py/observer.py

   1 #!/usr/bin/env python
   2 from pprint import pformat
   3
   4 class Observer (object):
   5         '''Observers receive notify(*attributes) messages from their
   6         subjects whenever new data arrives.  This notify() message signifies
   7         that an observer should update its internal state/view.'''
   8
   9         def __init__ (self):
  10                 self.__attribute_adapter = {}
  11                 self.__subjects = {}
  12                 self.__debug = False
  13
  14         def set_debug (self, enabled):
  15                 self.__debug = enabled
  16
  17         def notify (self, *attributes):
  18                 '''Called by the model to notify Observers about changes.'''
  19                 # We can be notified about multiple attribute changes at once
  20                 for attr in attributes:
  21
  22                         if attr not in self.__subjects: continue
  23
  24                         # The model corresponding to attribute
  25                         model = self.__subjects[attr]
  26
  27                         # The new value for updating
  28                         value = model.getattr (attr)
  29
  30                         # Allow mapping from model to observer attributes
  31                         if attr in self.__attribute_adapter:
  32                                 attr = self.__attribute_adapter[attr]
  33
  34                         # Call the concrete observer's notification method
  35                         notify = model.get_notify()
  36                         model.set_notify (False)
  37
  38                         self.subject_changed (model, attr, value)
  39
  40                         model.set_notify (notify)
  41
  42                         if not self.__debug: continue
  43
  44                         print ("Objserver::notify ("
  45                                         + pformat (attributes) + "):")
  46                         print model, "\n"
  47
  48
  49         def subject_changed (self, model, attr, value):
  50                 '''This method handles updating of the observer/UI.
  51                 This must be implemented in each concrete observer class.'''
  52
  53                 msg = 'Concrete Observers must override subject_changed().'
  54                 raise NotImplementedError, msg
  55
  56         def add_subject (self, model, model_attr):
  57                 self.__subjects[model_attr] = model
  58
  59         def add_attribute_adapter (self, model_attr, observer_attr):
  60                 self.__attribute_adapter[model_attr] = observer_attr