This is a Python implementation of the observer pattern described by Gamma et. al. It defines a one-to many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.
The example should output: Setting Data 1 = 10 DecimalViewer: Subject Data 1 has data 10 HexViewer: Subject Data 1 has data 0xa Setting Data 2 = 15 HexViewer: Subject Data 2 has data 0xf DecimalViewer: Subject Data 2 has data 15 Setting Data 1 = 3 DecimalViewer: Subject Data 1 has data 3 HexViewer: Subject Data 1 has data 0x3 Setting Data 2 = 5 HexViewer: Subject Data 2 has data 0x5 DecimalViewer: Subject Data 2 has data 5 Detach HexViewer from data1 and data2. Setting Data 1 = 10 DecimalViewer: Subject Data 1 has data 10 Setting Data 2 = 15 DecimalViewer: Subject Data 2 has data 15
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Copy to clipboard1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 | class Subject:
def __init__(self):
self._observers = []
def attach(self, observer):
if not observer in self._observers:
self._observers.append(observer)
def detach(self, observer):
try:
self._observers.remove(observer)
except ValueError:
pass
def notify(self, modifier=None):
for observer in self._observers:
if modifier != observer:
observer.update(self)
# Example usage
class Data(Subject):
def __init__(self, name=''):
Subject.__init__(self)
self.name = name
self.data = 0
def setData(self, data):
self.data = data
self.notify()
def getData(self):
return self.data
class HexViewer:
def update(self, subject):
print 'HexViewer: Subject %s has data 0x%x' % (subject.name, subject.getData())
class DecimalViewer:
def update(self, subject):
print 'DecimalViewer: Subject %s has data %d' % (subject.name, subject.getData())
# Example usage...
def main():
data1 = Data('Data 1')
data2 = Data('Data 2')
view1 = DecimalViewer()
view2 = HexViewer()
data1.attach(view1)
data1.attach(view2)
data2.attach(view2)
data2.attach(view1)
print "Setting Data 1 = 10"
data1.setData(10)
print "Setting Data 2 = 15"
data2.setData(15)
print "Setting Data 1 = 3"
data1.setData(3)
print "Setting Data 2 = 5"
data2.setData(5)
print "Detach HexViewer from data1 and data2."
data1.detach(view2)
data2.detach(view2)
print "Setting Data 1 = 10"
data1.setData(10)
print "Setting Data 2 = 15"
data2.setData(15)
if __name__ == '__main__':
main()
|
The classes shown are meant for subclassing. The subject class is a bit modified in relation to the pattern desribed by Gamma et. al. When the notify method is called it also takes a modifier argument. This is convenient if you don't want an observer which has modified the subject to be updated again.
Excellent idea and solution. There is a way to make things even simpler and avoid having to call Notify & SetData (you can still call them if you prefer/need it). Python allows you to intercept the modification of attributes. :)