ROS Learning Notes

ROS Learning Notes 05

Keypoints

• ROS Client

ROS Client

• A client is a node that calls a service.
• Example: to spawn a turtle in turtlesim
  • cpp:
    • cpp file:

      #include <ros/ros.h>
      #include <turtlesim/Spawn.h>
      
      int main(int argc, char **argv)
      {
        // Initialize the ROS system and become a node.
        ros::init(argc, argv, "turtle_spawn");
        ros::NodeHandle nh;
      
        // Wait for the service to appear. Otherwise, the client will stop here.
        ros::service::waitForService("/spawn");  
        // Create a service client object. Name the client as add_turtle. <> is the type of service. "/spawn" is the name of service.
        ros::ServiceClient add_turtle = nh.serviceClient<turtlesim::Spawn>("/spawn");
      
        // Initialize the service object. Name it as srv.
        turtlesim::Spawn srv;
        srv.request.x = 2.0;
        srv.request.y = 2.0;
        srv.request.name = "Leo";
      
        // Call the service.
        ROS_INGO("Call service to spawn turtle Leo [x: %0.6f, y: %0.6f, name: %s]", srv.request.x, srv.request.y, srv.request.name.c_str());
        add_turtle.call(srv); // Wait for the response from the service. If the service is not available, the client will wait forever and stop here. So, client/server is synchronous.
      
        // Print the service response.
        ROS_INFO("Spawn turtle successfully [name: %s]", srv.response.name.c_str());
      
        return 0;
      }
      

    • add this to CMakeLists.txt

      add_executable(turtle_spawn src/turtle_spawn.cpp) ## compile the cpp file turtle_spawncpp into an executable file turtle_spawn in /rosleaning/devel/lib/learning_service/turtle_spawn
      target_link_libraries(turtle_spawn ${catkin_LIBRARIES}) ## link the executable file turtle_spawn with the libraries in catkin_LIBRARIES
      

    • catkin_make to compile the cpp file
    • source the workspace: source devel/setup.bash
    • roscore to start the master node
    • rosrun turtlesim turtlesim_node to start the turtlesim node
    • rosrun learning_service turtle_spawn to start the client node
  • python:
    • python file:

      #!/usr/bin/env python
      # -*- coding: utf-8 -*-
      
      import rospy
      import sys
      from turtlesim.srv import Spawn
      
      def turtle_spawn():
        # Initialize the ROS system and become a node.
        rospy.init_node('turtle_spawn')
      
        # Wait for the service to appear. Otherwise, the client will stop here.
        rospy.wait_for_service('/spawn')
        try:
            add_turtle = rospy.ServiceProxy('/spawn', Spawn)  # Create a service client object. Name the client as add_turtle. Spawn is the type of service. "/spawn" is the name of service.
      
            # Call the service. Input the parameters.
            response = add_turtle(2.0, 2.0, 0.0, "Leo")  # Wait for the response from the service. If the service is not available, the client will wait forever and stop here. So, client/server is synchronous.
            return response.name
        except rospy.ServiceException, e:
            print ("Service call failed: %s"%e)
      
      if __name__ == "__main__":
        print ("Spawn turtle successfully [name: %s]" % turtle_spawn())
      

    • Change the permission of the python file: chmod +x turtle_spawn.py or right click the file -> properties -> permissions -> allow executing file as program

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