How to get information from the navfn planner execution

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NavFn can be used standalone just to generate plans, or as a plugin to move_base. I assume you mean in the latter case. What I would do in your place is replace navfn_node with a custom global planner, that additionally sends all information you need to a topic.

Basically you would copy the whole navfn ROS package, then remove all c++ source file except src/navfn_node.cpp. Then add a topic publisher and change makePlanService() service to also publish on your custom topic.

Then you need to adapt filenames, the package manifest, plugin file etc. Create a launchfile for move_base (by copying the one you used so far), and declare your custom global planner as a replacement to navfn. Use this launchfile instead of the one you used so far, and move_base should use your custom global planner, and every time move_base calls the makePlan Service, you would get a message on your topic telling you what you need to know.