Topic

In ROS 2, topics are an essential communication method.

On this page, you will learn the following:

• message
• publisher, subscriber
• ros2 topic command
• ros2 interface command

Message

The information exchanged over topics is called a message.

You can freely define the type of a message.

For example, using msg/Bool from the std_msgs package allows you to communicate boolean values over a topic.

In minecraft_ros2, custom message types are defined in the minecraft_msgs package.

Publisher and Subscriber

The sender is called a publisher, and the receiver is called a subscriber.

In other words, when a publisher sends a message to a topic, any subscriber that listens to the same topic will receive it and perform some action.

ros2 topic Command

The ros2 topic command allows you to inspect and interact with topics.

Here, we introduce the frequently used subcommands: list, echo, info, and pub.

list

Running ros2 topic list shows a list of currently available topics.

With the -v option, you can see more detailed information.

For example, after launching minecraft_ros2 and joining the world, you should see output like this:

    $ ros2 topic list
/cmd_vel
/parameter_events
/player/image_raw
/player/imu
/player/nearby_living_entities
/player/pointcloud
/player/status
/rosout
/tf

echo

Running ros2 topic echo <topic_name> lets you monitor the data flowing through a topic.

For instance, after launching minecraft_ros2, try:

    $ ros2 topic echo /player/status
name: Dev
dimension: minecraft:overworld
food_level: 20
score: 0
.....

This shows the player's status.

info

Running ros2 topic info <topic_name> provides information about the publishers and subscribers of that topic.

    $ ros2 topic info /player/status
Type: minecraft_msgs/msg/PlayerStatus
Publisher count: 1
Subscription count: 0

pub

Running ros2 topic pub <topic_name> <type> <message> publishes a message to a topic.

For example, to send a velocity command of 1 m/s forward and 0.3 m/s to the left on /cmd_vel:

ros2 topic pub /cmd_vel geometry_msgs/msg/Twist "{linear: {x: 1.0, y: 0.3}}"

The message type geometry_msgs/msg/Twist represents translation and rotation in 3D. Here, linear.x corresponds to forward translation, and linear.y to leftward translation.

Since this command continues publishing, you can stop it with Ctrl + C. Then, publish an empty message to stop the movement.

If you omit the message argument, an empty message will be sent.

ros2 interface Command

The ros2 interface command provides information about message definitions. Commonly used subcommands are package and show.

package

Running ros2 interface package <package_name> lists the messages available in that package.

    $ ros2 interface package minecraft_msgs
minecraft_msgs/srv/Command
minecraft_msgs/msg/BlockArray
minecraft_msgs/msg/LivingEntity
minecraft_msgs/msg/LivingEntityArray
minecraft_msgs/msg/PlayerStatus
minecraft_msgs/msg/Item
minecraft_msgs/msg/Block
minecraft_msgs/msg/MobCategory

show

Running ros2 interface show <message_type> displays the structure of that message.

    $ ros2 interface show minecraft_msgs/msg/Item
string name
uint8 count
int16 damage
int16 max_damage