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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.

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