I'm confused with callback_groups and threads

edit

Hi,

I want to get a value from a topic and after perform some operations on the value retrieved (can't be in the callback function).

I imagine a subscriber and this simple loop (python)

while self.__var is None:
    self.get_logger().info("__var has no value yet.")
    # wait

On my first attempt, I used time.sleep for the "wait", which pause the whole Node, including the subscriber if I don't misunderstand. Then I used a callback_group to handle the subscription, it didn't work either. I read that time.sleep should never be used and prefer rate.sleep instead, inside another thread. So i tried this

def retrieve_value(self):
    self.__var = None
    # Spin in a separate thread
    thread = threading.Thread(target=rclpy.spin, args=(self, ), daemon=True)
    thread.start()
    self.get_logger().info("thread starts.")

    rate = self.create_rate(1)
    self.__sub_var = self.create_subscription(
        String,
        f'/var_values',
        self.__set_var_value,  # simply self.__var = msg.data
        10,
        callback_group=self.__listener_cb_grp
    )
    while self.__var is None:
        self.get_logger().info("¤¤¤__var has not value yet.")
        rate.sleep()

    # rclpy.shutdown()  # the node crash if uncomment and the node seem to stop when commented
    self.get_logger().info("thread stops.")
    thread.join()

Last I used a MultiThreadedExecutor and it still did not work. Obviously I don't understand the thread management here.

Can someone help me ?