ROS的初步学习（五）--自己写一个简单的发布（Publisher）、订阅(Subscriber)程序

1 写一个发布（Publisher）节点

节点（node）是连接到ROS网络中可执行的基本单元。我们在这创建一个发布者—“talker”节点，这个节点持续对外发布消息。

首先我们要把目录切换到我们的beginner_tutorials工程包中

$ cd ~/catkin_ws/src/beginner_tutorials

因为我们已经编译过这个工程包了，所以会在beginner_tutorials文件夹下看到CmakeList.txt、package.xml文件和include、src这两个目录。接下来进入src子目录

在src目录中创建一个talker.cpp文件，里面的内容如下：

#include "ros/ros.h"
#include "std_msgs/String.h"

#include <sstream>
int main(int argc, char **argv)
{
/**
* The ros::init() function needs to see argc and argv so that it can perform
* any ROS arguments and name remapping that were provided at the command line. For programmatic
* remappings you can use a different version of init() which takes remappings
* directly, but for most command-line programs, passing argc and argv is the easiest
* way to do it.  The third argument to init() is the name of the node.
*
* You must call one of the versions of ros::init() before using any other
* part of the ROS system.
*/
ros::init(argc, argv, "talker");

/**
* NodeHandle is the main access point to communications with the ROS system.
* The first NodeHandle constructed will fully initialize this node, and the last
* NodeHandle destructed will close down the node.
*/
ros::NodeHandle n;

/**
* The advertise() function is how you tell ROS that you want to
* publish on a given topic name. This invokes a call to the ROS
* master node, which keeps a registry of who is publishing and who
* is subscribing. After this advertise() call is made, the master
* node will notify anyone who is trying to subscribe to this topic name,
* and they will in turn negotiate a peer-to-peer connection with this
* node.  advertise() returns a Publisher object which allows you to
* publish messages on that topic through a call to publish().  Once
* all copies of the returned Publisher object are destroyed, the topic
* will be automatically unadvertised.
*
* The second parameter to advertise() is the size of the message queue
* used for publishing messages.  If messages are published more quickly
* than we can send them, the number here specifies how many messages to
* buffer up before throwing some away.
*/
ros::Publisher chatter_pub = n.advertise<std_msgs::String>("chatter", 1000);

ros::Rate loop_rate(10);

/**
* A count of how many messages we have sent. This is used to create
* a unique string for each message.
*/
int count = 0;
while (ros::ok())
{
/**
* This is a message object. You stuff it with data, and then publish it.
*/
std_msgs::String msg;

std::stringstream ss;
ss << "hello world " << count;
msg.data = ss.str();

ROS_INFO("%s", msg.data.c_str());

/**
* The publish() function is how you send messages. The parameter
* is the message object. The type of this object must agree with the type
* given as a template parameter to the advertise<>() call, as was done
* in the constructor above.
*/
chatter_pub.publish(msg);

ros::spinOnce();

loop_rate.sleep();
++count;
}

return 0;
}

2**写一个订阅（Subscriber）节点**

还是在src目录下，创建一个listener.cpp文件。内容如下：

#include "ros/ros.h"
#include "std_msgs/String.h"

/**
* This tutorial demonstrates simple receipt of messages over the ROS system.
*/
void chatterCallback(const std_msgs::String::ConstPtr& msg)
{
ROS_INFO("I heard: [%s]", msg->data.c_str());
}

int main(int argc, char **argv)
{
/**
* The ros::init() function needs to see argc and argv so that it can perform
* any ROS arguments and name remapping that were provided at the command line. For programmatic
* remappings you can use a different version of init() which takes remappings
* directly, but for most command-line programs, passing argc and argv is the easiest
* way to do it.  The third argument to init() is the name of the node.
*
* You must call one of the versions of ros::init() before using any other
* part of the ROS system.
*/
ros::init(argc, argv, "listener");

/**
* NodeHandle is the main access point to communications with the ROS system.
* The first NodeHandle constructed will fully initialize this node, and the last
* NodeHandle destructed will close down the node.
*/
ros::NodeHandle n;

/**
* The subscribe() call is how you tell ROS that you want to receive messages
* on a given topic.  This invokes a call to the ROS
* master node, which keeps a registry of who is publishing and who
* is subscribing.  Messages are passed to a callback function, here
* called chatterCallback.  subscribe() returns a Subscriber object that you
* must hold on to until you want to unsubscribe.  When all copies of the Subscriber
* object go out of scope, this callback will automatically be unsubscribed from
* this topic.
*
* The second parameter to the subscribe() function is the size of the message
* queue.  If messages are arriving faster than they are being processed, this
* is the number of messages that will be buffered up before beginning to throw
* away the oldest ones.
*/
ros::Subscriber sub = n.subscribe("chatter", 1000, chatterCallback);

/**
* ros::spin() will enter a loop, pumping callbacks.  With this version, all
* callbacks will be called from within this thread (the main one).  ros::spin()
* will exit when Ctrl-C is pressed, or the node is shutdown by the master.
*/
ros::spin();

return 0;
}

3 编译创建的节点

在编译我们创建的节点之前，我们还需要编辑Cmakelist.txt文件（注意：是beginner_tutorials项目包下的CMakelist文件），告诉编辑器我们需要编辑什么文件，需要什么依赖。

$ gedit CMakeLists.txt

在文件末尾添加如下语句：

include_directories(include ${catkin_INCLUDE_DIRS})

add_executable(talker src/talker.cpp)
target_link_libraries(talker ${catkin_LIBRARIES})
add_dependencies(talker beginner_tutorials_generate_messages_cpp)

add_executable(listener src/listener.cpp)
target_link_libraries(listener ${catkin_LIBRARIES})
add_dependencies(listener beginner_tutorials_generate_messages_cpp)

将目录切换到工作区目录，并执行catkin_make运行命令：

$ cd ~/catkin_ws
$ catkin_make

至此，程序已经创建完成，而接下来我们要检查一下我们创建的程序是否正确。

测试程序的正确性

首先，我们得要启动ROS核心程序

roscore

.

在使用我们的程序之前，需要先把程序注册

$ cd ~/catkin_ws
$ source ./devel/setup.bash

运行talker节点：

$ rosrun beginner_tutorials talker

出现如下图所示：



这就表示发布（Publisher）节点已经正确的运行了。

接下来运行listener节点：

$ rosrun beginner_tutorials listener

这说明订阅节点（listener）已经成功的接收到了

发布节点（talker）

发布的信息。至此，整个程序结束！