http://wiki.ros.org/actionlib

http://wiki.ros.org/actionlib/DetailedDescription

http://wiki.ros.org/actionlib_tutorials/Tutorials/Writing%20a%20Callback%20Based%20Simple%20Action%20Client#But_I_want_to_use_Classes.21

Calling Actions from a SMACH state machine
The goal messages and action interfaces for states…
http://wiki.ros.org/smach/Tutorials/Calling%20Actions

http://wiki.ros.org/actionlib/DetailedDescription#Threading%20Model%20(C++) – Threading

https://answers.ros.org/question/220181/how-to-safely-use-actionlibs-goal-handles-in-a-real-time-concurrent-system/

Simple Action Client – Doxygen
http://docs.ros.org/melodic/api/actionlib/html/classactionlib_1_1SimpleActionClient.html

Simple Action Server – Doxygen
http://docs.ros.org/melodic/api/actionlib/html/classactionlib_1_1SimpleActionServer.html

http://docs.ros.org/melodic/api/actionlib/html/simple__action__server__imp_8h_source.html

actionlib_msgs/GoalStatus
http://docs.ros.org/melodic/api/actionlib_msgs/html/msg/GoalStatus.html