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

Am 17.09.2011 19:32, schrieb Tristan Van Berkom:
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...

No problem. Thanks for the comments!

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).

I used the translated names because these are what the property 
inspector shows, so it would be arguably inconsistent if we highlighted 
the internal property names as the primary identification means. I think 
better would be:

     Tooltip Text    /GtkWidget:tooltip-text/

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.

I chose this location so that all property set operations are recorded 
in the command system (grouped with the original set command, 
naturally), which makes undo trivial.

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...).

I think prohibit cycles, we would need to provide explicit support for 
two-way bindings (such as offered by a g_object_bind_property() flag) to 
retain the whole power, but this probably wouldn't be worth the 
complication in the code.

I don't know how well GBinding handles cycles currently. It would be 
best if we could avoid infinite loops on that level (e.g. by suspending 
the binding while syncing the target's value to the source, so that a 
resulting update of the source itself doesn't trigger a new sync), but 
I'm not sure how feasible that is.

- 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 ?

I don't set the sensitivity in the sense of 
glade_property_set_sensitive(). Rather, I just control the sensitivity 
of the associated GladeEditorProperty in 
glade_editor_property_sensitivity_cb(). Thus, the example you described 
works correctly.

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).

I will look into that.

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).

What about enabling/disabling? Does that invoke "set property" commands 

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.

*If* we have a GBinding transformation function facility based on 
function, naturally, GtkBuilderConnectFunc is just fine. My comment 
about it unsuitability was based on the assumption that GBinding is like 
it is now, with transformation functions having to be specified on 

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 assumes that GValue provides default conversions between all pairs 
of types. Is this the case? If not, creating the property bindings first 
and adding transformation functions later won't work. Assuming that 
GValue does handle all of this, however, this seems like a good plan.

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.

Thanks for your guidance.

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

No problem. :)


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