[SR-7992] JSONEncoder should allow custom key ordering behavior

[subdigital]

Previous ID	SR-7992
Radar	rdar://problem/41127529
Original Reporter	@subdigital
Type	Improvement

Additional Detail from JIRA

Votes	4
Component/s	Foundation
Labels	Improvement
Assignee	@itaiferber
Priority	Medium

md5: b325e74ffe4c72ee5ea685cb2c3ee15b

Issue Description:

I am working with a server that requires a certain key to be present first in an object ("for performance reasons" they said)...

Unfortunately, implementing `encode(to🙂` manually and encoding in a specific order does not end up having any effect on the key order of the resulting document.

Discussions about key order not being a valid JSON concern aside (which I agree with) it would be nice to have functionality for determining the order of keys by providing a custom function, similar to how we handle custom date formatting logic.

I'd love to have something like this:

let encoder = JSONEncoder()
encoder.outputFormatting = .customKeyOrdering { keys in
  // return a new array of sorted keys however I need

}

(And if Radar is a better place for this I'd be happy to submit it there instead.)

[belkadan]

cc @itaiferber

[itaiferber]

Although this sounds like a relatively simple request, I think in practice there would be quite a bit of complication here due to the fact that JSONEncoder currently produces JSON by going through JSONSerialization. JSONEncoder collects the entire encoded tree (storing dictionaries as {{NSDictionary}}s, which don't allow for ordering constraints like this) and passes that off to JSONSerialization; to support this, we'd need JSONSerialization to be able to make use of this sorting strategy and call back into Swift (potentially through an Objective-C shim). JSONSerialization also only gets dictionaries with string keys, and we'd have to map those back to CodingKeys to call the strategy; not impossible, but likely lossy.

I'm not sure how generally applicable this feature would be—as you say yourself, this isn't really a valid requirement on the JSON—but something we could definitely consider. This would be easier if JSONEncoder started using its own serializer (which we've considered doing), but that's not certain.

[itaiferber]

@swift-ci create

[subdigital]

Ah, that's a good explanation. I had assumed that because lexographic key sorting is offered that this was something that might be easy to hook into.

[itaiferber]

@subdigital Yeah, that was significantly easier to do because we introduced a similar option on JSONSerialization and simply toggle that option in JSONEncoder with no additional work needed (JSONSerialization does the lexicographic sorting on its own there). Calling back into JSONEncoder from JSONSerialization would be more difficult though.

[swift-ci]

Comment by Damian Mehers (JIRA)

I've just hit this issue, converting .NET code to Swift.

I'm talking to an Amazon service over which I have no control, and it gives an error if the keys are not ordered as they specify.

I lost many many hours because of this - most of which were caused by me not believing that the service could require a specific field ordering ... but it does ... is there any recommended approach if ordering is required to be preserved, other than rolling my own encoder?