编写一个 ROS2 service程序，包含2个节点： 客户端调用一个名为把两个整数相乘的服务，该服务接受两个整数并返回它们的乘积。

编写ROS2 (Robot Operating System version 2) 的 service 程序涉及到几个关键步骤，这里是一个简化的示例，演示如何创建一个服务提供者和服务消费者。在这个例子中，我们将创建一个名为 `multiply_numbers` 的服务，它接收两个整数并返回它们的乘积。

首先，你需要安装 ROS2，并了解其包管理工具 `ros2 pkg` 和 `ros2 run`。

**1. 创建服务提供者（server_node.py）**

# server_node.py
import rclpy
from rclpy.node import Node
from rclpy.service import Service, ServiceException
from std_msgs.msg import Int32

class MultiplyNumbersService(Node):
    def __init__(self):
        super().__init__('multiply_numbers_server')
        self.service = self.create_service(
            Int32, 'multiply_numbers', self.multiply_callback)

    def multiply_callback(self, request, response):
        try:
            if not isinstance(request.a, int) or not isinstance(request.b, int):
                raise ServiceException('Both inputs must be integers.')
            result = request.a * request.b
            response.result = result
            return response
        except Exception as e:
            self.get_logger().error(f'Mulitply failed: {e}')
            return response

def main(args=None):
    rclpy.init(args=args)
    multiply_numbers_server = MultiplyNumbersService()
    rclpy.spin(multiply_numbers_server)
    rclpy.shutdown()

if __name__ == '__main__':
    main()

**2. 客户端调用（client_node.py）**

# client_node.py
import rclpy
from rclpy.node import Node
from rclpy.client import Client
from std_msgs.msg import Int32
from std_srvs.srv import Empty, MultiplyRequest, MultiplyResponse

class MultiplyNumbersClient(Node):
    def __init__(self):
        super().__init__('multiply_numbers_client')
        self.client = self.create_client(MultiplyRequest, 'multiply_numbers')

    def call_multiply_service(self, a, b):
        while not self.wait_for_service(timeout_sec=5.0):
            self.get_logger().info('Waiting for multiply_numbers service...')
        request = MultiplyRequest()
        request.a = a
        request.b = b
        future = self.call_async(request)
        rclpy.spin_until_future_complete(self, future)

        if future.result() is not None:
            response = future.result()
            print(f'Result of multiplication: {response.result}')
        else:
            self.get_logger().err('Service call failed.')

def main(args=None):
    rclpy.init(args=args)
    multiply_numbers_client = MultiplyNumbersClient()
    try:
        multiply_numbers_client.call_multiply_service(4, 5)
    except KeyboardInterrupt:
        pass
    finally:
        rclpy.shutdown()

if __name__ == '__main__':
    main()

运行这两个节点，客户端会向服务器发送请求，服务器接收到请求后计算两个数字的乘积并返回结果。