JSON and GVariant

Last week, my patch to provide GVariant integration into json-glib was merged. I filed a bug for it back in October, and after some patching iterations it finally got its way in.

This mean that now you can obtain a JSON node tree from a GVariant value and the opposite, with a single API call. I want to thank Emmanuele for reviewing my patches promptly (they were kind of lengthy), and for the positive feedback. The new API will be available in next release 0.14.

Why do we need that, anyway?

This integration seems quite natural if you give it a thought:

  • json-glib and GVariant are both general purpose data structure holders,
  • with serializing/deserializing capabilities
  • and both are GLib based.

My motivation to hack on that comes from my work in EventDance. I’m trying to bring D-Bus APIs into a Web page so that it is possible to connect to any server-side message bus from a page’s script and talk to the exported objects, export JavaScript objects to the bus so that they can be consumed by server applications, and own bus names. This feature and the rationale behind its usefulness goes beyond the reach of this post. I will devote another entry just to talk about that and show some cool demos.

So lets focus back on the matter: As I’m using the GIO’s GDBus API for the server-side, and GDBus uses GVariant for the data packing, I needed a way to convert JavaScript data structures to GVariant and vice-versa, so that the arguments in the JavaScript context can be constructed naturally using JS native objects and arrays, and passed seamlessly to the remote APIs without the need to worry about GVariant or whatever.

Also, GVariant stores data in a binary format and there are contexts unable to handle that (like JavaScript and other scripting languages). With this API, programmers can convert their GVariants to JSON, have them processed, and then converted back to GVariant.

How does the conversion work?

Well, as you probably know, GVariant features a data-type set that is quite richer than JSON. Thus, conversion from JSON to GVariant is an ambiguous operation. To solve this, I included an optional type signature as an argument in the API, so that when a JSON tree is converted to GVariant, the signature, if provided, is used to disambiguate the data types. This mean that if a signature is provided, the resulting GVariant is guaranteed to comply with it.

GVariant * json_gvariant_deserialize (JsonNode     *json_node,
                                                         const gchar  *signature,
                                                         GError         **error);

If the signature is not provided, the conversion can still be done, and a fixed mapping is used. For detailed information on how to use the new API, you can check my build of the json-glib documentation.

I hope other people find this useful.

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