//Marcus Daniels Sept. 19, 1999 swarm-support list
DS> I've added a linking method to my agents.  It compiles, but it
DS> when I run it I get the following error:

DS> (Tcl -eval:) invalid command name "(null)" while executing
DS> "unknown_pre_tclObjc $name $args" (procedure "unknown" line 6)
DS> invoked from within "(null) create line 0 0 0 0 -arrow last" 

Looks like the `graphCanvas' is null.  Here's an example of using LinkItem.

#import <simtools.h>
#import <gui.h>

int
main (int argc, const char **argv)
{
  id <Canvas> canvas;
  id <NodeItem> node1, node2, node3;
  id <LinkItem> link13, link23;

  initSwarm (argc, argv);

  canvas = [Canvas create: globalZone];
  [canvas setWidth: 400 Height: 400];
  [canvas setWindowTitle: "Canvas"];
  [canvas pack];

  node1 = [OvalNodeItem createBegin: globalZone];
  [node1 setCanvas: canvas];
  [node1 setX: 100 Y: 100];
  [node1 setString: "N1"];
  node1 = [node1 createEnd];

  node2 = [OvalNodeItem createBegin: globalZone];
  [node2 setCanvas: canvas];
  [node2 setX: 300 Y: 100];
  [node2 setString: "N2"];
  node2 = [node2 createEnd];

  node3 = [OvalNodeItem createBegin: globalZone];
  [node3 setCanvas: canvas];
  [node3 setX: 200 Y: 200];
  [node3 setString: "N3"];
  node3 = [node3 createEnd];

  link13 = [LinkItem createBegin: globalZone];
  [link13 setCanvas: canvas];
  [link13 setFrom: node1];
  [link13 setTo: node3];
  link13 = [link13 createEnd];

  link23 = [LinkItem createBegin: globalZone];
  [link23 setCanvas: canvas];
  [link23 setFrom: node2];
  [link23 setTo: node3];
  link23 = [link23 createEnd];

  while (1) { while (GUI_EVENT_ASYNC ()) {} }
}

/*
Local Variables:
compile-command: "$SWARMHOME/bin/libtool-swarm --mode=link gcc -o canvas2 -g -Wno-import
-I$SWARMHOME/include -L$SWARMHOME/lib canvas2.m -lswarm -lobjc"
End:
*/