Can I pass a nav_msgs/Path message directly to local planner?

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If you want to use all of the functionality of move_base, you're going to have to write your own global planner plugin. base_local_planner does not subscribe to a path, so you cannot publish it over a topic. In fact, base_local_planner has no external access at all. It can only be accessed by using move_base and interface methods, and move_base provides no methods for externally setting the plan.