Re: Enable drag and drop for treeview?

[Jef Driesen <jefdriesen hotmail com>]

Jef Driesen wrote:
> Paul Davis wrote:
> > On 5/2/07, Jef Driesen <jefdriesen hotmail com> wrote:
> > > For my application, I want to be able to re-order rows in a treeview
> > > (treestore model) using drag and drop. According to the gtkmm book, I
> > > can use set_reorderable(). But how can I detect when a drag and drop
> > > operation is finished (to update the underlying database)? I can't find
> > > any signals to connect to. Am I missing something?
> > I think to do this, you have to subclass Gtk::TreeModel and override
> > Gtk::TreeDragDest::drag_data_received_vfunc()
> >
> > I did something similar here:
> > http://trac.lsdcas.engineering.uiowa.edu/browser/trunk/cas2/plugins/viewer/event_analysis/treemodel.hh
> > http://trac.lsdcas.engineering.uiowa.edu/browser/trunk/cas2/plugins/viewer/event_analysis/treemodel.cc
> >
> > There's also an example in the book about doing this stuff which is
> > what I based my implementation on:
> >
> > http://www.gtkmm.org/docs/gtkmm-2.4/examples/book/treeview/drag_and_drop/
>
> Does this method also work well with a sorted treemodel? Because the 
> simple set_reorderable() does not seem to work in that case. Once the 
> model is sorted (by clicking on a treeview column header), drag and drop 
> is not possible anymore.

I tried that approach and the row_drop_possible_vfunc is not very 
difficult. I ported the default gtk function to gtkmm, disabled the 
check for the sorting and added some extra constraints (see code below). 
 But I'm having problems with implementing drag_data_received_vfunc, 
because there is no gtk_tree_store_copy function. And I can't easily 
re-use the existing code from gtk+ because it depends on the treestore 
(private) internals. Any suggestions?

The changes that are required to make my custom dnd work with a sorted 
treestore is not very complicated (I think). Instead of adding the row 
to a specific location (using gtk_tree_store_prepend or 
gtk_tree_store_insert_after) I would have to add it to a parent. Because 
if you do that, the row is automatically sorted, and that is exactly 
what I need.

While testing, I also found some very strange behavior for the 
row_drop_possible_vfunc. If you have this very simple treestore:

Item 0
Item 1
Item 2

and try to drop item 0 between item 1 and 2, the path passed to the 
function is 2, as I expected. But if you drop after item 2, the path is 
also 2. But once you drop the item there, drag_data_received_vfunc 
receives the path 3. Shouldn't row_drop_possible_vfunc also receive the 
path 3?

bool
DndTreeStore::row_drop_possible_vfunc (const Gtk::TreeModel::Path& 
dest_path, const Gtk::SelectionData& selection_data) const
{
   // don't accept drops if the tree has been sorted
   /*if (GTK_TREE_STORE_IS_SORTED (this))
      return false;*/

   Glib::RefPtr<Gtk::TreeModel> src_model;
   Gtk::TreePath src_path;
   if (!Gtk::TreePath::get_from_selection_data (selection_data,
         src_model,
         src_path))
      return false;

   // can only drag to ourselves
   /*if (src_model != GTK_TREE_MODEL (this))
      return false;*/

    // Can't drop into ourself.
   if (src_path.is_ancestor (dest_path))
      return false;

   // Can't drop if dest_path's parent doesn't exist
   if (dest_path.get_depth() > 1) {
      Gtk::TreePath tmp = dest_path; tmp.up();

      if (!((Gtk::TreeStore*)this)->get_iter (tmp))
           return false;
   }

   // Can't drop into to the same parent.
   if (dnd_equal_parent(src_path, dest_path))
      return false;

   // Allow new parent only
   if (dest_path.back() != 0)
      return false;

   // Can otherwise drop anywhere.
   return true;
}

static bool
dnd_equal_parent (const Gtk::TreeModel::Path& src, const 
Gtk::TreeModel::Path& dst)
{
   // Paths with a different depth never have the same parent.
   if (src.size() != dst.size())
      return false;

   // Top-level paths always have the same parent.
   // Note: There is no need to check both depths,
   // because they are always equal at this point.
   if (src.size() == 1)
      return true;

   // Compare parent paths for all other cases.
   Gtk::TreeModel::Path src_parent = src; src_parent.up();
   Gtk::TreeModel::Path dst_parent = dst; dst_parent.up();
   return (src_parent == dst_parent);
}