[pox-dev] Event handling and listeners

[Murphy McCauley]

On Nov 12, 2013, at 5:08 AM, Giannis Sapountzis <johnsub1985 at gmail.com> wrote:

> Thank you Murphy.
> 
> I noticed that you also added this info in the POX wiki yesterday. That should be really helpful. Much appreciated.

And yet more on revent in the wiki today. :)

> On Nov 11, 2013, at 9:26 PM, Murphy McCauley <murphy.mccauley at gmail.com> wrote:
> 
>> On Nov 11, 2013, at 4:31 AM, Giannis Sapountzis <johnsub1985 at gmail.com> wrote:
>> 
>>> Hello everyone. 
>>> 
>>> I'm looking for some explanation on event handling in POX. 
>> 
>> There's quite a bit of explanation on this subject in the POX manual and in the reference (docstrings).
>> 
>>> Lets say that i have 2 components registered in core with names Component_1 and Component_2. Component_1 raises some event called Event_1.
>> 
>> Just a sidenote: components and events in POX are both classes, and POX's naming convention uses TitleCase for these -- with no underscores. Names with underscores like yours should work fine with the more explicit functions (e.g., addListener), but you may find that some of the convenience features of POX expect POX style naming (e.g., listen_to_dependencies).
>> 
>>> I want to handle some events in Component_2 both from core.openflow and from core.Component_1.
>>> 
>>> How should i name the handler methods in Component_2 and how should i register the listeners? Is the following example correct?
>> 
>> As described in the manual, you can name them whatever you want if you set up the bindings yourself using addListener() or addListenerByName().  If you want to use addListeners(), you have to name them _handle_<EventName> or _handle_<Prefix>_<EventName>, where EventName is the name of the event type and Prefix is the prefix as specified in the call to addListeners.
>> 
>> If you want to use core.listen_to_dependencies, you have to name them _handle_<ObjectOnCore>_<EventName> by default, where ObjectOnCore is the name of the object (which is registered on core) which is sourcing the events you want. (More on this below.)
>> 
>>> In Component_2:
>>> 
>>> __init__(self):
>>> 	core.Component_1.addListeners(self) 
>>> 	core.openflow.addListeners(self)
>>> 
>>> def _handle_openflow_ConnectionUp(self):
>>> 	….
>>> 
>>> def _handle_Component_1_Event_1(self):
>>> 	….
>> 
>> Since you're using addListeners here with no prefix, it'd just be _handle_ConnectionUp.  You could use the prefix argument of addListeners() to make it _handle_WhateverYouLike_ConnectionUp.  If you're going to be handling events from multiple sources, this isn't a bad idea (to make sure that two sources don't try to fire the same handler).  listen_to_dependencies() just automatically makes the prefix the name of the component on core.
>> 
>>> Secondly, i only want a component to start handling events afar having received a notification from another component. Can i add the listeners only after i have "caught" an event? Something like the following:
>>> 
>>> __init__(self):
>>> 	core.Component_1.addListeners(self) 
>>> 
>>> def _handle_Component_1_Event_1(self):
>>> 	core.openflow.addListeners(self)
>> 
>> Indeed, you can do this (though the above example has prefix/name issues).
>> 
>>> Third, can someone explain to me what the core.listen_to_dependencies does?
>> 
>> Here's the docstring.  You can get it yourself by reading the code, starting POX with the Python shell (py component) and doing "help(core.listen_to_dependencies)", or by building the pydoc or Sphinx documentation (neither of which are as accessible/used as they should be, but see POX's custom pydoc in the tools directory for the former and the pox-doc repo for the latter).
>> 
>> listen_to_dependencies(self, sink, components=None, attrs=True, short_attrs=False, listen_args={}) method of pox.core.POXCore instance
>>   Look through *sink* for handlers named like _handle_component_event.
>>   Use that to build a list of components, and append any components
>>   explicitly specified by *components*.
>> 
>>   listen_args is a dict of "component_name"={"arg_name":"arg_value",...},
>>   allowing you to specify additional arguments to addListeners().
>> 
>>   When all the referenced components are registered, do the following:
>>   1) Set up all the event listeners
>>   2) Call "_all_dependencies_met" on *sink* if it exists
>>   3) If attrs=True, set attributes on *sink* for each component
>>      (e.g, sink._openflow_ would be set to core.openflow)
>> 
>>   For example, if topology is a dependency, a handler for topology's
>>   SwitchJoin event must be defined as so:
>>      def _handle_topology_SwitchJoin (self, ...):
>> 
>> 
>> -- Murphy
>