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1.流程

话题通信是ROS中使用频率最高的一种通信模式，话题通信是基于发布订阅模式的，也即:一个节点发布消息，另一个节点订阅该消息。在ROS中，实现话题通信只需要如下几步：

1. 确定要发布的数据类型，一般都需要自定义.msg文件，修改好CMakeLists.txt文件和package.xml文件并重编译
2. 编写发布方和订阅方的cpp文件，修改好CMakeLists.txt文件并重编译
3. 分别启动发布方节点和订阅方节点，顺序无所谓

2.自定义发布数据

在 ROS 通信协议中，数据载体是一个较为重要组成部分，ROS 中通过 std_msgs 封装了一些原生的数据类型。但是，这些数据一般只包含一个 data 字段，结构的单一意味着功能上的局限性，当传输一些复杂的数据， std_msgs 由于描述性较差而显得力不从心，这种场景下必须通过编写.msg文件来自定义消息类型。

2.1 std_msgs内置类型

• 内置类型与 C++ 和 Python 中的对应关系:

• 内置类型的数组与 C++ 和 Python 中的对应关系:

2.2 编写.msg文件

示例如下：

#文件名Person.msg
string name
uint16 age
float64 height

2.3 修改package.xml文件

• 查看是否存在如下编译依赖

<build_depend>message_generation</build_depend>

• 查看是否存在如下执行依赖

<exec_depend>message_generation</exec_depend>

2.3.1 完整的package.xml文件

<?xml version="1.0"?>
<package format="2">
  <name>chat</name>
  <version>0.0.0</version>
  <description>The chat package</description>

  <!-- One maintainer tag required, multiple allowed, one person per tag -->
  <!-- Example:  -->
  <!-- <maintainer email="jane.doe@example.com">Jane Doe</maintainer> -->
  <maintainer email="xu736946693@todo.todo">xu736946693</maintainer>


  <!-- One license tag required, multiple allowed, one license per tag -->
  <!-- Commonly used license strings: -->
  <!--   BSD, MIT, Boost Software License, GPLv2, GPLv3, LGPLv2.1, LGPLv3 -->
  <license>TODO</license>


  <!-- Url tags are optional, but multiple are allowed, one per tag -->
  <!-- Optional attribute type can be: website, bugtracker, or repository -->
  <!-- Example: -->
  <!-- <url type="website">http://wiki.ros.org/chat</url> -->


  <!-- Author tags are optional, multiple are allowed, one per tag -->
  <!-- Authors do not have to be maintainers, but could be -->
  <!-- Example: -->
  <!-- <author email="jane.doe@example.com">Jane Doe</author> -->


  <!-- The *depend tags are used to specify dependencies -->
  <!-- Dependencies can be catkin packages or system dependencies -->
  <!-- Examples: -->
  <!-- Use depend as a shortcut for packages that are both build and exec dependencies -->
  <!--   <depend>roscpp</depend> -->
  <!--   Note that this is equivalent to the following: -->
  <!--   <build_depend>roscpp</build_depend> -->
  <!--   <exec_depend>roscpp</exec_depend> -->
  <!-- Use build_depend for packages you need at compile time: -->
  <!--   <build_depend>message_generation</build_depend> -->
  <!-- Use build_export_depend for packages you need in order to build against this package: -->
  <!--   <build_export_depend>message_generation</build_export_depend> -->
  <!-- Use buildtool_depend for build tool packages: -->
  <!--   <buildtool_depend>catkin</buildtool_depend> -->
  <!-- Use exec_depend for packages you need at runtime: -->
  <!--   <exec_depend>message_runtime</exec_depend> -->
  <!-- Use test_depend for packages you need only for testing: -->
  <!--   <test_depend>gtest</test_depend> -->
  <!-- Use doc_depend for packages you need only for building documentation: -->
  <!--   <doc_depend>doxygen</doc_depend> -->
  <buildtool_depend>catkin</buildtool_depend>
  <build_depend>message_generation</build_depend>
  <build_depend>roscpp</build_depend>
  <build_depend>rospy</build_depend>
  <build_depend>std_msgs</build_depend>
  <build_export_depend>roscpp</build_export_depend>
  <build_export_depend>rospy</build_export_depend>
  <build_export_depend>std_msgs</build_export_depend>
  <exec_depend>message_generation</exec_depend>
  <exec_depend>roscpp</exec_depend>
  <exec_depend>rospy</exec_depend>
  <exec_depend>std_msgs</exec_depend>


  <!-- The export tag contains other, unspecified, tags -->
  <export>
    <!-- Other tools can request additional information be placed here -->

