signals, objects, user_data and stuff.

[Thomas Mailund <mailund mail1 stofanet dk>]

Hi all.

I'm messing around with signals right now, and I've run into the
following problem.  I need to connect a signal, as in
'connect_object', but I want to pass user data along with the signal,
as in the usual signal_connect.  I can probably get around this using
a struct and put the stuff I need into that, but it'll get quite ugly
for my current situation and I'm looking for a more elegant solution.

A little context for the problem:  I have a gnome canvas where I've
drawn a number of "nodes", items which I can move around, and a number
of "connectors", lines connecting the nodes.  For the nodes I've
subclassed the canvas item and added the signal "moved" which is
emitted whenever I move the node.  My connectors listens for these
signals, so they will be informed when one of their endpoints move.
So I can do something like

  gtk_signal_connect_object (GTK_OBJECT (node), "moved",
			     GTK_SIGNAL_FUNC (connector_moved),
			     GTK_OBJECT (connector));

where the callback moves the connector

static void
connector_moved (Connector *conn, gdouble dx, gdouble dy)
{
  <<move "the right" point>>
}

(here the (dx,dy) are parameters from the nodes "moved" signal).

Of course, getting "the right point" is the tricky one.  If I only
have two points on a connector, i.e. connectors are straight lines
between nodes, I can hack it doing something like

  gtk_signal_connect_object (GTK_OBJECT (node1), "moved",
			     GTK_SIGNAL_FUNC (connector_moved1),
			     GTK_OBJECT (connector));
  gtk_signal_connect_object (GTK_OBJECT (node2), "moved",
			     GTK_SIGNAL_FUNC (connector_moved2),
			     GTK_OBJECT (connector));

static void
connector_moved1 (Connector *conn, gdouble dx, gdouble dy)
{
  connector_move_point (conn, 1);
}

static void
connector_moved2 (Connector *conn, gdouble dx, gdouble dy)
{
  connector_move_point (conn, 2);
}

It's not nice, but it would work.  But if I add bend points to
connectors it gets tricky.  Then I need a special callback for each
bend point, just to pass an integer parameter along.  If I don't know
the number of bend points statically (which I don't, since the
connectors are read in from a file) I don't see how I can do this.

Does anyone have any suggestions on how to achieve what I want here?
Either getting the extra parameter passed along or perhaps redesigning
it all and do something more clever?

        /mailund