Example of using custom messages in the ros project

[mjcarroll]

No description provided.

[jrutgeer]

There is this part:

gz_msgs_generate_messages(
  # The cmake target to be generated for libraries/executables to link
  TARGET msgs

And then further on:

target_link_libraries(FullSystem PRIVATE
  gz-sim${GZ_SIM_VER}::gz-sim${GZ_SIM_VER}
  ros_gz_example_gazebo-msgs
)

Doest that mean that the TARGET name specified in the gz_msgs_generate_messages() call is combined with the project() name to form the library name?

If so, I think this should be explicitly mentioned.
I suggest to add the following comment:

target_link_libraries(FullSystem PRIVATE
  gz-sim${GZ_SIM_VER}::gz-sim${GZ_SIM_VER}
  # Downstream libraries/executables using custom messages
  # need to link with the custom message library.
  # The name of this library is the concatenation of the project name and
  # the target name as specified in the gz_msgs_generate_messages() call
  # (in this example 'ros_gz_example_gazebo' and 'msgs'):
  ros_gz_example_gazebo-msgs
)

[jrutgeer]

In this example the custom messages and the system plugin that uses them are in the same package.

However in a ROS / Gazebo setting it is typically a more common use case that the messages and systems are in different packages, and the downstream package calls ament_target_dependencies() to load the package dependencies:

ament_target_dependencies(my_system
  PUBLIC
  custom_messages_package
  gz-transport${GZ_TRANSPORT_VER}
  gz-sim${GZ_SIM_VER} 
  rclcpp
  #possible other dependencies
)

I am trying this for my project, and it seems that an extra statement is needed in the CMakeLists.txt of the custom message definition package:

ament_export_include_directories(include)

I am not sure if this is the correct/recommended way to do this, as the documentation states that this should be 'superfluous':

There are two additional functions which are available, but are superfluous for target based installs:
ament_export_include_directories("include/${PROJECT_NAME}")
ament_export_libraries(my_library)

Also note that in my case it seems I need:

• ament_export_include_directories(include)

as opposed to:

• ament_export_include_directories(include/${PROJECT_NAME}).

I think it would be valuable to extend this example to also cover this use case.

[jrutgeer]

Above did not work, I did not manage to get it to link.

After another 2h of toiling and plodding, I finally have a working CMakeLists.txt.

I conclude that it is not intended to use ament_export_include_directories(include), but rather:

• Add following commands to the CMakeLists.txt file of the custom messages package:

  install(
    TARGETS ros_gz_example_gazebo-msgs
    EXPORT arbitrary_name_for_export
  )
  
  target_include_directories(ros_gz_example_gazebo-msgs
    PUBLIC
    "$<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>"
    "$<INSTALL_INTERFACE:include>")
  
  ament_export_targets(arbitrary_name_for_export HAS_LIBRARY_TARGET)

• And add this in the downstream package:

  find_package(ros_gz_example_gazebo REQUIRED COMPONENTS ros_gz_example_gazebo-msgs)
  # Not sure if this could be just 'find_package(ros_gz_example_gazebo REQUIRED)'
  
  add_executable(my_executable src/executable.cc)
  ament_target_dependencies(my_executable ros_gz_example_gazebo)
  
  target_link_libraries(my_executable
    gz-transport${GZ_TRANSPORT_VER}::core
    ros_gz_example_gazebo::ros_gz_example_gazebo-msgs)

[jrutgeer]

It would be great if there were more documentation on the use of CMake / ament for ROS and Gazebo, as currently one has to resort to copy-paste and trial-and-error until it works.