Add a check (predicate) callable to the signal connect

This allows another signal condition to the connection, so that, in addition
to the (sub) class, another check can be made before the signal is delivered
to the connected callable

include/class.signal.php

@@ -46,18 +46,27 @@ class Signal {
      * Optionally, if $object is a class and is passed into the ::connect()
      * method, only instances of the named class or subclass will actually
      * be connected to the callable function.
+     *
+     * A predicate function, $check, can be used to filter calls to the
+     * signal handler. The function will receive the signal data and should
+     * return true if the signal handler should be called.
      */
-    /*static*/ function connect($signal, $callable, $object=null) {
+    /*static*/ function connect($signal, $callable, $object=null,
+            $check=null) {
         global $_subscribers;
         if (!isset($_subscribers[$signal])) $_subscribers[$signal] = array();
         // XXX: Ensure $object if set is a class
         if ($object && !is_string($object))
             trigger_error("Invalid object: $object: Expected class");
-        $_subscribers[$signal][] = array($object, $callable);    
+        elseif ($check && !is_callable($check)) {
+            trigger_error("Invalid check function: Must be callable");
+            $check = null;
+        }
+        $_subscribers[$signal][] = array($object, $callable, $check);
     }
 
     /**
-     * Publish a signal. 
+     * Publish a signal.
      *
      * Signal::send('user.login', $this, array('username'=>'blah'));
      *
@@ -81,9 +90,11 @@ class Signal {
         if (!isset($_subscribers[$signal]))
             return;
         foreach ($_subscribers[$signal] as $sub) {
-            list($s, $callable) = $sub;
+            list($s, $callable, $check) = $sub;
             if ($s && !is_a($object, $s))
                 continue;
+            elseif ($check && !call_user_func($check, $data))
+                continue;
             call_user_func($callable, $data);
         }
     }