little event mechanism for changed positions

lib/risc/position/object_position.rb

@@ -6,6 +6,7 @@ module Risc
       def initialize( object, at)
         @at = at
         @object = object
+        @listeners = []
         raise "not int #{self}-#{at}" unless @at.is_a?(Integer)
       end
 
@@ -32,6 +33,25 @@ module Risc
         @at = pos
         true
       end
+
+      # Register a handler position change event.
+      # The object calls position_changed on the handler object
+      #
+      #   obj.position_changed( changed_position )
+      #
+      # @param [Object] object handling position_changed
+      def register_listener( handler)
+        @listeners << handler
+      end
+
+      def unregister_listener(handler)
+        @listeners.delete handler
+      end
+
+      # Trigger position change  and pass self to position_changed
+      def trigger()
+        @listeners.each { |handler| handler.position_changed( self ) }
+      end
     end
   end
 end