You can use ``@broker.observer()`` as a decorator to your callback
        instead of :func:`Broker.add_observer`.

def observer(self, component_type=ComponentType):
        """
        You can use ``@broker.observer()`` as a decorator to your callback
        instead of :func:`Broker.add_observer`.
        """
        def inner(func):
            self.add_observer(func, component_type)
            return func
        return inner