1 写一个发布（Publiser）功能的Node
Node是连接到ROS网络的可执行程序，是ROS的一个术语。现在我们要创建一个node，它可以不断地广播消息。
首先打开我们的package，具体方法已经很熟练了，不再赘余。
1.1 源码
创建一个src文件夹，这个文件夹包含所有源文件。
好像本来就有了，不重复创建了。
在src文件夹里创建talker.cpp：
vim talker.cpp
然后粘贴以下内容：
#include "ros/ros.h"
#include "std_msgs/String.h"
#include
/**
* This tutorial demonstrates simple sending of messages over the ROS system.
*/
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. （advertise()函数是告诉ROS想要在哪个主题上发布的方式）
* 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("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;
}
1.2 源码解析
#include "ros/ros.h"
ros/ros.h 是一个简单的头文件包含方法，这样能把ROS里大部分常用的头文件都包含进来。
#include "std_msgs/String.h"
这个包含了std_msgs/String message，存在于std_msgs package内。这个头文件从package包内的String.msg文件自动生成的。
ros::init(argc, argv, "talker");
初始化ROS，这个现在还不重要，它允许ROS通过命令行进行名称重绘。这里也是我们可以指定node名称的地方，需要注意的是node名称必须唯一。
命名规则差不多就是变量的命名规则。
ros::NodeHandle n;
为正在运行的node创建handle。第一个handle就是用来初始化node的，最后一个会清除掉node所使用的所有资源。
ros::Publisher chatter_pub = n.advertise
告诉主线master我们将要推送一个类型为std_msgs::String的message到topic chatter。这个使master告诉所有node接听chatter，因为我们将会推送数据到这个topic。第二个参数是我们推送队列的大小。防止我们推送的太快了，所以建立一个1000的缓冲区。
NodeHandle::advertise() 返回一个ros::Publisher对象，有两个目的：1）它包含了一个publish()方法，可以用来推送消息到它建立的topic。2）当所有的这个对象都被销毁时，主题会自动的被回收。
ros::Rate loop_rate(10);
一个ros::Rate 对象允许你去制定一个循环运行的频率。它会根据
Rate::sleep()出现的位置，去消耗时间来使程序运行总时间满足设定的频率。
int count = 0;
while (ros::ok())
{
下列情况ros::ok()将会返回 false：
-接收到一个SIGNT（比如Ctrl+C时）
-我们被另一个同名node踢出了网络
-应用的另一部分调用了ros::shutdown()
-所有ros::NodeHandles都已经被销毁了
只要ros::ok()返回了FALSE，所有ROS调用都会失败。
std_msgs::String msg;
std::stringstream ss;
ss << "hello world " << count;
msg.data = ss.str();
我们使用一个消息适应类在ROS上广播了一条消息，通常从msg file 生成。更加复杂的类型也是可以的，不过现在我们就用标准的String类型message，只有一个成员：“data”
chatter_pub.publish(msg);
现在我们相当于把消息发给了连接过来的每一个人。
ROS_INFO("%s", msg.data.c_str());
ROS_INFO 和扩展的函数是替代 printf/cout函数的。
ros::spinOnce();
我们这个简单的程序其实没必要调用这个函数，因为我们没有接受任何回调。当我们需要在程序里增加一个订阅功能时，就必须用这个了，如果没有，你的回调就永远不会被调用。所以，保险起见还是加上这个。
loop_rate.sleep();
使用ros::Rate 对象使剩余时间睡眠来让我们进行10Hz的推送。
下面是运行流程的概要：
-初始化ROS系统
-声明我们要向主题发布消息
-循环使消息以10Hz推送给chatter主题。
2 写一个订阅node
2.1 源码
在 beginner_tutorials package里创建 src/listener.cpp 文件，然后输入以下内容。
vim src/listener.cpp
#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;
}
2.2 源码解析
上面讲过的片段就不再重复叙述了。
void chatterCallback(const std_msgs::String::ConstPtr& msg)
{
ROS_INFO("I heard: [%s]", msg->data.c_str());
}
这是一个回调函数，当有新的message发布到chatter topic上面，这个message会经过boost shared_ptr，也就是说如果有需要的话，你可以把它储存起来。
ros::Subscriber sub = n.subscribe("chatter", 1000, chatterCallback);
订阅chatter topic。当有新message到达的时候，ROS会调用chatterCallback。第二个参数就是队列大小，就是用来防止消息来的太快，作为缓冲区使用。
NodeHandle::subscribe() 返回一个ros::Subscriber对象，必须持续接收，除非退订topic。Subscriber 对象被销毁时，他会自动退订topic。
ros::spin();
ros::spin();进入一个循环，尽快调用message回调。不用担心的是，如果没什么工作要做，它不会占用太多CPU。当 ros::ok() 返回false的时候，它就会退出。或者手动跳出。还有替他方法跳出回调。
- There are other ways of pumping callbacks, but we won't worry about those here. The roscpp_tutorials package has some demo applications which demonstrate this. The roscpp overview also contains more information.*
概要：- 初始化ROS系统
- 订阅chatter topic
- Spin,等待message到达
- 当message到达时，调用 chatterCallback() 回调函数
3 编译自己的node
我们之前使用catkin_creat_pkg创建了一个package.xml和CMakeLists.txt文件。
生成的CMakeLists.txt看起来是这样的：
cmake_minimum_required(VERSION 2.8.3)
project(beginner_tutorials)
## Find catkin and any catkin packages
find_package(catkin REQUIRED COMPONENTS roscpp rospy std_msgs genmsg)
## Declare ROS messages and services
add_message_files(DIRECTORY msg FILES Num.msg)
add_service_files(DIRECTORY srv FILES AddTwoInts.srv)
## Generate added messages and services
generate_messages(DEPENDENCIES std_msgs)
## Declare a catkin package
catkin_package()
在CMakeLists.txt底部加入以下内容：
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)
这个将会创建两个可执行文件，talker和listener，默认将会放在devel文件夹里，位置在~/catkin_ws/devel/lib/
需要注意的是，你必须为message生成可执行程序添加依赖项目标：
add_dependencies(talker beginner_tutorials_generate_messages_cpp)
这确保了这个package 的message header在使用前被生成。如果你想在这个工作区里的其他package内使用这个message，你就需要把依赖放入他们各自的文件里，因为catkin是平行编译所有项目的。
你可以直接调用可执行程序或者通过rosrun调用。他们不会被放在 '
现在运行catkin_make:
# In your catkin workspace
$ catkin_make
如果你建立了新的pkg,你需要告诉catkin强制make，使用（--force-cmake）选项。
官网链接
下面我们来运行他们,记得开新窗口：
#新窗口
cd catkin_ws
source ./devel/setup.bash
rosrun beginner_tutorials talker
#新窗口
cd catkin_ws
source ./devel/setup.bash
rosrun beginner_tutorials listener