Add documentation on custom tracers

doc/tracers_all.rst

@@ -157,3 +157,94 @@ Cell
 ----
 
 The cell tracer is discussed separately, as it has very specific behaviour and is only applicable to a select number of models.
+
+Custom
+------
+
+Additionally, it might necessary to define your own custom tracer.
+This can be done by defining a class similar to the following template.
+For each trace method, an *aDEVS* parameter is passed, being a reference to the atomic DEVS model doing the transition.
+On this *aDEVS* object, the following functions and attributes can be accessed:
+
+    - *aDEVS.getModelFullName()*: full hierarchical name of the model
+    - *aDEVS.IPorts*: reference to all input ports
+    - *aDEVS.OPorts*: reference to all output ports
+    - *aDEVS.state*: state of model
+    - *aDEVS.time_last[0]*: Time of next transition
+    - *aDEVS.time_next[0]*: Time of next transition
+    - *aDEVS.my_output*: dictionary of output events
+    - *aDEVS.my_input*: dictionary of input events
+    - *aDEVS.elapsed*: elapsed time before transition
+
+A custom tracer can be defined as follows::
+
+    class TracerCustom(object):
+        def __init__(self, uid, server, filename):
+            """
+            Both uid and server can be ignored, as these are only required for distributed simulation
+            filename contains the name of the file in which we should write the trace
+            """
+            pass
+
+        def startTracer(self, recover):
+            """
+            Recover is a boolean representing whether or not this is a recovered call (e.g., should the file be overwritten or appended to?)
+            """
+            pass
+
+        def stopTracer(self):
+            """
+            Stops the tracer (e.g., flush the file)
+            """
+            pass
+
+        def traceInternal(self, aDEVS):
+            """
+            Called for each atomic DEVS model that does an internal transition.
+            """
+            pass
+
+        def traceExternal(self, aDEVS):
+            """
+            Called for each atomic DEVS model that does an external transition.
+            """
+            pass
+
+        def traceConfluent(self, aDEVS):
+            """
+            Called for each atomic DEVS model that does a confluent transition.
+            """
+            pass
+
+        def traceInit(self, aDEVS, t):
+            """
+            Called upon initialization of a model.
+            The parameter *t* contains the time at which the model commences (likely 0).
+            """
+            pass
+
+        def traceUser(self, time, aDEVS, variable, value):
+            """
+            Called upon so called *god events* during debuggin, where a user manually alters the state of an atomic DEVS instance.
+            """
+            pass
+
+For some "example" tracers, have a look at the built-in tracers of PythonPDEVS, which can be found in *src/tracers*.
+
+Note that in optimistic synchronization the destructive parts of this operation should be separated.
+This can be done using the *runTraceAtController* function::
+
+    runTraceAtController(server, uid, aDEVS, [time, trace_text])
+
+Both the *server* and *uid* are those passed to the constructor of the tracer.
+
+Finally, after the tracer is defined, it needs to be registered for the simulator to use it.
+This is done using the following call on the instantiated simulator::
+
+    sim.setCustomTracer(self, tracerfile, tracerclass, args):
+
+Where:
+
+    - *tracerfile*: the Python class containing the implementation of the tracer, which is dynamically imported.
+    - *tracerclass*: the name of the class implementing the tracing functionality.
+    - *args*: the list of arguments  that must additionally be passed to the tracer (e.g., filename)