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シンプルなサービスとクライアントを書く
Contents
Writing a Service Node
Here we'll create the service ("add_two_ints_server") node which will receive two ints and return the sum.
Change directory into the beginner_tutorials package, you created in the earlier tutorial, creating a package::
roscd beginner_tutorials
Please make sure you have followed the directions in the previous tutorial for creating the service needed in this tutorial, creating the AddTwoInts.srv.
The Code
Create the src/add_two_ints_server.cpp file within the beginner_tutorials package and paste the following inside it:
1 #include "ros/ros.h"
2 #include "beginner_tutorials/AddTwoInts.h"
3
4 bool add(beginner_tutorials::AddTwoInts::Request &req,
5 beginner_tutorials::AddTwoInts::Response &res )
6 {
7 res.sum = req.a + req.b;
8 ROS_INFO("request: x=%ld, y=%ld", (long int)req.a, (long int)req.b);
9 ROS_INFO("sending back response: [%ld]", (long int)res.sum);
10 return true;
11 }
12
13 int main(int argc, char **argv)
14 {
15 ros::init(argc, argv, "add_two_ints_server");
16 ros::NodeHandle n;
17
18 ros::ServiceServer service = n.advertiseService("add_two_ints", add);
19 ROS_INFO("Ready to add two ints.");
20 ros::spin();
21
22 return 0;
23 }
The Code Explained
Now, let's break the code down.
beginner_tutorials/AddTwoInts.h is the header file generated from the srv file that we created earlier.
This function provides the service for adding to ints, it takes in the request and response type defined in the srv file and returns a boolean.
Here the two ints are added and stored in the response. Then some information about the request and response are logged. Finally the service returns true when it is complete.
18 ros::ServiceServer service = n.advertiseService("add_two_ints", add);
Here the service is created and advertised over ROS.
Writing the Client Node
The Code
Create the src/add_two_ints_client.cpp file within the beginner_tutorials package and paste the following inside it:
1 #include "ros/ros.h"
2 #include "beginner_tutorials/AddTwoInts.h"
3 #include <cstdlib>
4
5 int main(int argc, char **argv)
6 {
7 ros::init(argc, argv, "add_two_ints_client");
8 if (argc != 3)
9 {
10 ROS_INFO("usage: add_two_ints_client X Y");
11 return 1;
12 }
13
14 ros::NodeHandle n;
15 ros::ServiceClient client = n.serviceClient<beginner_tutorials::AddTwoInts>("add_two_ints");
16 beginner_tutorials::AddTwoInts srv;
17 srv.request.a = atoll(argv[1]);
18 srv.request.b = atoll(argv[2]);
19 if (client.call(srv))
20 {
21 ROS_INFO("Sum: %ld", (long int)srv.response.sum);
22 }
23 else
24 {
25 ROS_ERROR("Failed to call service add_two_ints");
26 return 1;
27 }
28
29 return 0;
30 }
The Code Explained
Now, let's break the code down.
15 ros::ServiceClient client = n.serviceClient<beginner_tutorials::AddTwoInts>("add_two_ints");
This creates a client for the add_two_ints service. The ServiceClient object is used to call the service later on.
Here we instantiate an autogenerated service class, and assign values into its request member. A service class contains two members, "request" and "response". It also contains two class definitions, "Request" and "Response".
19 if (client.call(srv))
This actually calls the service. Since service calls are blocking, it will return once the call is done. If the service call succeeded, call() will return true and the value in srv.response will be valid. If the call did not succeed, call() will return false and the value in srv.response will be invalid.
Building your nodes
Again edit the beginner_tutorials CMakeLists.txt:
$ rosed beginner_tutorials CMakeLists.txt
and add the following at the end:
rosbuild_add_executable(add_two_ints_server src/add_two_ints_server.cpp) rosbuild_add_executable(add_two_ints_client src/add_two_ints_client.cpp)
This will create two executables, "add_two_ints_server" and "add_two_ints_client", which by default will go into the "bin" directory.
For more information on using CMake with ROS, see CMakeLists Now run make:
$ make