ROS notes (06) - definition and use of topic messages

ROS Pass through std_msgs Encapsulates some native data types , such as : String、Int32、Int64、Char、Bool、Empty… however , These data generally contain only one data Field , The singleness of structure means the limitation of function , When transmitting some complex data , such as ： Lidar information … std_msgs Because of the poor description, it seems to be inadequate , In this scenario, you can use custom message types .

msgs Just a simple text file , Each row has a field type and a field name , The field types that can be used are ：

• int8, int16, int32, int64 ( Or unsigned type : uint*)
• float32, float64
• string
• time, duration
• other msg files
• variable-length array[] and fixed-length array[C]

ROS There is also a special type in ：Header , The header contains a timestamp and ROS Coordinate frame information commonly used in . Will often see msg The first line of the file has Header header .

1. Topic model

The topic model we want to implement is as follows ：
among Message Medium Person Message format customized for us , The message is passed between the subscriber and the publisher .

2. Custom message implementation

2.1 Definition msg file

In Feature Pack topic_demo Create one in the directory msg Folder , Create a... In this folder Person.msg file , As shown in the figure below ：

Person.msg There is our custom message format in

string name
uint8  age
uint8  gender

uint8 unknown = 0
uint8 male    = 1
uint8 female  = 2

2.2 Add Feature Pack dependencies

stay package.xml Add the following exec_depend Operational dependency ：

  <exec_depend>message_generation</exec_depend>  
  <exec_depend>message_runtime</exec_depend>

2.3 Add compile options

find_package(catkin REQUIRED COMPONENTS
  geometry_msgs
  rospy
  std_msgs
  turtlesim
  message_generation   #  The new content 

add_message_files(
   FILES
   Person.msg
 )

generate_messages(
   DEPENDENCIES
   std_msgs
 )

catkin_package(
# INCLUDE_DIRS include
# LIBRARIES topic_demo
   CATKIN_DEPENDS geometry_msgs rospy std_msgs turtlesim message_runtime
# DEPENDS system_lib
)

2.4 Compile and generate language related files

To the root directory of the project catkin_make command

$ catkin_make
Base path: /home/wohu/project/ros/ros_demo
Source space: /home/wohu/project/ros/ros_demo/src
Build space: /home/wohu/project/ros/ros_demo/build
Devel space: /home/wohu/project/ros/ros_demo/devel
Install space: /home/wohu/project/ros/ros_demo/install
####
#### Running command: "make cmake_check_build_system" in "/home/wohu/project/ros/ros_demo/build"
####
####
#### Running command: "make -j12 -l12" in "/home/wohu/project/ros/ros_demo/build"
####
[  0%] Built target std_msgs_generate_messages_py
[  0%] Built target std_msgs_generate_messages_eus
[  0%] Built target std_msgs_generate_messages_lisp
[  0%] Built target std_msgs_generate_messages_nodejs
[  0%] Built target std_msgs_generate_messages_cpp
[  0%] Built target _topic_demo_generate_messages_check_deps_Person
[ 57%] Built target topic_demo_generate_messages_py
[ 57%] Built target topic_demo_generate_messages_eus
[ 71%] Built target topic_demo_generate_messages_cpp
[ 85%] Built target topic_demo_generate_messages_nodejs
[100%] Built target topic_demo_generate_messages_lisp
[100%] Built target topic_demo_generate_messages

You can see that code files in various languages will be generated . among Python The relevant code files are in

/devel/lib/python2.7/dist-packages

Under the table of contents

2.5 Run code

Publisher code person_publisher.py :

#!/usr/bin/env python
# -*- coding: utf-8 -*-

#  The routine will publish /person_info topic of conversation , Custom message type learning_topic::Person

import rospy
from topic_demo.msg import Person

def velocity_publisher():
	# ROS Node initialization 
    rospy.init_node('person_publisher', anonymous=True)

	#  Create a Publisher, The release is called /person_info Of topic, The message type is learning_topic::Person, The queue length 10
    person_info_pub = rospy.Publisher('/person_info', Person, queue_size=10)

	# Set the frequency of the cycle 
    rate = rospy.Rate(10) 

    while not rospy.is_shutdown():
		#  initialization learning_topic::Person Type of message 
    	person_msg = Person()
    	person_msg.name = "Tom";
    	person_msg.age  = 18;
    	person_msg.gender  = Person.male;

		#  Release the news 
        person_info_pub.publish(person_msg)
    	rospy.loginfo("Publsh person message[%s, %d, %d]", 
				person_msg.name, person_msg.age, person_msg.gender)

		#  Delay according to the cycle frequency 
        rate.sleep()

if __name__ == '__main__':
    try:
        velocity_publisher()
    except rospy.ROSInterruptException:
        pass

Subscriber code person_subscriber.py Content ：

#!/usr/bin/env python
# -*- coding: utf-8 -*-

#  This routine will subscribe to /person_info topic of conversation , Custom message type learning_topic::Person

import rospy
from topic_demo.msg import Person

def personInfoCallback(msg):
    rospy.loginfo("Subcribe Person Info: name:%s age:%d gender:%d", 
			 msg.name, msg.age, msg.gender)

def person_subscriber():
	# ROS Node initialization 
    rospy.init_node('person_subscriber', anonymous=True)

	#  Create a Subscriber, The subscription is called /person_info Of topic, Register callback function personInfoCallback
    rospy.Subscriber("/person_info", Person, personInfoCallback)

	#  Loop waiting for callback function 
    rospy.spin()

if __name__ == '__main__':
    person_subscriber()

Run the code command ：

$ cd  ~/catkin_ws
$ catkin_make 
$ source ./devel/setup.bash
$ rescore
$ rosrun topic_demo person_subscriber.py
$ rosrun topic_demo person_publisher.py