  </export>
</package>

2.4 修改CMakeLists.txt文件

2.4.1 修改find_package指令

# 需要加入 message_generation,必须有 std_msgs
find_package(catkin REQUIRED COMPONENTS
  roscpp
  rospy
  std_msgs
  message_generation
)

2.4.2 添加add_message_files指令

## 配置 msg 源文件
add_message_files(
  FILES
  Person.msg
)

2.4.3 添加generate_messages指令

generate_messages(
  DEPENDENCIES
  std_msgs
)

2.4.4 修改catkin_package指令

#执行时依赖
catkin_package(
#  INCLUDE_DIRS include
#  LIBRARIES demo02_talker_listener
  CATKIN_DEPENDS roscpp rospy std_msgs message_runtime
#  DEPENDS system_lib
)

其中，add_message_files指令必须要在generate_messages指令的前面，然后在工作空间目录编译即可。

2.5 查看头文件

经过以上几步，在${workspace}/devel/include/${package}/目录下应该会出现一个头文件，如图：

• 如果没有出现，是无法进行接下来的步骤的。这时，只需要把${workspace}/目录下的build目录和devel目录全部删除，然后重新编译即可。

rm -rf build/
rm -rf devel/
catkin_make

3.编写cpp文件

3.1 功能包目录文件树

3.2 修改CMakeLists.txt文件

3.2.1 添加add_executable指令

add_executable(publisher
        src/publisher.cpp)
add_executable(listener
        src/listener.cpp)

3.2.2 添加add_dependencies指令

add_dependencies(publisher ${PROJECT_NAME}_generate_messages_cpp)
add_dependencies(listener ${PROJECT_NAME}_generate_messages_cpp)

3.2.3 添加target_link_libraries指令

target_link_libraries(person_talker
  ${catkin_LIBRARIES}
)
target_link_libraries(person_listener
  ${catkin_LIBRARIES}
)

3.2.4 完整的CMakeLists.txt

• 其中很多语句都是catkin_make自动生成的

cmake_minimum_required(VERSION 3.0.2)
project(chat)

## Compile as C++11, supported in ROS Kinetic and newer
# add_compile_options(-std=c++11)

## Find catkin macros and libraries
## if COMPONENTS list like find_package(catkin REQUIRED COMPONENTS xyz)
## is used, also find other catkin packages
find_package(catkin REQUIRED COMPONENTS
  roscpp
  rospy
  std_msgs
  message_generation
)


## System dependencies are found with CMake's conventions
# find_package(Boost REQUIRED COMPONENTS system)


## Uncomment this if the package has a setup.py. This macro ensures
## modules and global scripts declared therein get installed
## See http://ros.org/doc/api/catkin/html/user_guide/setup_dot_py.html
# catkin_python_setup()

################################################
## Declare ROS messages, services and actions ##
################################################

## To declare and build messages, services or actions from within this
## package, follow these steps:
## * Let MSG_DEP_SET be the set of packages whose message types you use in
##   your messages/services/actions (e.g. std_msgs, actionlib_msgs, ...).
## * In the file package.xml:
##   * add a build_depend tag for "message_generation"
##   * add a build_depend and a exec_depend tag for each package in MSG_DEP_SET
##   * If MSG_DEP_SET isn't empty the following dependency has been pulled in
##     but can be declared for certainty nonetheless:
##     * add a exec_depend tag for "message_runtime"
## * In this file (CMakeLists.txt):
##   * add "message_generation" and every package in MSG_DEP_SET to
##     find_package(catkin REQUIRED COMPONENTS ...)
##   * add "message_runtime" and every package in MSG_DEP_SET to
##     catkin_package(CATKIN_DEPENDS ...)
##   * uncomment the add_*_files sections below as needed
##     and list every .msg/.srv/.action file to be processed
##   * uncomment the generate_messages entry below
##   * add every package in MSG_DEP_SET to generate_messages(DEPENDENCIES ...)

