[Glade-devel] Property Binding Support: Present and Future

Hi Denis,
   Sorry it took me a while to get back to you, I just got back to
montreal and should have a little time...

On Thu, 2011-09-01 at 09:58 +0200, Denis Washington wrote:

As you may know, I took part in Google Summer of Code this year (thanks 
to Juan for mentoring me!) and worked on "GObject property binding 
support for GtkBuilder and Glade":


While talking on IRC with Tristan yesterday, I realized that while I 
sent out weekly reports to gnome-soc-list and blogged about my work 
twice, I never actually wrote anything about the code's status or 
technical details on this mailing list. I would like to apologize for my 
failure to do so and will try to make up for it by telling you 
*everything* in this message: what I have done, how I have done it, what 
works now, where the remaining problems are, and how these issues could 
be overcome (this is where I really need your feedback!).

For those who don't know, the objective of my work is to extend Glade 
with support for creating bindings between widget properties in a 
project. What this means is that you can define a property's value as 
being directly dependent on the value of another property - whenever the 
"source" property's value is set, the value of the "target" property is 
automatically updated, either to the same value or a user-defined 
transformation thereof (more on this later). GLib supports this through 
its GBinding API [1]. The goal is to expose this functionality in Glade 
so that the user can create, modify and delete property bindings in 
Glade and save them as part of a UI file.

This feature requires changes to both GTK+ and Glade, so I created 
git.gnome.org branches for both of them, named "gtkbuilder-gbinding" and 
"gbinding", respectively [2][3]. The GTK+ branch adds support for a new 
bit of GtkBuilder syntax - the <binding> element - which makes it 
possible for GtkBuilder objects to read property bindings from UI files. 
It is used like this:

   <object name="button">
     <binding to="sensitive" from="active" source="checkbutton"/>

which means: "Let the 'sensitive' property of 'button' always have the 
same value as the 'active' property of 'checkbutton'." Thanks to the 
existence of GBinding, the code changes required for this are pretty 
small. No new API is introduced; all bindings are automatically created 
at the time they are read in by gtk_builder_add_from*(). (But this might 
change slightly; see the problem discussion later in this epic mail 

The "gbinding" Glade branch is where the bulk of my work happened. It 
adds a new "binding-source" property to GladeProperty for representing 
property bindings in the data model and supports serialization and 
deserialization of this information to <binding> elements. (See 
glade_property_binding_read() and glade_property_binding_write() in 
glade-property.c, respectively.) Furthermore, it augments Glade's 
undo/redo framework with a glade_command_bind_property() command for 
creating and deleting property bindings. On the UI side, this command is 
exposed through a "Bind to source..." context menu item in the property 
inspector. (For screenshots of how the UI currently looks, see my blog 
posts [4][5].)

The dialog is nice enough in general, I think it needs to expose the
real untranslated property name as well, perhaps with the translated
name in italic/grey beside it. It would even be interesting to include
the proper class name introducing that property in the list.


  GtkWidget:tooltip-text   /Tooltip Text/
  GtkEntry:text            /Text/

(its nice to have the translated titles around, but it's usually
more important to show something untranslated and informative here).

The UI and data model has been adapted to reflect the defined property 

- glade_command_set_property() was modified to recursively set the value 
of all properties bound to the originally set property. This means that 
the effect of a property binding is immediately visible in the Glade 

It sounds like a nice feature but I'm not sure it's the right place.

Perhaps the implementation of glade_property_sync() would be a better
location for this.

Which also brings to mind, there should be some protection to avoid
circular references and feedback loops which could be easily introduced
by using bindings, perhaps Glade should simply allow the user to create
dangerous documents and forcefully avoid feedback internally, or just
refuse to create circular loops all together (not such a bad limitation
at first thought...).

- In the property inspector, the edit widgets for bound properties are 
insensitive (setting the value of a bound property doesn't make much 
sense). Also, the tooltip of a bound property shows which other property 
it is bound to.
(See glade-editor-property.c)

If I understand correctly, you are updating property sensitivity
directly based on whether a property is bound or not, this will
probably fail in some conditions as the backend is responsible
currently for updating state.

If for instance, you bind the "label" property of a GtkButton
and then set the button to be "custom content" or such (using
the radio buttons in the button editor), then undo... will the
tooltip of the insensitive "label" property be correct ?

Also, I made some precautions to avoid invalid property bindings:

- The "Bind to source..." dialog for choosing the source of a property 
binding only allows you to select properties that have the same, or a 
compatible, type, and are enabled (if they are optional) and sensitive 
(if there are one of multiple alternative properties, e.g. "text", 
"stock" and "embedded widget" in GtkButton). All other properties are 
greyed out and moved to the end of the list.
(See glade_editor_property_show_bind_dialog() and its helper functions 
in glade-editor-property.c)

Right, this should at least take care of possible runtime warnings.

- If the source or target of a property binding disappears because the 
widget it belongs to is deleted, the binding is automatically removed 
too. This is properly integrated into the undo/redo system, so undoing 
the widget removal also brings the property binding back.
(See glade_command_delete_binding_refs() in glade-command.c)

Will have to eventually review that in detail, but sounds like the
right approach there.

