My timing accuracy is related to the number of spin_once.

edit

I try to call service 'compute_rectangle_area' at a constant rate. Unfortunately, if I set the rate to 1 Hz, I have to do spin_once four times. If the time of repeats is smaller than four, the programming will get stuck.

If the time of repeats is bigger than four, the programming sleeps longer than expected.

#!/usr/bin/python3
from asyncio import Future
from functools import partial

import rclpy
from rclpy.node import Node
from robot_interfaces1.srv import ComputeRectangleArea

class ComputeRectangleAreaClientNode(Node):
    def __init__(self, node_name: str) -> None:
         super().__init__(node_name)
         self.client_ = self.create_client(ComputeRectangleArea,"compute_rectangle_area")

    def call_compute_rectangle_area_server(self,length:float,width:float)->bool:
         if(self.client_.wait_for_service(5)):
              msg_request = ComputeRectangleArea.Request()
              msg_request.length = length
              msg_request.width = width
              future = self.client_.call_async(msg_request)
              future.add_done_callback\
                   (partial(self.callback_call_compute_rectangle_area_server,length=length,width=width))
              return True
         else:
             self.get_logger().error("Server is not ready.")
             return False


    def callback_call_compute_rectangle_area_server \
          (self,future:Future,length:float,width:float):
          if future.exception() == None:
               response_:ComputeRectangleArea.Response = future.result()
               self.get_logger().info(f"{length} * {width} = {response_.area}.")

          else:
               self.get_logger().error(f"The {self.get_name()} returns {future.exception()}.")
def main(args=None):
       rclpy.init(args=args)
       n = ComputeRectangleAreaClientNode("compute_rectangle_area_client")
       loop_rate = n.create_rate(1)

       counter = 0
       while rclpy.ok():
               if(n.call_compute_rectangle_area_server(67.4,45.6)):
               n.get_logger().info(f"Call {counter} times.")
               counter += 1
               rclpy.spin_once(n)
               rclpy.spin_once(n)
               rclpy.spin_once(n)
               rclpy.spin_once(n)
               rclpy.spin_once(n)
               rclpy.spin_once(n)

      loop_rate.sleep()
rclpy.shutdown()

if __name__ == "__main__": main()