##########
# pattern_impl.py
##########
from installmethod import installmethod # the installmethod from recipe: 223613
class ObserverPattern:
"""
A reusable implementation of the Observer pattern.
"""
theSubject = None
observers = {}
class Subject:
def __init__(self):
self.observers = []
def attach(self, observer):
self.observers.append(observer)
def detach(self, observer):
self.observers.remove(observer)
def notify(self):
for observer in self.observers:
observer.update(self)
def decoration(self):
self.decorated_trigger()
self.notify()
class Observer:
def __init__(self, subject):
subject.attach(self)
def update(self, observer):
currentState = observer.get_current_state()
self.react_to_observation(currentState)
def specify_subject(self, subject):
self.theSubject = subject
self.make_generalization(subject, self.Subject)
def add_observer(self, observer):
self.observers[observer.__name__] = observer
self.make_generalization(observer, self.Observer)
def make_generalization(self, childClass, parentClass):
bases = list(childClass.__bases__)
bases.append(parentClass)
childClass.__bases__ = tuple(bases)
def make_observation(self, changeObservation, changeReaction):
func = getattr(self.theSubject, changeObservation)
installmethod(func, self.theSubject, "get_current_state")
for observer in self.observers.keys():
func = getattr(self.observers[observer], changeReaction)
installmethod(func, self.observers[observer], "react_to_observation")
def add_trigger(self, trigger):
func = getattr(self.theSubject, trigger)
installmethod(func, self.theSubject, "decorated_trigger")
func = getattr(self.theSubject, "decoration")
installmethod(func, self.theSubject, trigger)
##########
# example.py
##########
class ClockTimer:
def get_time(self):
# get current state of the subject
return self.currentTime
def tick(self):
# update internal time-keeping state
import time
self.currentTime = time.ctime()
class DigitalClock:
def draw(self, currentTime):
# display currentTime as a digital clock
print "DigitalClock: current time is", currentTime
class AnalogClock:
def draw(self, currentTime):
# display currentTime as an analog clock
print "AnalogClock: current time is", currentTime
if __name__ == '__main__':
from pattern_impl import ObserverPattern
observerPattern = ObserverPattern()
observerPattern.specify_subject(ClockTimer)
observerPattern.add_observer(DigitalClock)
observerPattern.add_observer(AnalogClock)
observerPattern.make_observation("get_time", "draw")
observerPattern.add_trigger("tick")
aTimer = ClockTimer()
dClock = DigitalClock(aTimer)
aClock = AnalogClock(aTimer)
import time
for i in range(10):
print "\nTick!"
aTimer.tick()
time.sleep(1)
