Getting to a flexible components management from URDF in `/robot_description` topic

[destogl]

This description of issues is related to the following PRs:

• adjust example 1 to use hardware_spawner. ros2_control_demos#277
• [CM] Use robot_description topic instead of parameter and don't crash on empty URDF 🦿 #940

To solve this knot, I figured out the following 4 steps that will be done in separate PRs to make them manageable:

1. Rename parameters for setting components in unconfigured and inactive state after CM is started. This feature does not break anything, and it should be back ported to humble as well. See: Enable setting of initial state in HW compoments #1046
2. Enable loading components from topic if they are not loaded from parameter. Only the first callback will be considered to avoid any complexity with managing potential changes of components in URDF (fixing [CM] Use robot_description topic instead of parameter and don't crash on empty URDF 🦿 #940).
3. Enable not_loaded components after start. This needs some changes in RM (ABI breaking) that will separate some things better. Enable non-loaded hardware components after start of CM. #1049
4. Enable updates of /robot_description and components in URDF. The default behavior will be that if a new component exists it will be added to the system, but none component will be removed if it doesn't exist in URDF. User will be notified about this and they can manually remove the component.

[christophfroehlich]

Ad #940: Can it be that it was forgotten to update the documentation

ros2_control/controller_manager/doc/userdoc.rst

Lines 58 to 60 in db34dfa

	robot_description (mandatory; string)
	String with the URDF string as robot description.
	This is usually result of the parsed description files by ``xacro`` command.

according to this deprecation warning?

ros2_control/controller_manager/src/controller_manager.cpp

Lines 162 to 166 in db34dfa

	RCLCPP_WARN(
	get_logger(),
	"[Deprecated] Passing the robot description parameter directly to the control_manager node "
	"is deprecated. Use '~/robot_description' topic from 'robot_state_publisher' instead.");
	init_resource_manager(robot_description);

[destogl]

Yes, you are correct, docs has to be updated.

[fmauch]

I can try to update the docs.