Re: GVariant for prez!

Hi David

Thanks for your reply.

David Zeuthen wrote:
 - GVariant brings in a completely new type system

That's not entirely true. The DBus type system is being used and loved by many people already and GVariant is using that type system. GVariant merely brings this type system into glib.

 - The API looks very similar to DBusMessage and DBusMessageIter

I'd disagree here for two main reasons:

1) It's possible to remove GVariants from their parents. ie: you can take one argument of a DBus method call (or one item of an array) and pass it around as a separate value. This is not possible with DBusMessage.

For applications like dconf, this is critically important. It allows using the same type system that is used to implement the message bus to pass data on your API without unpacking/repacking between two separate value container formats (say: deserialising from a DBusMessage to GConfValue type of thing).

On the flip side of this, it is also possible to construct more complex GVariants out of GVariants that are given to you by someone else (ie: no code for serialising a GConfValue into a DBusMessage along with other stuff).

2) The varargs construction/deconstruction vastly simplifies construction of complex value types when using C. Consider how much code you need in DBus to create a structure containing a string and a varaint (say, containing an integer).


open struct subiter
  add string
  open variant subiter
    add integer
  close variant subiter
close struct subiter


g_variant_new ("(sv)", "foo", g_variant_new_int (123));

While I can see the general usefulness of having a DBusMessage-ish API
being used outside libdbus let us focus on what it is we need (or a
least what I think we need)

[I don't address most of these because they are not directly related to GVariant, although they could easily be potential users of it].

 - Some kind of replacement for GConf. I understand your proposal is
   not dconf per se, it's GSettings, right? Do you expect people to
   use the GVariant API in GSettings?

It's possible, but it would be unusual. One of the nice features of GVariant varargs support is that you can use it in your library to build APIs that are similar to g_object_get/set(). In the usual case, you would make a call that looks something like:

  // assuming "key" is boolean
  g_settings_set (settings, "key", TRUE);

which would use GVariant internally to collect the varargs list of arguments before calling a lowel level function like g_settings_set_value() which actually takes a GVariant *.

This is vaguely similar to how gobject can collect into a GValue before passing that to a call like g_object_set_property ().

Basically my impression is that GVariant is a lot like DBusMessage and
frankly, for application writers, I'm not sure that is good API (while
for bindings / low-level stuff it is fine). Then again, I'm known to
favor type-safety / generated code which some people regard as an
extremist view. And I haven't tried GVariant so in some way I'm just
speaking out of my ass.

Your key contention that GVariant is just like DBusMessage is something that I very strongly disagree with. I created it mostly because I was unable to deal with DBusMessage and still do the things that I wanted to do.

Please try using it. :)

btw: after a lot of talk with some people who really know about DBus (like Telepathy developers) there is some consensus that demarshaling DBus messages into the "equivalent" glib types is extreme pain and tends not to work well. Even in a code-generating approach, the real obvious cases can easily be dealt with: strings, ints, even arrays but it the sanity ends there.

When you get complicated types like arrays of structures, or really anything that contains a variant (array of pairs of strings and variant are common on a lot of APIs) then you're really in a bad place. We don't currently have anything in glib to turn these things into without it getting messy. I believe GVariant is an ideal candidate for the "closest convenient glib type" for a lot of types of data that you could get from the bus, and it's not that difficult or weird to use it either.

Btw, the copyright headers in your glib branch still says GPLv3. You
should probably change that...

Yes. Thanks for noticing. There are also some style issues with respect to things like placements of '*'s, etc. These issues will obviously be fixed up.

Thanks again for the discussion.


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