ROS message

1. Simply put, the message of ROS is similar to the structure of c language. The structure contains all the standard data types and can be nested between messages. ROS brings a series of messages, such as std_msgs. It can also be user-defined. The data types supported by messages are as follows, and between c++ language and python language. Correspondence.

Message data type	serialize	Corresponding C++
bool	uint8_t	bool
int8	int8_t	int
uint8	uint8_t	int
int16	int16_t	int
uint16	uint16_t	int
int32	int32_t	int
uint32	uint32_t	int
int64	int64_t	long
uint64	uint64_t	long
float32	float	float
float64	double	double
string	std::string	string
time	ros::Time	rospy.Time
duration	ros::Duration	rospy.Duration

Custom message

1. In the first program of ROS, we used the first message of ROS, the String message of std_msgs. So, how do we define a message ourselves?
For example: define person, name, age, gender.
2. In the previous project, we can create a msg folder under the ros_test folder. Note that the case is exactly the same. Create a new person.msg file under the MSG folder.

Create a custom person-type message in the following way. You can define variables or constants.

string name   #variable
uint8 age     #variable
uint8 sex     #variable

uint8 male = 0    #constant
uint8 female = 1  #constant

Generate message

Modify package.xml file

Added to the <-package> node of package.xml for automatic message generation.

  <!--Custom message type, function package dependency-->
  <build_depend>message_generation</build_depend>
  <exec_depend>message_runtime</exec_depend>

Modify the CMakeLists.txt file

1, find_package adds:

find_package(catkin REQUIRED COMPONENTS
  roscpp
  std_msgs
  message_generation #Custom message type dependency
  }

2, add_message_files, add person.msg file here

add_message_files(
 FILES
 person.msg
)

3. Add generate_messages.

generate_messages(
  DEPENDENCIES
  std_msgs   #Custom message dependency
  )

4. Add catkin_package.

catkin_package(
INCLUDE_DIRS include
LIBRARIES ros_test
CATKIN_DEPENDS roscpp std_msgs message_runtime  #Custom dependencies
DEPENDS system_lib
)

5. Use catkin_make in the terminal of vscode to generate person message. When the generation is completed, you can view the generated person.h file in the devel/include/ros_test folder.

Create Publishers and Subscribers

1. Create under the src folder, personTalker.cpp and personListener.cpp files.
2. Paste the following code in personTalker.cpp.

#include <iostream>
#include "ros/ros.h"
#include "ros_test/person.h" //Automatic generation
int main(int argc, char **argv)
{
    setlocale(LC_ALL, ""); //Chinese support
    //Ros node initializes the "personTalker" node name, and two are not allowed in ROS at the same time.
    ros::init(argc,argv,"personTalker");
    //Create node handles
    ros::NodeHandle h;
    //Create a publisher, topic:personChat, message type ros_test::person
    ros::Publisher chatter_pub = h.advertise<ros_test::person>("personChat",1000); // "personChat" topic name, message caching
    //Setting the frequency of a single cycle
    ros::Rate looprate(10);

    while (ros::ok())
    {
        ros_test::person per;
        per.name = "Mu Shi Ning soul";
        per.age = 20;
        per.sex = per.male;
        chatter_pub.publish(per);
        //Waiting for callback function
        ros::spinOnce();
        //Follow the previous cycle
        looprate.sleep();
    }
    
}

3. Paste the following code in personTalker.cpp.

#include <ros/ros.h>
#include "ros_test/person.h"
#include "std_msgs/String.h"

void chatterCallBack(const ros_test::person::ConstPtr &msg)
{

    if(msg->sex==msg->male)
    {
        ROS_INFO("I get a man : name [%s], age[%d]}",msg->name.c_str(), msg->age);
    }
    else
    {
        ROS_INFO("I get a woman: name [%s], age[%d]}",msg->name.c_str(), msg->age);
    }
    
}

int main(int argc, char **argv)
{
    setlocale(LC_ALL, "");
    //Ros node initializes the "personListener" node name, and two are not allowed in ROS at the same time.
    ros::init(argc,argv,"personListener");
    //Create node handles
    ros::NodeHandle h;

    //Create a subscriber, topic:chatter, message type ros_test::person, receive the message, and respond to chatterCallBack
    ros::Subscriber listener_sub = h.subscribe<ros_test::person>("personChat",1000,chatterCallBack);
    
    //Loop waiting callback function
    ros::spin();

    return 0;
}

4. For specific explanations, please see the explanations.
5. Modify the CMakeLists.txt file

add_executable(personListener src/personListener.cpp)
target_link_libraries(personListener ${catkin_LIBRARIES})
add_dependencies(personListener ${PROJECT_NAME}_gen_cpp) #Contains custom message type customization

add_executable(personTalker src/personTalker.cpp)
target_link_libraries(personTalker ${catkin_LIBRARIES})
add_dependencies(personTalker ${PROJECT_NAME}_gen_cpp) #Contains custom message type customization

Compile and run

Using catkin_make in the terminal of vscode, compile the whole project, use the shortcut key Ctrl + Shift + T, pop up the system terminal, start the roscore, run the personListener and personTalker nodes, test results:

Epilogue

More ROS-related robots, public-public-number search "robot station".