ROS Topic

Notes about an ROS Topic. This will describe a basic publisher and subscriber topic. Where to find the list of topics and the list of possible messages for a topic. How to install a new message for a topic.

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rostopic list
rosmsg list
rostopic type <topic>
rostopic echo <topic>
rostopic info <topic>
rostopic hz <topic>

Publisher

Publish messages into a topic

Publish in Code

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import rospy
from geometry_msgs.msg import Twist

rospy.init_node('robot_mover')      # Create Publisher
pub = rospy.Publisher('/cmd_move', Twist, queue_size=1)     # Set the data type for the publisher
rate = rospy.Rate(2)                # 2hz
move = Twist()                      # Create a Twist object
move.linear.x = 0.5                 # Move forward on x 0.5
move.angular.z = 0.5                # Move at an anglular rate of 0.5

while not rospy.is_shutdown():
    pub.publish(move)               # Publish the move
    rate.sleep()                    # Maintain the refresh rate

Publish on Commandline

Manually publish in command line

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rostopic pub /cmd_move geometry_msgs/Twist "linear: x: 0.5 y: 0.0 z: 0.0 angular: x: 0.0 y: 0.0 z: 0.5"

rostopic pub <topic_name> <message_type> <value>

You can use the ‘double tab’ to autofil the value.

Subscriber

Read messages from a topic

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import rospy
from std_msgs.msg import Int32

def callback(msg)
    print(msg.data)

rospy.init_node('topic_subscriber')
sub = rospy.Subscriber('/counter', Int32, callback)
rospy.spin()

Determine the message type of the topic

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rostopic info /counter 

This will return back the data type, publishers and subscribers for the given topic.

Create a Custom Data Type

Create a directory in the package

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roscd my_new_package
mkdir msg

Create the Data type

Create a file within the new directory for the new data type Call it Vector.msg. Add the following in to the file

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float32 magnitude
float32 direction

Edit CMakeLists.txt

Edit CMakeLists.txt in the package to find the new data type. Look for find_package(catkin REQUIRED COMPONENTS …)

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find_package(catkin REQUIRED COMPONENTS
    rospy
    std_msgs
    message_generation
)

Again within CMakeLists.txt search for catkin_package(…) and uncomment the following line and append it if it contains other things passed CAKTIN_DEPENDS.

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catkin_package(
        CATKIN_DEPENDS message_runtime
)

Again within CMakeLists.txt search for add_message_files(…) and uncomment it and add the new messages.

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add_message_files(
    FILES
    Vector.msg
)

Again within CMakeLists.txt search for generate_messages(…) and uncomment it.

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generate_messages(
    DEPENDENCIES
    std_msgs
)

Save the CMakeLists.txt file.

Edit package.xml

Edit package.xml in the package to find our new data type. Find build_depend…

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<build_depend>message_generation</build_depend>
<run_depend>message_runtime</run_depend>

Compile Code

Go to your catkin workspace and run catkin_make

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cd catkin_ws
catkin_make

Verify Data Type Exist

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rosmsg show Vector

Stop Topic

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rosnode kill /cmd_move

All the details for this can be found in this video.