Mastering ROS for Robotics Programming（Second Edition）

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Building the nodes

We have to edit the CMakeLists.txt file in the package to compile and build the source code. Navigate to mastering_ros_demo_pkg to view the existing CMakeLists.txt file. The following code snippet in this file is responsible for building these two nodes:

include_directories( 
   include 
   ${catkin_INCLUDE_DIRS} 
   ${Boost_INCLUDE_DIRS} 
) 
#This will create executables of the nodes 
add_executable(demo_topic_publisher src/demo_topic_publisher.cpp) 
add_executable(demo_topic_subscriber src/demo_topic_subscriber.cpp) 
 
#This will generate message header file before building the target 
add_dependencies(demo_topic_publisher mastering_ros_demo_pkg_generate_messages_cpp) 
add_dependencies(demo_topic_subscriber mastering_ros_demo_pkg_generate_messages_cpp) 
 
#This will link executables to the appropriate libraries  
target_link_libraries(demo_topic_publisher ${catkin_LIBRARIES}) 
target_link_libraries(demo_topic_subscriber ${catkin_LIBRARIES})

We can add the preceding snippet to create a new a CMakeLists.txt file for compiling the two codes.

The catkin_make command is used to build the package. We can first switch to a workspace:

$ cd ~/catkin_ws

Build mastering_ros_demo_package as follows:

$ catkin_make

We can either use the preceding command to build the entire workspace, or use the the -DCATKIN_WHITELIST_PACKAGES option. With this option, it is possible to set one or more packages to compile:

$ catkin_make -DCATKIN_WHITELIST_PACKAGES="pkg1,pkg2,..."

Note that is necessary to revert this configuration to compile other packages or the entire workspace. This can be done using the following command:

$ catkin_make -DCATKIN_WHITELIST_PACKAGES=""

If the building is done, we can execute the nodes. First, start roscore:

$ roscore

Now run both commands in two shells. In the running publisher:

$ rosrun mastering_ros_demo_package demo_topic_publisher

In the running subscriber:

$ rosrun mastering_ros_demo_package demo_topic_subscriber

We can see the output as shown here:

Figure 2: Running topic publisher and subscriber

The following diagram shows how the nodes communicate with each other. We can see that the demo_topic_publisher node publishes the /numbers topic and then subscribes to the demo_topic_subscriber node:

Figure 3: Graph of the communication between publisher and subscriber nodes

We can use the rosnode and rostopic tools to debug and understand the working of two nodes:

$ rosnode list: This will list the active nodes.
$ rosnode info demo_topic_publisher: This will get the info of the publisher node.
$ rostopic echo /numbers: This will display the value sending through the /numbers topic.
$ rostopic type /numbers: This will print the message type of the /numbers topic.