[swift-evolution] Support for a KeyCodingStrategy option in JSONEncoder and JSONDecoder

Tony Parker anthony.parker at apple.com
Thu Oct 19 10:09:34 CDT 2017

Hi Morten,

I’ve actually been working on this same idea already and will have something to propose soon.

- Tony

> On Oct 19, 2017, at 2:03 AM, Morten Bek Ditlevsen via swift-evolution <swift-evolution at swift.org> wrote:
> Hi all,
> At work we have just added Codable support for a whole bunch of model objects in our code base.
> Many places we have added CodingKeys enumeration in order to convert the camel cased property names to snake case for our JSON keys.
> As an experiment I have tried adding a KeyCodingStrategy option to a copy of the JSONEncoder and JSONDecoder implementations.
> This is currently an enumeration with the following values
> .original
> .snakeCase
> .custom((String) -> String)
> I just extended CodingKey as follows:
> extension CodingKey {
>     func stringValue(with encodingStrategy: StructuralEncoder.KeyEncodingStrategy) -> String {
>         switch encodingStrategy {
>         case .original:
>             return stringValue
>         case .snakeCase:
>             let pattern = "([a-z0-9])([A-Z])"
>             let regex = try! NSRegularExpression(pattern: pattern, options: [])
>             let range = NSRange(location: 0, length: stringValue.characters.count)
>             return regex.stringByReplacingMatches(in: stringValue, options: [], range: range, withTemplate: "$1_$2").lowercased()
>         case .custom(let t):
>             return t(stringValue)
>         }
>     }
> }
> and then I replaced all references to key.stringValue with key.stringValue(with: self.encoder.options.keyCodingStrategy)
> This seems to work very nicely.
> So my question is: Do anyone else see the benefit of such an addition to the JSONEncoder and JSONDecoder?
> The downside as I see it, is that the current CodingKeys are guaranteed to be unique by the compiler, and it would be possible to create collisions by using key name transforms.
> Is this downside bigger than the gains?
> One advantage is that one could argue that one CodingKey strategy may not fit all serialization mechanisms. For instance one might wish to have upper camel cased keys in Plists (just an example) and snake case in JSON. This method could easily support this, while the current CodingKeys strategy cannot...
> Looking forward to hearing feedback.
> Sincerely,
> /morten
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