def observed(cls, _func):
        """
        Decorate methods to be observable. If they are called on an instance
        stored in a property, the model will emit before and after
        notifications.
        """

        def wrapper(*args, **kwargs):
            self = args[0]
            assert(isinstance(self, Observable))

            self._notify_method_before(self, _func.__name__, args, kwargs)
            res = _func(*args, **kwargs)
            self._notify_method_after(self, _func.__name__, res, args, kwargs)
            return res

        return wrapper