One remaining issue with the current code is that it does not react to 
property binding sources becoming disabled or insensitive. This is 
currently not possible in a sane way as changes to a property's 
enabled/sensitivity state are not tracked with the undo/redo framework 
at the moment. (I had code to do this in the branch before, but it 
worked with manual signal handling hackery and was removed later on 
Tristan's request.) The obvious solution would be to change this by 
introducing glade_command_set_property_enabled() and 
glade_command_set_property_sensitive() and porting all of Glade to that. 
If there is general agreement to do so, I would be willing to do that 
work. In any case, this has to be fixed in some way before the branch is 
ready to be merged into master.

Those are the blockers for this integration.

Porting that will take time and effort though, take a look at
glade-gtk.c and note how sensitivity is generally driven, all of
that needs to be ported to control sensitivity while updating
property values at the editor level (that is, things now belong 
on the 'calling' side of GladeCommand instead of the other, 
'data model' side).

This might mean extending GladeEditorProperty api to give the
widget class adaptor code easy control on when to control
sensitivity (the pre/post-commit signals sound like the right
place to couple in sensitivity commands with property commands).

It's also important that sensitivity cannot be controlled alone
in a single command... because commands usually should represent
an action taken in the document (a document is dirty after executing
any command).

Other than that, I don't know of any other major showstoppers, but an 
extensive code review by Juan and Tristan might very well reveal some. ;)

The big question, however, it how to support transformation functions 
for property bindings. This GBinding feature allows you to define a 
function that processes the value of a binding's source property before 
it is applied to the target, which allows you to create bindings between 
properties of different types and generally make property bindings much 
more useful and interesting. Adding this into the GTK+ and Glade 
branches is not a big problem in principle, and in fact I did just that 
during Summer of Code. However, I had to later remove the code again, 
the reason being on the GTK+/GLib side.

The problem is when and how to resolve the transformation function names 
that would be stored in the GtkBuilder file to the actual function 
implementations. In my code, I moved all property binding creation to 
two new API functions, gtk_builder_create_bindings() and 
gtk_builder_create_bindings_full(), which take the same arguments as 
gtk_builder_connect_signals*() and locate transformation functions the 
same way (GModule or a custom callback, called GtkBuilderBindingFunc).

Unfortunately, this setup means more work for language binding authors: 
because transformation functions are specified as an argument to 
g_object_bind_property_full() rather than by connecting a signal, a 
language-specific GtkBuilderConnectFunc for 
gtk_builder_connect_signals*() cannot be reused and each language 
binding would be required to provide a GtkBuilderBindingFunc to replace 
the use of GModule with something appropriate for the language. Also, 
the introduction of another function to be called for every loaded 
GtkBuilder function is really not ideal.

Talking with Juan and Tristan, we concluded that a proper solution 
probably requires changes to GBinding itself. More specifically, if 
GBinding implemented the transformation function as being the handler of 
a "transform" signal instead of an anonymous callback, one could reuse 
gtk_builder_connect_signals*() to locate transformation functions and 
move property binding creation there. For backwards compatibility and 
convenience, the g_object_bind_property() API could still stay as it is 
now. There would be some issues to sort out - for instance, how to 
behave if multiple signal handlers are connected to the "transform" 
signal - but it might be doable without breaking anything. This still 
needs to be talked about with both the GLib and GTK+ team, though.

For reference, you can still find my code with transformation function 
support in the "gtkbuilder-gbinding" GTK+ and "gbinding-transform" Glade 

I don't want to discuss transformation functions in depth until 
finished landing this code in GTK+/Glade.

If transformation functions are implemented at a later date, they should
use similar semantics as signals, actually reading the code at this
moment I don't see any reason why the normal GtkBuilderConnectFunc
couldnt be used by the GtkBuilder during gtk_builder_connect_signals()
to resolve any gbinding transform function signals.

So *when* we look into this, my vague proposal runs like this:
  a.) Introduce "transform" signals on GBinding object and
      probably prefer the signal (the default handler falls
      back on invoking any registered transform function for
      backwards compatibility and then defaults to usual GValue
      transforms and copies).

  b.) Extend GtkBuilder binding parsing slightly to allow specification
      of a transform function... collect the GBinding objects and 
      transform function data during the parse and cache them for use
      in gtk_builder_connect_signals() (possibly even in the same cache
      as the actual <signals>).

  c.) Connect the transform signal callbacks later in the normal way
      from gtk_builder_connect_signals()

This will imply an api extension in GtkBuilder, so when it gets
introduced in GTK+ Glade will need to be careful not to register
transform functions for projects targeting a too old version of GTK+.

Which leads me to another observation, the binding feature itself
should not be available of the project is targeting < GTK+ 3.2
(or whatever GTK+ version we get the feature into).

Well, this is all there is to say about the code now. (This mail is 
already way too long as it is. ;) I hope you now have a better insight 
wrt what I have done during GSoC and what is still to do.

As I wrote in the beginning, I need your feedback! If you have any 
questions, remarks, or suggestions regarding the issues I outlined - 
especially regarding the transformation function situation - I would be 
excited to hear them! Thanks. :)

Thanks a lot for the great effort done this year.

Again, sorry for taking so long to give you some attention.

Best Regards,

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