Is it possible to launch multiple nodes from different packages in ROS2 (Crystal)?

edit

thank you for the reference. after adding a launch package with generate_launch_description function to return the description, how can one use it to actually launch multiple nodes?

Not sure if I have understood your question, but to launch the nodes included in the launch file you should use:

ros2 launch [package] [name_of_the_launch_file]

For the MARA example it should be: ros2 launch mara_gazebo mara.launch.py

I am sorry, I was not very clear. In the MARA example, find_package(urdf REQUIRED) and ament_export_dependencies(urdf) are used´in the launch CMake. urdf is found inside another package MARA_description. I have the following folder structure: -ros2_ws

           |- state_machine     
                    |- state_machine_pkg
                          |- state_machine_node.py
                          |- __init__.py
                    |- setup.py
                    |- package.xml
           |- launch
                    |- launch
                          |- sm.launch.py
                    |- CMakeList.txt
                    |- package.xml

I get cmake error if I use ament_export_dependencies(state_machine_pkg) and find_package(state_machine_pkg REQUIRED). I have just taken the exact CMake and xml files from MARA example and just changed urdf to state_machine_pkg.

I understand now. Would you please provide somewhere (Github or Gist) your code and CMakeLists?? That would help me to see where is the problem ;)

https://github.com/I3Tech/Temporary.git

I can see that your code is in Python so there is no reason to use CMakeLists.txt in this case

are you saying to use setup.py ?

Keep the CMakeLists.txt but delete the ament_export_depencies(state_machine) line and try again.

Remember compiling: colcon build --merge-install