Events - Design Patterns

Hi there,
i found a nice article about a custom implementation of events by Duncan Booth here:

http://www.suttoncourtenay.org.uk/duncan/accu/pythonpatterns.html#observer


The code is here:

STEP 1) Define a Delegate Class

class Delegate:
'''Handles a list of methods and functions
Usage:
d = Delegate()
d += function # Add function to end of delegate list
d(*args, **kw) # Call all functions, returns a list of results
d -= function # Removes last matching function from list
d -= object # Removes all methods of object from list
'''
def __init__(self):
self.__delegates = []

def __iadd__(self, callback):
self.__delegates.append(callback)
return self

def __isub__(self, callback):
# If callback is a class instance,
# remove all callbacks for that instance
self.__delegates = [ cb
for cb in self.__delegates
if getattr(cb, 'im_self', None) != callback]

# If callback is callable, remove the last
# matching callback
if callable(callback):
for i in range(len(self.__delegates)-1, -1, -1):
if self.__delegates[i] == callback:
del self.__delegates[i]
return self
return self

def __call__(self, *args, **kw):
return [ callback(*args, **kw)
for callback in self.__delegates]STEP 2) Define the Event Classclass Event(property):
'''Class event notifier
Usage:
class C:
TheEvent = Event()
def OnTheEvent(self):
self.TheEvent(self, context)

instance = C()
instance.TheEvent += callback
instance.OnTheEvent()
instance.TheEvent -= callback
'''
def __init__(self):
self.attrName = attrName = "__Event_" + str(id(self))
def getEvent(subject):
if not hasattr(subject, attrName):
setattr(subject, attrName, Delegate())
return getattr(subject, attrName)
super(Event, self).__init__(getEvent)

def call(self, subject, *args, **kw):
if hasattr(subject, self.attrName):
getattr(subject, self.attrName)(subject, *args, **kw)

STEP 3) Use this logic in an object (ObjectA)class ClockTimer:
def GetHour(self):
return self._hour
def GetMinute(self):
return self._minute
def GetSecond(self):
return self._second

TickEvent = Event()
def OnTick(self):
ClockTimer.TickEvent.call(self, self.GetHour(),
self.GetMinute(), self.GetSecond())

def Tick(self):
# update internal time-keeping state
# ...
self.OnTick()
STEP 4) Subscribe the ObjectA Event and react...class DigitalClock(Widget):
def __init__(self, clockTimer):
self.__subject = clockTimer
clockTimer.TickEvent += self.Update

def close(self):
self.__subject.TickEvent -= self.Update

def Update(self, subject, hour, min, sec):
self.displayedTime = (hour, min, sec)
self.Draw()

def Draw(self):
# draw the digital clock
I've tried to use it but i failed. From mypoint of view my event is never been fired and it seem that my event subscription is not accepted and my OnTick Event never raise to execute the Update method,i've made some changes to the original code.. jjust inserted a timer to have this event fired many times:class ClockTimer: _hour = 1 _minute = 1 _second = 1 _timer = Timers.timer(0) def __init__(self): self._timer.start(10) while 1: if(self._timer.isexpired()): self._hour = self._hour + 1 self._minute = self._minute +1 self._second = self._second +1 self.Tick() self._timer.start(10) def getHour(self): return self._hour def getMinutes(self): return self._minute def getSecond(self): return self._second TickEvent = Event() def OnTick(self): ClockTimer.TickEvent.call(self, self.getHour(), self.getMinutes(), self.getSecond()) def Tick(self): print "Tick in CLockTimer" self.OnTick()and a little modification to the object that uses this event:class DigitalClock(): clockTimer = ClockTimer() def __init__(self): clockTimer.TickEvent += self.update def close(self): self.__subject.TickEvent -= self.update def update(self, subject, hour, min, sec): print hour print min print secthe code is really eas as you can see... so in the main i added this simpleline of code...c = DigitalClock()but nothing really happen... i'm sure i'm going wrong somewhere.. but cant understand where...any Idea???I've tried to ask to Duncan directly by mail.. but the reported mail address is no more valid...Someone used this?? Someone uses an other way for events??? Regards,Gianmaria

.

Relevant Pages

  • Potential improvement on delegation via explicit calls and super
    ... Derived classes sometimes need to delegate portions of the work in overridden ... def should_call(self, pos, supr): ... delegate(self, Der, Right.__init__) ...
    (comp.lang.python)
  • Re: whats wrong with "lambda x : print x/60,x%60"
    ... >> One perhaps needs to be a little more careful with instance variables, ... > def callback(): ... But this seems to defeat the main argument for removing lambda ...
    (comp.lang.python)
  • Re: Delegates and pointers
    ... > public static extern int ptRegisterLinkStateCallback(int callbackID, ... This, of course, means that you have to declare the delegate in your library and not the consuming program. ...
    (microsoft.public.dotnet.languages.csharp)
  • Re: beginners question about UI threading
    ... It is periodically polling another DB for updates and needs to inform my service when interesting changes have taken place. ... The component will call a callback function when a change is detected. ... IMHO the simplest way is to just create a producer/consumer queue, where the thread that should be executing the delegate is the consumer, and the threads that want the delegate to be executed are the producers. ...
    (microsoft.public.dotnet.languages.csharp)
  • Re: Cant find the DbAsyncResult type. Whats going on?
    ... There I learned that the delegate instance was lurking inside the System.IAsyncResult parameter of the callback method. ... delegate int MyDelegate; ... static void CallBack ...
    (microsoft.public.dotnet.languages.csharp)