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Codable synthesis for enums with associated values

Available from Swift 5.5

Paul Hudson      @twostraws

SE-0295 upgrades Swift’s Codable system to support writing enums with associated values. Previously enums were only supported if they conformed to RawRepresentable, but this extends support to general enums as well as enum cases with any number of Codable associated values.

For example, we could define a Weather enum like this one:

enum Weather: Codable {
    case sun
    case wind(speed: Int)
    case rain(amount: Int, chance: Int)

That has one simple case, one case with a single associated values, and a third case with two associated values – all are integers, but you could use strings or other Codable types.

With that enum defined, we can create an array of weather to make a forecast, then use JSONEncoder or similar and convert the result to a printable string:

import Foundation

let forecast: [Weather] = [
    .wind(speed: 10),
    .rain(amount: 5, chance: 50)

do {
    let result = try JSONEncoder().encode(forecast)
    let jsonString = String(decoding: result, as: UTF8.self)
} catch {
    print("Encoding error: \(error.localizedDescription)")

Behind the scenes, this is implemented using multiple CodingKey enums capable of handling the nested structure that results from having values attached to enum cases, which means writing your own custom coding methods to do the same is a little more work.

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