Send a change event notification if the property is set to a
        value is not equal to ``old``.

        Args:
            obj (HasProps)
                The object the property is being set on.

            old (obj) :
                The previous value of the property to compare

        Returns:
            None

def trigger_if_changed(self, obj, old):
        ''' Send a change event notification if the property is set to a
        value is not equal to ``old``.

        Args:
            obj (HasProps)
                The object the property is being set on.

            old (obj) :
                The previous value of the property to compare

        Returns:
            None

        '''
        new_value = self.__get__(obj, obj.__class__)
        if not self.property.matches(old, new_value):
            self._trigger(obj, old, new_value)