ROS学习手记 - 8 编写ROS的Publisher and Subscriber

上一节我们完成了 message & srv 文件的创建和加入编译，这次我们要玩简单的Publisher 和 Subscriber

要玩 Publisher 和 Subscriber， 需要具备的条件有哪些呢？先总结一下：

1.  创建并生成自己的Package，本次是 beginner_tutorials
2.  创建并生成ROS message & srv

详细版的手记要看我上传到百度文库的文件了：http://wenku.baidu.com/view/f872755a26fff705cc170ade
这里会写个总结~

1. Writing a Simple Publisher and Subscriber (C++)

   This tutorial covers how to write a publisher and subscriber node in C++.

ROS学习笔记10 - 编写编译和检验Service Node

百度文库：http://wenku.baidu.com/view/e14d5e18dd88d0d232d46a37

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 directories to your beginner_tutorialspackage you created in your catkin workspace previous tutorials:

cd ~/catkin_ws/src/beginner_tutorials

Please make sure you have followed the directions in the previous tutorialfor creating the service needed in this tutorial,creating the AddTwoInts.srv (be sure to choose the rightversion of build tool you're using at the top of wiki page in the link).

The Code

Create the src/add_two_ints_server.cpp file within the beginner_tutorialspackage and paste the following inside it:

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#include "ros/ros.h"

#include "beginner_tutorials/AddTwoInts.h"

 

bool add(beginner_tutorials::AddTwoInts::Request  &req,

        beginner_tutorials::AddTwoInts::Response &res)

{

  res.sum = req.a + req.b;

  ROS_INFO("request:x=%ld, y=%ld", (long int)req.a, (long int)req.b);

  ROS_INFO("sendingback response: [%ld]", (long int)res.sum);

  return true;

}

 

int main(int argc, char **argv)

{

  ros::init(argc, argv, "add_two_ints_server");

  ros::NodeHandle n;

 

  ros::ServiceServer service = n.advertiseService("add_two_ints",add);

  ROS_INFO("Readyto add two ints.");

  ros::spin();

 

  return 0;

}

The Code Explained

Now, let's break the code down.

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#include "ros/ros.h"

#include "beginner_tutorials/AddTwoInts.h"

 

beginner_tutorials/AddTwoInts.h is the header file generated fromthe srv file that we created earlier. 位置：~/catkin_ws/include/beginner_tutorials

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bool add(beginner_tutorials::AddTwoInts::Request &req,

        beginner_tutorials::AddTwoInts::Response &res)

This function provides the service for adding two ints,it takes in the request and response type defined in the srvfile(AddTwoInts.srv) and returns a boolean.

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{

  res.sum = req.a + req.b;

  ROS_INFO("request:x=%ld, y=%ld", (long int)req.a, (long int)req.b);

  ROS_INFO("sendingback response: [%ld]", (long int)res.sum);

  return true;

}

Here the two ints are added and stored in theresponse. Then some information about the request and response are logged.Finally the service returns true when it is complete.

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ros::ServiceServerservice = n.advertiseService("add_two_ints", add);

Here the service is created and advertised over ROS.
对比一下前一节publisher nodes的创建：

 ros::Publisher chatter_pub = n.advertise<std_msgs::String>("chatter", 1000);  //topic name “chatter”, bufferSize:1000

对比一下后面client相关声明语句：

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.

 

Writing the Client Node

The Code

Create the src/add_two_ints_client.cpp file within the beginner_tutorialspackage and paste the following inside it:

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#include "ros/ros.h"

#include "beginner_tutorials/AddTwoInts.h"

#include <cstdlib>

 

int main(int argc, char **argv)

{

  ros::init(argc, argv, "add_two_ints_client");

  if (argc != 3)

  {

    ROS_INFO("usage:add_two_ints_client X Y");

    return1;

  }

 

  ros::NodeHandle n;

  ros::ServiceClient client = n.serviceClient<beginner_tutorials::AddTwoInts>("add_two_ints");

  beginner_tutorials::AddTwoInts srv;

  srv.request.a= atoll(argv[1]);

  srv.request.b= atoll(argv[2]);

  if (client.call(srv))

  {

    ROS_INFO("Sum:%ld", (long int)srv.response.sum);

  }

  else

  {

    ROS_ERROR("Failedto call service add_two_ints");

    return1;

  }

 

  return 0;

}

The Code Explained

Now, let's break the code down.

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  ros::ServiceClient client = n.serviceClient<beginner_tutorials::AddTwoInts>("add_two_ints");

This creates a client for the add_two_intsservice. The ros::ServiceClient object is used to call the service later on.

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  beginner_tutorials::AddTwoInts srv;

  srv.request.a= atoll(argv[1]);

  srv.request.b= atoll(argv[2]);

Here we instantiate列举 an autogenerated service class, and assignvalues into its request member.Aservice class contains two members, requestand response.It alsocontains two class definitions, Requestand Response.

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  if (client.call(srv))

This actually calls the service. Since service calls are blocking, it willreturn once the call is done. If the service call succeeded, call()will return true and the value in srv.response willbe valid. If the call did not succeed, call() willreturn false and the value in srv.response will beinvalid.

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:

https://raw.github.com/ros/catkin_tutorials/master/create_package_srvclient/catkin_ws/src/beginner_tutorials/CMakeLists.txt

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add_executable(add_two_ints_server src/add_two_ints_server.cpp)

target_link_libraries(add_two_ints_server ${catkin_LIBRARIES})

add_dependencies(add_two_ints_server beginner_tutorials_gencpp)

 

add_executable(add_two_ints_client src/add_two_ints_client.cpp)

target_link_libraries(add_two_ints_client ${catkin_LIBRARIES})

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 intopackage directory of yourdevel space, located by default at ~/catkin_ws/devel/lib/share/<package name>. You can invoke executablesdirectly or you can use rosrun to invoke调用 them. They are not placed in '<prefix>/bin' because that would pollute thePATH when installing your package to the system. If you wish for your executableto be on the PATH at installation time, you can setup an install target, see:catkin/CMakeLists.txt

For more detailed discription 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.

Examining the Simple Service and Client

Running the Service

Let's start by running the service:

$ rosrunbeginner_tutorials add_two_ints_server     (C++)

$ rosrunbeginner_tutorials add_two_ints_server.py  (Python)

You should see something similar to:

Ready to add two ints.

Running the Client

Now let's run the client with the necessaryarguments:

$ rosrunbeginner_tutorials add_two_ints_client1 3     (C++)

$ rosrunbeginner_tutorials add_two_ints_client.py 1 3  (Python)

You should see something similar to:

Requesting 1+3

1 + 3 = 4

Now that you've successfully run your first serverand client, let's learn how torecord and play back data.

Further examples on Service and Client nodes

If you want to investigate further and get ahands-on example, you can get one here. A simple Client and Servicecombination shows the use of custom message types. The Service node is writtenin C++ while the Client is available in C++, Python and LISP.