# Generate messages in the 'msg' folder
 add_message_files(
   FILES
   Person.msg
 )

## Generate services in the 'srv' folder
# add_service_files(
#   FILES
#   Service1.srv
#   Service2.srv
# )

## Generate actions in the 'action' folder
# add_action_files(
#   FILES
#   Action1.action
#   Action2.action
# )

## Generate added messages and services with any dependencies listed here
# 生成消息时依赖于 std_msgs
generate_messages(
        DEPENDENCIES
        std_msgs
)


################################################
## Declare ROS dynamic reconfigure parameters ##
################################################

## To declare and build dynamic reconfigure parameters within this
## package, follow these steps:
## * In the file package.xml:
##   * add a build_depend and a exec_depend tag for "dynamic_reconfigure"
## * In this file (CMakeLists.txt):
##   * add "dynamic_reconfigure" to
##     find_package(catkin REQUIRED COMPONENTS ...)
##   * uncomment the "generate_dynamic_reconfigure_options" section below
##     and list every .cfg file to be processed

## Generate dynamic reconfigure parameters in the 'cfg' folder
# generate_dynamic_reconfigure_options(
#   cfg/DynReconf1.cfg
#   cfg/DynReconf2.cfg
# )

###################################
## catkin specific configuration ##
###################################
## The catkin_package macro generates cmake config files for your package
## Declare things to be passed to dependent projects
## INCLUDE_DIRS: uncomment this if your package contains header files
## LIBRARIES: libraries you create in this project that dependent projects also need
## CATKIN_DEPENDS: catkin_packages dependent projects also need
## DEPENDS: system dependencies of this project that dependent projects also need
#执行时依赖
catkin_package(
        #  INCLUDE_DIRS include
        #  LIBRARIES demo02_talker_listener
        CATKIN_DEPENDS roscpp rospy std_msgs message_runtime
        #  DEPENDS system_lib
)


###########
## Build ##
###########

## Specify additional locations of header files
## Your package locations should be listed before other locations
include_directories(
# include
  ${catkin_INCLUDE_DIRS}
)

## Declare a C++ library
# add_library(${PROJECT_NAME}
#   src/${PROJECT_NAME}/pkg1.cpp
# )

## Add cmake target dependencies of the library
## as an example, code may need to be generated before libraries
## either from message generation or dynamic reconfigure
# add_dependencies(${PROJECT_NAME} ${${PROJECT_NAME}_EXPORTED_TARGETS} ${catkin_EXPORTED_TARGETS})

## Declare a C++ executable
## With catkin_make all packages are built within a single CMake context
## The recommended prefix ensures that target names across packages don't collide
# add_executable(${PROJECT_NAME}_node src/pkg1_node.cpp)
add_executable(publisher
        src/publisher.cpp)
add_executable(listener
        src/listener.cpp)

## Rename C++ executable without prefix
## The above recommended prefix causes long target names, the following renames the
## target back to the shorter version for ease of user use
## e.g. "rosrun someones_pkg node" instead of "rosrun someones_pkg someones_pkg_node"
# set_target_properties(${PROJECT_NAME}_node PROPERTIES OUTPUT_NAME node PREFIX "")

## Add cmake target dependencies of the executable
## same as for the library above
# add_dependencies(${PROJECT_NAME}_node ${${PROJECT_NAME}_EXPORTED_TARGETS} ${catkin_EXPORTED_TARGETS})
add_dependencies(publisher ${PROJECT_NAME}_generate_messages_cpp)
add_dependencies(listener ${PROJECT_NAME}_generate_messages_cpp)

## Specify libraries to link a library or executable target against
# target_link_libraries(${PROJECT_NAME}_node
#   ${catkin_LIBRARIES}
# )

#############
## Install ##
#############

# all install targets should use catkin DESTINATION variables
# See http://ros.org/doc/api/catkin/html/adv_user_guide/variables.html

## Mark executable scripts (Python etc.) for installation
## in contrast to setup.py, you can choose the destination
# catkin_install_python(PROGRAMS
#   scripts/my_python_script
#   DESTINATION ${CATKIN_PACKAGE_BIN_DESTINATION}
# )

## Mark executables for installation
## See http://docs.ros.org/melodic/api/catkin/html/howto/format1/building_executables.html
# install(TARGETS ${PROJECT_NAME}_node
#   RUNTIME DESTINATION ${CATKIN_PACKAGE_BIN_DESTINATION}
# )

## Mark libraries for installation
## See http://docs.ros.org/melodic/api/catkin/html/howto/format1/building_libraries.html
# install(TARGETS ${PROJECT_NAME}
#   ARCHIVE DESTINATION ${CATKIN_PACKAGE_LIB_DESTINATION}
#   LIBRARY DESTINATION ${CATKIN_PACKAGE_LIB_DESTINATION}
#   RUNTIME DESTINATION ${CATKIN_GLOBAL_BIN_DESTINATION}
# )

## Mark cpp header files for installation
# install(DIRECTORY include/${PROJECT_NAME}/
#   DESTINATION ${CATKIN_PACKAGE_INCLUDE_DESTINATION}
#   FILES_MATCHING PATTERN "*.h"
#   PATTERN ".svn" EXCLUDE
# )

