#
# Base class for observable objects.
#
class Observable(object):

    #
    # Ensures that the observable is initialized.
    # The user doesn't have to call the constructor explicitly.
    #
    def __ensure_init(self):

        try:
            self.__handlers
        except:
            self.__handlers = []



    def add_observer(self, observer):

        self.__ensure_init()
        self.__handlers.append(observer)



    def remove_observer(self, observer):

        self.__ensure_init()
        self.__handlers.remove(observer)



    def drop_observers(self):

        self.__ensure_init()
        self.__handlers = []



    def update_observer(self, *args):

        self.__ensure_init()

        for h in self.__handlers[:]:
            h(self, *args)