GtkBuilder syntax for property bindings

[Denis Washington <denisw online de>]

Hello,

My name is Denis Washington, and I was selected as a Google Summer of 
Code student this year (yay!). My project is to add support for GObject 
property binding (a.k.a. GBinding [1]) to Glade; see [2] for my complete 
proposal.

A core component of this work is a new bit of GtkBuilder file syntax to 
encode property bindings. I have deliberately not proposed a concrete 
syntax in my original proposal as I believe that this is a point that 
really needs consensus with the GTK+ community. This is why I would now 
like to discuss this with you. I hope we can agree on something 
worthwhile before the official coding period starts (May 23).

My initial proposal is to allow <property> tags to contain a new <bind> 
element describing a binding to a source property instead of a concrete 
value. For instance, the following would make the value of property 
"prop" dependent on the "prop2" property value of object "source":

<property name="prop">
<bind source="source" property="property2" />
</property>

If "prop" should have an initial value before the value of source::prop 
is changed the first time, that value could be nested inside of <bind>, 
with the same semantics as putting it into <property> directly:

<property name="prop">
<bind source="source" property="property2">initial-value</bind>
</property>

The advantage of this syntax is that it fits naturally into the existing 
syntax, is right at the target site - which is where a property binding 
actually does anything, anyway - and could be extended for other types 
of bindings in the future, e.g. GSettings bindings (with <bind 
type="settings" ...> or something similar). It is also easy to implement 
the way that GTK+'s GtkBuilder parser is written.

The main issue with this approach is what to do with two-way bindings. 
One approach would be to make this a property of <bind>, but then, 
<bind> wouldn't appear in every property it influences anymore. 
Alternatively, the parser could be required to recognize direct cyclic 
binding declarations (A binds to B, B binds to A) and collapse them into 
a single two-way binding, which retains the 
<bind>-only-affects-the-property-it-appears-in property; the parser 
would be complicated by this, though.

I would love to hear your feedback on this matter!

Regards,
Denis Washington

[1]
[2] http://www.google-melange.com/gsoc/project/google/gsoc2011/denisw/12001