Codable with references?

[itaiferber]

There was some discussion of this in Codable != Archivable - #5 by itaiferber, and specifically:

@itaiferber

You can consider this a bug. Although not explicitly called out in the Codable proposals, we planned on supporting and maintaining reference semantics both through our own encoders ( JSONEncoder , PropertyListEncoder ) and through NSKeyedArchiver support. However, we didn’t manage to finish this aspect of the feature in time, and have not yet had the bandwidth to follow through, unfortunately. I’m planning on incorporating this explicitly in the next update to the Codable feature.

Not all formats support reference semantics, and one of the things we planned was a way to express whether a given format/encoder supports them or not. JSON, for instance, does not support references natively, so we'd have to add an additional encoding layer on top of JSON to support it.

This is something that's planned, but we didn't have time to push it through in the Swift 4 timeframe.

In the meantime, there are two main approaches you can take to cover your needs:

1. If you really want to keep synthesized conformances and make support for this automatic for your format, you can implement an encoder/decoder pair which does the work to implement references. The specifics of how this is done or represented depends on the format you're looking to support; you can look at the implementation of JSONEncoder and JSONDecoder as a basis for how to get started — there isn't an easy template at the moment, but this is something we're also looking into potentially supporting
2. If you are willing to give up synthesized conformances in order to avoid the work needed to implement an encoder/decoder, you can add your own compatibility layer here — you can create a ReferenceTable type (or similar) that you give reference types to; it can give each value (by identity) an identifier which you encode in place of the object. After you've encoded all the UIDs, the last step can be to encode the ReferenceTable itself, which encodes the actual objects you have, once. On decode, you decode the ReferenceTable first, then any object UID types you find can go through the reference table to give you objects back out

Approach #2 is what JSONEncoder/JSONDecoder would essentially do on your behalf, but unfortunately, we haven't gotten there yet. If you want to do that yourself at the moment, you'll likely need buy-in from your types, which isn't great.

[If you're interested in following either of these approaches, I can give more concrete information about how to get started/what the process might look like.]