Event handling on ROS2 Nodes across Launch files

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Thank you so much for your response.

but, all these examples focus on conditioning on nodes that are initiated in the same Launch file, so we know the object name on which we can condition our actions on easily.

But what about the cases where we load the launch description of another launch file that initiates nodes inside them, and those node objects are not visible to our launch file. how do we condition on those nodes?

To make sure I understand the question, you are referring to the "talker_node" object from this line not being available?

arget_lifecycle_node=talker_node, goal_state='active',

No. So, in this example, the Event Handlers are registered on the node objects defined in the same launch file. What about the case, where for example, the talker_node is defined in a child launch file, that is then included in this description. This makes the word 'talker_node' not defined in this parent launch file. Can we then somehow register the event handler on the 'talker_node's state?

There is a get_sub_entities() method on all Actions that might be able to accomplish this. Something along the lines of

my_included_launch = IncludeLaunchDescription(...)
sub_entities = my_included_launch.get_sub_entities()
target_node = None
for entity in sub_entities:
   # Somehow identify the node entity maybe with entity.describe()
   if (some check):
      target_node = entity

handler = launch.actions.RegisterEventHandler(
    launch_ros.event_handlers.OnStateTransition(
        target_lifecycle_node=target_node, goal_state='active', ...))

Something like that might conceivably work, but it will take some experimentation on your end and looking at the launch source code. Its possible you may also need to wrap the get_sub_entities() call and for loop search in a new Substitution class in order for all the entities to be properly found.

This looks promising! Will try it. Thank you very much :)

No problem. Hope it works!