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doc/listener.rst

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+..
+    Copyright 2014 Modelling, Simulation and Design Lab (MSDL) at 
+    McGill University and the University of Antwerp (http://msdl.cs.mcgill.ca/)
+
+    Licensed under the Apache License, Version 2.0 (the "License");
+    you may not use this file except in compliance with the License.
+    You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+    Unless required by applicable law or agreed to in writing, software
+    distributed under the License is distributed on an "AS IS" BASIS,
+    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+    See the License for the specific language governing permissions and
+    limitations under the License.
+
+Listening to realtime simulation
+================================
+
+During realtime simulation, there frequently exists the need to listen to specific ports, monitoring the events passing over them. In PythonPDEVS, it is possible to register a function that is invoked for each event passing over that port. This is called a *listener* in PythonPDEVS.
+
+Example model
+-------------
+
+We can simply reuse the traffic light model from the previous section on realtime simulation (:download:`trafficLightModel.py <trafficLightModel.py>`). All that needs to be changed now, is adding a listener on an output port of a *coupled* DEVS model. First and foremost, therefore, we must put our traffic light in a container::
+
+    class TrafficLightSystem(CoupledDEVS):
+        def __init__(self):
+            CoupledDEVS.__init__(self, "System")
+            self.light = self.addSubModel(TrafficLight("Light"))
+            self.observed = self.addOutPort(name="observed")
+
+In the experiment file, it is then possible to register a listener on this port as follows (:download:`injection.py <injection.py>`)::
+
+    def my_function(event):
+        print("Observed the following event: " + str(event))
+        
+    sim.setListenPorts(model.observed, my_function)
+
+Since this function is invoked with the complete bag, the received event will contain a list of events, exactly as how it would be invoked in the external transition function.
+
+.. note:: Listeners are only supported on output ports of Coupled DEVS models. This is because the listeners are invoked upon event routing, for which the source port is not checked.