cola/observer.py

   1 # Copyright (c) 2008 David Aguilar
   2 """This module provides the Observer design pattern.
   3 """
   4
   5 class Observer(object):
   6     """Implements the Observer pattern for a single subject"""
   7
   8     def __init__(self, model):
   9         self.model = model
  10         self.model.add_observer(self)
  11
  12     def __del__(self):
  13         self.model.remove_observer(self)
  14
  15     def notify(self, *attributes):
  16         """Called by the model to notify Observers about changes."""
  17         # We can be notified about multiple attribute changes at once
  18         model = self.model
  19         for attr in attributes:
  20             notify = model.notification_enabled
  21             model.notification_enabled = False # NOTIFY OFF
  22
  23             value = model.param(attr)
  24             self.subject_changed(attr, value)
  25
  26             model.notification_enabled = notify # NOTIFY ON
  27
  28     def subject_changed(self, attr, value):
  29         """
  30         Updates the observer/view
  31
  32         This must be implemented by concrete observers class.
  33         """
  34
  35         msg = 'Concrete Observers must override subject_changed().'
  36         raise NotImplementedError(msg)