本系列教程作者：小鱼 公众号：鱼香ROS QQ交流群：139707339 教学视频地址：小鱼的B站 完整文档地址：鱼香ROS官网 版权声明：如非允许禁止转载与商业用途。 
大家好，我是小鱼。上一章节中，我们采用了面向对象的方法创建了作家李四和单身狗王二，本节课我们就通过ROS2的话题机制，让作家李四发布小说，让单身狗王二订阅小说并付费。
1.背景故事李四是一名擅长写小说的作家，既然是作家，肯定要写文章才行，所以李四就写了一本带颜色的小说《艳娘传奇》。
王二是一名单身多年的单身狗,单身狗没啥爱好，但是《艳娘传奇》肯定是必看的（单身多年的你肯定懂）。
所以我们就要想办法让王二可以看到李四写的小说，有什么好的办法吗？
办法就是ROS2的话题机制。
我们让王二（Node）通过订阅（Subscribe）李四（Node）发布（Publish）的《艳娘传奇》（Topic）来欣赏小说的。
2.话题介绍故事其实就是讲了一下ROS2中Topic通信方式，Topic通信模型是一种发布订阅模型。
2.1 话题的发布订阅模型这里的王二和李四两个节点，通过话题来互相通信（传递数据）。
李四节点会创建一个发布者（Publisher）来发布一个话题（艳娘传奇,小鱼取个英文名叫sexy_girl）。单身汉王二节点，他创建了一个订阅者（Subscriber）来订阅李四发布的话题sexy_girl。
那艳娘传奇的内容是什么呢？我们暂且规定为由文字组成的字符串（连插图都没的那种）。
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发布
订阅
Node李四
艳娘传奇
Node王二
李四王二通信模型是一个一对一（一个发布者，一个订阅者）的模型，除此之外ROS2中话题通信其实还可以是1对n,n对1,n对n的。
需要满足以下规则：
- 话题名字是关键,发布订阅接口类型要相同，发布的是字符串，接受也要用字符串来接收;
- 同一个人(节点)可以订阅多个话题，同时也可以发布多个话题，就像一本书的作者也可以是另外一本书的读者;
- 同一个小说不能有多个作者（版权问题），但跟小说不一样，同一个话题可以有多个发布者。
ROS2作为一个强大的工具，在运行过程中，我们是可以通过命令来看到节点和节点之间的数据关系的。
运行我们第二章中的你说我听小demo。依次打开三个终端，分别输入下面三个命令。
ros2 run demo_nodes_py listener
ros2 run demo_nodes_cpp talker
rqt_graph
你将看到下面这张图
你可以尝试改变菜单栏的选项，看一看下面图的变化，感受一下rqt_graph工具的强大。
这是一个很重要的工具，小鱼在学习和使用ROS2的过程中经常会用到它，来看一看数据到底是怎么走的，它可以帮我们搞清楚一个节点的输入和输出是什么。
3.2 ROS2话题相关命令行界面（CLI）工具还记得上一节的ros2 node指令吗？ros2也支持很多强大的topic指令。可以使用下面的指令查看。
ros2 topic -h
本着学以致用的目的，小鱼先对比较常用的几个命令进行介绍，其他的我们用到的时候小鱼再介绍（现在介绍反而不好理解）。
3.2.1 ros2 topic list 返回系统中当前活动的所有主题的列表命令
ros2 topic list
结果
命令
ros2 topic list -t
结果
命令
ros2 topic echo /chatter
结果
命令
ros2 topic info /chatter
结果
上面一个指令告诉大家这个消息是std_msgs/msg/String，那String里面有什么呢？不妨来试一试。
命令
ros2 interface show std_msgs/msg/String
结果
关闭发布者，我们受到来发布
命令
ros2 topic pub /chatter std_msgs/msg/String 'data: "123"'
结果
了解完话题，下一节小鱼就会带大家来手动写Python版本的话题通信。
参考链接：Understanding ROS 2 topics — ROS 2 Documentation: Foxy documentation
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