Sure, some comments:

  a.) When launched for a file, it should also show what toplevel it's going to 
      preview in the title
  b.) We should send the project name to the previewer process
      when launching the preview *and* when updating it

That way the title can be always consistent:
  "Previewing %s (%s)" = "Previewing ~/myfile.ui (window1)"

Furthermore, when running the previewer program standalone, we should
make the filename look nicer 
(so it never says ../../../home/tristan/opt/share/...")

To do so, use first:
   glade_util_canonical_path() 
to get a canonical path and then:
   glade_utils_replace_home_dir_with_tilde ()
to strip the leading "/home/username"

Created attachment 182438 [details] [review]
Set window preview title.

The attached patch implements the requirements that were asked. Please review it.

Created attachment 182439 [details] [review]
Properly set glade previewer window title.

Same as before, but with Changelog entry. Please review.

Ok, looks pretty good, one main thing could be improved and some minor details.

Main thing that should be improved:

     Currently it looks like the preview window title is not updated
     when the preview is updated (i.e. the window title only gets
     updated once when the preview is initially launched).

     This should be fixed, actually ideally it should be fixed
     by adding a new token to the protocol, when the project name
     changes, it should send the new project name to all of it's
     running previews so those previews can update the window title.

     (This is to ensure running previews have the correct title
     when the project gets saved under a different name, or initially
     saved when priorly the project name was "Untitled 1" or such).

Other than that, just a few easier to fix problems:

 
+  gwidget = glade_widget_get_toplevel (gwidget);
+  object = glade_widget_get_object (gwidget);
+  if (!GTK_IS_WIDGET (object))
+    return;
+
+  /* Temporarily turn visibility off */
+  visible = gtk_widget_get_visible (GTK_WIDGET (object));
+  gtk_widget_set_visible (GTK_WIDGET (object), FALSE);
+

This code, as I said on irc, has to use glade_widget_property_get()
and glade_widget_property_set()... the actual visible state of the
GtkWidget that the GladeWidget happens to be wrapping has no effect
on the GladeProperty that is serialized (actually in this case,
the GladeProperty does not even have any effect on the GtkWidget's
runtime state, all widgets appear visible in the workspace, however
Glade's data model is what we're after here).

So we just need to change the gtk_widget_get/set_visible() for
glade_widget_property_get/set() and this will work as intended.

  
   if (!preview)
     {
-      preview = glade_preview_launch (gwidget, text);
+      //preview = glade_preview_launch (gwidget, text);
+      project_name = glade_project_get_name (project);
+      preview = glade_preview_launch (gwidget, text, project_name);

Here we are adding some garbage comments, please clean that out.

Otherwise the patch looks fine.

I would prefer if launching the preview initially had some stronger 
syntax in the protocol (i.e. using explicit keywords to depict data 
instead of a simple g_strsplit() call), but the protocol is a completely
private detail and we don't need to ensure any api stability there so 
it's not really an issue immediately.

Looking at the code, I see that the project name comes from project->priv->path. So, I need to update previews whenever it changes. Is there a simple way to detect it?

According to what was agreed on with Tristan (on IRC), there must be a new function on glade-project.c called glade_project_set_path(). This function must call glade_preview_update_title() for all the previews of such project. The glade_preview_update_title() function will then send a token to the preview to update its title.

We'll have to changes all accesses that modify priv->path to call glade_project_set_path(). Also, glade_preview_update_title() should be called if the toplevel widget it is previewing is changed.

(In reply to comment #6)
> According to what was agreed on with Tristan (on IRC), there must be a new
> function on glade-project.c called glade_project_set_path(). This function must
> call glade_preview_update_title() for all the previews of such project. The
> glade_preview_update_title() function will then send a token to the preview to
> update its title.

Right, and sending the token to update the title should be implied when
initially creating the preview.

> 
> We'll have to changes all accesses that modify priv->path to call
> glade_project_set_path(). Also, glade_preview_update_title() should be called
> if the toplevel widget it is previewing is changed.

Right, if we want to put the toplevel widget name in the title then we
must also watch the toplevel widget name.

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