Oftentimes, you need to have objects that communicate with each other via events. This is a very useful setup, for example, in a GUI -- where these events represent things like mouse clicks, key strokes, or button presses. That's not what I developed these classes for, since I was more interested in simulating things and the event system seemed like the most natural fit, but the recipe is still relevant to other event handling needs.
We're going to build upon earlier discussions, most notably about
Stackless event loops by adding in some concrete examples of using that recipe.
The
Observer pattern strikes again, as I'm defining the relationship between the event generator and the event listener as one of Observable and Observer. We'll make use of the
channel_processor function decorator described in
Stackless event loops.
class EventObserver( object ):
def __init__( self, *args, **kwargs ):
super( EventObserver, self ).__init__( *args, **kwargs )
@channel_processor
def event_transieve( self, *args, **kwargs ):
evt = kwargs.get( 'data', None )
self.process_event( evt )
def process_event( self, event ):
pass
def event_notify( self, event ):
self.event_transieve( event )
This is straight-forward enough. The only trickery (if you could call it that) is in the
event_transieve method. And all that does is take whatever is passed as the
keyword argument data and call the method
process_event. In this base class implementation, that function does nothing.
One bit of niftiness does occur, however, when the
event_transieve method is invoked. Through the use of function decorators (and therefore transparent to the calling client) this method actually spans across
tasklets, granting some semblance of concurrency.
class EventObservable( object ):
def __init__( self, *args, **kwargs ):
super( EventObservable, self ).__init__( *args, **kwargs )
self.__event_observers = weakref.WeakKeyDictionary( )
self.__events = [ ]
def attach_event_observer( self, obs, level=1 ):
if obs not in self.__event_observers:
self.__event_observers[obs] = level
return self
def detach_event_observer( self, obs ):
if obs in self.__event_observers:
del self.__event_observers[obs]
return self
@channel_processor
def dispatcher( self, *args, **kwargs ):
data = kwargs.get( 'data', None )
wlist = []
for key in self.__event_observers.keys( ):
val = self.__event_observers[key]
seok = SortableEventObserverKey( key, val )
heapq.heappush( wlist, seok )
while len( wlist ):
obs = heapq.heappop( wlist )
obs( evt )
def dispatch_event( self, event ):
self.dispatcher( event )
return event
Now, we can safely ignore the
attach_event_observer and methods -- they only exist to implement the Observer pattern. The only method we really care about at the moment is
dispatcher.
In this method we simply loop over all the currently registered observers, invoking (ultimately) their
event_notify method. If you don't see how that happens, just be patient and wait until we look at the
SortableEventObserverKey helper class and it's definition of the
__call__ method.
class SortableEventObserverKey( object ):
def __init__( self, kval, weight, *args, **kwargs ):
super( SortableEventObserverKey, self ).__init__( *args, **kwargs )
self.__value = kval
self.__weight = weight
def __cmp__( self, other ):
return cmp( self.__weight, other.__weight )
def __call__( self, event ):
return self.__value.event_notify( event )
def __repr__( self ):
return "%s, %s" % ( self.__value, self.__weight )
Now, I hate that I had to throw something like that into the discussion. The helper class only exists to make the comparison functions easier when using the
heap queue. For anyone unfamiliar with heaps, a heap ensures that the highest weighted object is at the front of the queue and will be the first one taken out of the structure.
class EventParticipant( EventObservable, EventObserver ):
def __init__( self, *args, **kwargs ):
super( EventParticipant, self ).__init__( *args, **kwargs )
event_manager = kwargs.get( "event_manager", "events" )
self.event_manager = pytypes.get_event_manager( event_manager )
def generate_event( self, event_type, *data, **kwargs ):
evt = self.event_manager.create_event( self, event_type, *data, **kwargs )
return self.dispatch_event( evt )
Here's the easy class to implement. It defines the
EventParticipant, which is both the Observable and the Observer. This is, utlimately, the class that I extend for my simulations since my program domain requires for the objects to both generate events and be interested in other object's events. Simply extending from this class gives you that ability in a nice, clean, and concurrent fashion (or, at least as concurrent as
Stackless gets you).
:
. She hasn't done anything with it, but she sure keeps talking about it. 