## Mark other files for installation (e.g. launch and bag files, etc.)
# install(FILES
#   # myfile1
#   # myfile2
#   DESTINATION ${CATKIN_PACKAGE_SHARE_DESTINATION}
# )

#############
## Testing ##
#############

## Add gtest based cpp test target and link libraries
# catkin_add_gtest(${PROJECT_NAME}-test test/test_pkg1.cpp)
# if(TARGET ${PROJECT_NAME}-test)
#   target_link_libraries(${PROJECT_NAME}-test ${PROJECT_NAME})
# endif()
target_link_libraries(publisher
        ${catkin_LIBRARIES}
        )

target_link_libraries(listener
        ${catkin_LIBRARIES}
        )

## Add folders to be run by python nosetests
# catkin_add_nosetests(test)

3.3 发布方cpp

示例如下：

/*
    需求: 实现基本的话题通信，一方发布数据，一方接收数据，
         实现的关键点:
         1.发送方
         2.接收方
         3.数据

         PS: 二者需要设置相同的话题


    消息发布方:
        循环发布信息:HelloWorld 后缀数字编号

    实现流程:
        1.包含头文件
        2.初始化 ROS 节点:命名(唯一)
        3.实例化 ROS 句柄
        4.实例化 发布者 对象
        5.组织被发布的数据，并编写逻辑发布数据

*/
// 1.包含头文件
#include "ros/ros.h"
#include "chat/Person.h"

int main(int argc, char  *argv[])
{
    //设置编码
    setlocale(LC_ALL,"");

    //2.初始化 ROS 节点:命名(唯一)
    // 参数1和参数2 后期为节点传值会使用
    // 参数3 是节点名称，是一个标识符，需要保证运行后，在 ROS 网络拓扑中唯一
    ros::init(argc,argv,"talker");
    //3.实例化 ROS 句柄
    ros::NodeHandle nh;//该类封装了 ROS 中的一些常用功能

    //4.实例化 发布者 对象
    //泛型: 发布的消息类型
    //参数1: 要发布到的话题
    //参数2: 队列中最大保存的消息数，超出此阀值时，先进的先销毁(时间早的先销毁)
    ros::Publisher pub = nh.advertise<chat::Person>("chatter",10);

    //5.组织被发布的数据，并编写逻辑发布数据
    //数据(动态组织)
    chat::Person p;
    p.name = "sunwukong";
    p.age = 2000;
    p.height = 1.45;

    //逻辑(一秒1次)
    ros::Rate r(1);

    //节点不死
    while (ros::ok())
    {
        //发布消息
        pub.publish(p);
        //加入调试，打印发送的消息
        ROS_INFO("我叫:%s,今年%d岁,高%.2f米", p.name.c_str(), p.age, p.height);p.age++;
        //根据前面制定的发送贫频率自动休眠 休眠时间 = 1/频率；
        r.sleep();
        //暂无应用
        ros::spinOnce();
    }


    return 0;
}

3.4 订阅方cpp

示例如下：

/*
    需求: 实现基本的话题通信，一方发布数据，一方接收数据，
         实现的关键点:
         1.发送方
         2.接收方
         3.数据


    消息订阅方:
        订阅话题并打印接收到的消息

    实现流程:
        1.包含头文件
        2.初始化 ROS 节点:命名(唯一)
        3.实例化 ROS 句柄
        4.实例化 订阅者 对象
        5.处理订阅的消息(回调函数)
        6.设置循环调用回调函数

*/
// 1.包含头文件
#include "ros/ros.h"
#include "chat/Person.h"

void doMsg(const chat::Person::ConstPtr& person_p){
    ROS_INFO("订阅的人信息:%s, %d, %.2f", person_p->name.c_str(), person_p->age, person_p->height);
}
int main(int argc, char  *argv[])
{
    //设置编码
    setlocale(LC_ALL,"");
    //2.初始化 ROS 节点:命名(唯一)
    ros::init(argc,argv,"listener");
    //3.实例化 ROS 句柄
    ros::NodeHandle nh;

    //4.实例化 订阅者 对象
    ros::Subscriber sub = nh.subscribe<chat::Person>("chatter",10,doMsg);
    //5.处理订阅的消息(回调函数)

    //     6.设置循环调用回调函数
    ros::spin();//循环读取接收的数据，并调用回调函数处理

    return 0;
}

4.效果

• 如果找不到可执行文件，就把工作目录下的build文件夹和devel文件夹删了重新编译。

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