Observer pattern

Key 1: Spreading information to all listeners.

This is the basic pattern in which an object tells other objects about something interesting. It is very useful in GUI applications, pub/sub applications, and those applications where we need to notify a lot of loosely-coupled application components about a change occurring at one source node. In the following code, Subject is the object to which other objects register themselves for events via register_observer. The observer objects are the listening objects. The observers start observing the function that registers the observers object to Subject object. Whenever there is an event to Subject it cascades the event to all observers:

import weakref


class Subject(object):
    """Provider of notifications to other objects
    """

    def __init__(self, name):
        self.name = name
        self._observers = weakref.WeakSet()

    def register_observer(self, observer):
        """attach the observing object for this subject...