // Encodes the struct using the custom coding keys
  }

}

Example

In the example below a Book struct is declared and an instance book is created. Its data is then converted to JSON:

import Foundation

struct Book: Encodable {
  var title: String
  var author: String
  var price: Int
}

let book = Book(title: "Four Quartets", author: "T. S. Elliot", price: 9)

print("\(book)")

let jsonEncoder = JSONEncoder()
let bookJson = try jsonEncoder.encode(book)
let bookJsonString = String(data: bookJson, encoding: .utf8)

print("\(bookJsonString!)")

This will output book as a Swift object and then as a JSON string:

Book(title: "Four Quartets", author: "T. S. Elliot", price: 9)
{"title":"Four Quartets","author":"T. S. Elliot","price":9}

Example using CodingKeys

In the example below a Book struct is declared with a computed property description, Codingkeys and encode(to:) to specify how to encode the data to JSON. An instance book is then created and the specified data converted to JSON.

import Foundation

struct Book: Encodable {
  var title: String
  var author: String
  var price: Int

  var description: String {
    return "\(title), by \(author)"
  }

  private enum CodingKeys: String, CodingKey {
    case description, price
  }

  func encode(to encoder: Encoder) throws {
    var container = encoder.container(keyedBy: CodingKeys.self)
    try container.encode(description, forKey: .description)
    try container.encode(price, forKey: .price)
  }
}

let book = Book(title: "Four Quartets", author: "T. S. Elliot", price: 9)

print("\(book)")

let jsonEncoder = JSONEncoder()
let bookJson = try jsonEncoder.encode(book)
let bookJsonString = String(data: bookJson, encoding: .utf8)

print("\(bookJsonString!)")

This will output book as a Swift object and then as a JSON string using only the specified keys:

Book(title: "Four Quartets", author: "T. S. Elliot", price: 9)
{"price":9,"description":"Four Quartets, by T. S. Elliot"}