NEW: My new book Pro SwiftUI is out now – level up your SwiftUI skills today! >>

Key decoding strategies for Codable

Available from Swift 4.1

Paul Hudson      @twostraws

In Swift 4.0 a common problem was trying to use Codable with JSON that utilized snake_case for its key names rather than the camelCase we normally use in Swift. Codable was unable to understand how the two different name types were mapped, so you had to create a custom CodingKeys enum helping it out.

This is where Swift 4.1's new keyDecodingStrategy property comes in: it’s set to .useDefaultKeys by default, which does a direct mapping of JSON names to Swift properties. However, if you change it to .convertFromSnakeCase then Codable handles the name conversion for us.

For example:

let decoder = JSONDecoder()
decoder.keyDecodingStrategy = .convertFromSnakeCase

do {
    let macs = try decoder.decode([Mac].self, from: jsonData)
    print(macs)
} catch {
    print(error.localizedDescription)
}

When you want to go back the other way – to convert a Codable struct with camelCase properties back to JSON with snake_case keys, set the keyEncodingStrategy to .convertToSnakeCase like this:

let encoder = JSONEncoder()
encoder.keyEncodingStrategy = .convertToSnakeCase
let encoded = try encoder.encode(someObject)

Note: At the time of writing keyDecodingStrategy and keyEncodingStrategy are not available on Linux.

Hacking with Swift is sponsored by Stream

SPONSORED Build Chat messaging quickly with Stream Chat. The Stream iOS Chat SDK is highly flexible, customizable, and crazy optimized for performance. Take advantage of this top-notch developer experience, get started for free today!

Click here

Sponsor Hacking with Swift and reach the world's largest Swift community!

Other changes in Swift 4.1…

Download all Swift 4.1 changes as a playground Link to Swift 4.1 changes

Browse changes in all Swift versions

 
Unknown user

You are not logged in

Log in or create account
 

Link copied to your pasteboard.