[gst-devel] gst/gststructure.[ch] gst/gstvalue.[ch]

[in7y118 at public.uni-hamburg.de]

Hi there.
Since it's a recent addition to the core and you probably don't know much 
about it, I thought I'd give you a short rundown about what GstStructure is 
and what we are going to use it for.

David describes GstStructure as a runtime-defined equivalent to the C struct.

GstStructure is a list of values associated with names. You just use 
gst_structure_set (structure, "something", G_TYPE_INT, 15, NULL); to associate 
an integer with value 15 with the name "something". After that you can get 
this value with g_value_get_int (structure, "something", &some_int);
This is supposed to work for any GType you can think of.
Every GstStructure has a name. you can use gst_structure_{get|set}_name for 
this. To keep up with the C struct example, this would be the name of that 
structure.

Internally, GstStructure stores quarks and GValues so it is as flexible as 
that system is.

A really nice thing about GstStructure is the ability to convert a structure 
to/from strings. gst_structure_to_string (structure) will give you a nice 
representation that you can use when debugging - at least if you don't use 
totally weird GTypes that don't support string transformations. And 
serialization obviously allows you to send GstStructures via simpe IPC 
echanisms, via UDP or whatever you can imagine.


Now what will GStreamer use this for?

The idea that lead to the invention of GstStructure was the caps system and 
its usage of GstProps. David rewrites the caps system with GstStructure 
instead of GstProps. There are some extensions for GValues required to achieve 
this (unions, intersections and some additional GValues for ranges, fourccs 
etc) that reside in gstvalue.[ch].
So the new caps system will wrap GstStructure.

After David created this thing, I found it tremendously useful for the tag 
system. So I rewrote the tag system once again. It now completely wraps 
GstStructure, with some tag specific extensions.

The third thing where GstStructure should be really useful is the event 
system. I plan to (hopefully before feature freeze) replace the whole event 
union mess with GstStructures. This buys us serializable events (think events 
via network) that can be user defined. And we (or you) can define them just 
like we can now define new mime types.


Hm, I hope this wasn't too short and you're now even more clueless than you 
were before.
Ask away if something is still unclear :)

Benjamin