| Note: This tutorial assumes that you have completed the previous tutorials: examining the simple publisher and subscriber. |
 Please ask about problems and questions regarding this tutorial on answers.ros.org. Don't forget to include in your question the link to this page, the versions of your OS & ROS, and also add appropriate tags. |
Viết một Service và Client đơn giản (C++)
Description: Hướng dẫn làm thế nào để viết một node service và client đơn giản trong C++.Tutorial Level: BEGINNER
Next Tutorial: Examining the simple service and client
Contents
Tạo một node Service
Chúng ta sẽ tạo một node service ("add_two_ints_server") sẽ nhận hai đối số kiểu và trả về tổng.
Change directories to your beginner_tutorials package you created in your catkin workspace previous tutorials:
roscd beginner_tutorials
Hãy chắc rằng bạn làm theo hướng dẫn trước để tạo service cần trong hướng dẫn này, creating the AddTwoInts.srv (hãy chắc rằng chọn đúng phiên bản build tool với link ở đầu trang wiki).
Code
Tạo tập tin src/add_two_ints_server.cpp trong gói beginner_tutorials và thêm những dòng sau vào:
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 }
Giải thích Code
Bây giờ, phân tích từng đoạn mã.
1 #include "ros/ros.h"
2 #include "beginner_tutorials/AddTwoInts.h"
3
beginner_tutorials/AddTwoInts.h đây là tập tin header đã sinh ra từ tập tin srv trước đó.
4 bool add(beginner_tutorials::AddTwoInts::Request &req,
5 beginner_tutorials::AddTwoInts::Response &res)
Hàm này cung cấp service cho cộng hai số kiểu 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 ros::ServiceClient object is used to call the service later on.
16 beginner_tutorials::AddTwoInts srv;
17 srv.request.a = atoll(argv[1]);
18 srv.request.b = atoll(argv[2]);
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 located at ~/catkin_ws/src/beginner_tutorials/CMakeLists.txt and add the following at the end:
27 add_executable(add_two_ints_server src/add_two_ints_server.cpp)
28 target_link_libraries(add_two_ints_server ${catkin_LIBRARIES})
29 add_dependencies(add_two_ints_server beginner_tutorials_gencpp)
30
31 add_executable(add_two_ints_client src/add_two_ints_client.cpp)
32 target_link_libraries(add_two_ints_client ${catkin_LIBRARIES})
33 add_dependencies(add_two_ints_client beginner_tutorials_gencpp)
This will create two executables, add_two_ints_server and add_two_ints_client, which by default will go into package directory of your devel space, located by default at ~/catkin_ws/devel/lib/<package name>. You can invoke executables directly or you can use rosrun to invoke them. They are not placed in '<prefix>/bin' because that would pollute the PATH when installing your package to the system. If you wish for your executable to be on the PATH at installation time, you can setup an install target, see: catkin/CMakeLists.txt
For more detailed description of the CMakeLists.txt file see: catkin/CMakeLists.txt
Now run catkin_make:
# In your catkin workspace cd ~/catkin_ws catkin_make
If your build fails for some reason:
make sure you have followed the directions in the previous tutorial: creating the AddTwoInts.srv.
Now that you have written a simple service and client, let's examine the simple service and client.