In Gtk, when using Drag and Drop in a TreeView, how do I keep from dropping between rows?

I'm testing a window that looks something like this:

Dragging a Tag to a Card links the Tag to the Card. So does dragging a Card to a Tag.

It's meaningless to drop a tag between two cards, or a card between two tags. I can ignore these outcomes in the Handle...DataReceived function like this:

if (dropPos != TreeViewDropPosition.IntoOrAft开发者_如何学Cer &&
    dropPos != TreeViewDropPosition.IntoOrBefore)
    return;

However, when dragging, the user still sees the option to insert:

How do I prevent this from happening?

You need to connect to the drag-motion signal and change the default behaviour so it never indicates a before/after drop:

def _drag_motion(self, widget, context, x, y, etime):
    drag_info = widget.get_dest_row_at_pos(x, y)
    if not drag_info:
        return False
    path, pos = drag_info
    if pos == gtk.TREE_VIEW_DROP_BEFORE:
        widget.set_drag_dest_row(path, gtk.TREE_VIEW_DROP_INTO_OR_BEFORE)
    elif pos == gtk.TREE_VIEW_DROP_AFTER:
        widget.set_drag_dest_row(path, gtk.TREE_VIEW_DROP_INTO_OR_AFTER)
    context.drag_status(context.suggested_action, etime)
    return True

You can define different targets for tags and cards, and on the left widget accept only the target that represents the tags. Use Gtk.Drag.DestSet method. Maybe something like:

        Gtk.Drag.DestSet (widget, DestDefaults.All,
                      new TargetEntry[1] { new TargetEntry ("MYAPP_TAGS", TargetFlags.App, 1) },
                      DragAction.Default);

I tried to make the destination emit Motion events with:

        Gtk.Drag.DestSet (widget, DestDefaults.Motion,
                      new TargetEntry[1] { new TargetEntry ("MYAPP_TAGS", TargetFlags.App, 1) },
                      DragAction.Default);

theoretically, if I understand it correctly, it should work. But I couldn't make it fire motion events :(