[Vala] Question about signal connection

[Johan <johan wolke7 net>]

Hi,
I wanna check if a file has been changed, and then take some action.
However I am not getting the trick with the signal connection.
the code below returns:
(process:18773): GLib-GObject-WARNING **: IA__g_object_connect: invalid 
signal spec "changed"

well I have checked the specs of GLIB and everything seems to be 
correct, maybe something with the Vala notation?

I appreciate any help!
johan

using GLib;
using Gtk;

public class HelloVala: GLib.Object {
int handler2 (FileMonitor monitor,
                  File file, File other_file, FileMonitorEvent 
event_type) {
   return 0;
  }

public static int main (string[] args) {
   var file = File.new_for_path ("test.png");
   var monitor = file.monitor_file(FileMonitorFlags.NONE);
   monitor.connect("changed",handler2);
   return 0;
   }
}