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How to convert between camel case and snake case with Codable and keyEncodingStrategy

Swift version: 5.1

Paul Hudson    @twostraws   

Most Swift developers use camel case for naming properties, which means we start with a lowercase letter then capitalize the first letter of second and subsequent words: numberOfUsers or powerLevel. On the other hand, many web APIs prefer snake case, written as number_of_users and power_level, so if you need to convert camel case to snake case you need to use the keyEncodingStrategy of JSONEncoder, like this:

let encoder = JSONEncoder()
encoder.keyEncodingStrategy = .convertToSnakeCase

if let encoded = try? encoder.encode(yourData) {
    // continue with encoded data
}

With that set, Swift can convert property names as part of the encoding process, which makes Codable much easier to use.

A similar property exists on JSONDecoder, except now it’s called keyDecodingStrategy. It does the same thing in reverse: specifying .convertFromSnakeCase will convert names_like_this into namesLikeThis.

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Available from iOS 8.0

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