Feature request

There should be a c++ file importable by #include <package_name/action/actionname.hpp> which contains everything needed to use an action type in c++.

Feature description

It needs to be possible to generate a header file that describes an action, and has all the information needed to create topics and services. Since services already are an example of a typesupport that needs to include messages, actions should use the same strategy.

Implementation considerations

Output files

Services have a top level header which includes two files: traits and struct. Traits indicates whether either the request or response is bounded or unbounded in size. I don't think there needs to be a traits file for the whole action.

The struct file includes the request and response message, as well as defines a struct with a using directive that indicates which message is which

The action struct template could look something like this

#include "@(spec.pkg_name)/action/@(get_header_filename_from_msg_name(spec.action_name))__sendgoal.hpp"
#include "@(spec.pkg_name)/action/@(get_header_filename_from_msg_name(spec.action_name))__get_result.hpp"
#include "@(spec.pkg_name)/action/@(get_header_filename_from_msg_name(spec.action_name))__feedback.hpp"
// Convenience include for common service and message definition
#include <rcl_interfaces/srv/cancel_action.hpp>
#include <rcl_interfaces/srv/goal_status.hpp>

 struct @(spec.action_name) 
 { 
   using SendGoalService = @(spec.pkg_name)::action::@(spec.action_name)_SendGoal; 
   using GetResultService = @(spec.pkg_name)::action::@(spec.action_name)_GetResult; 
   using FeedbacMessage = @(spec.pkg_name)::action::@(spec.action_name)_Feedback; 
};

CMake targets

The CMake target needs to be aware of the generated output files so that it can rebuild stuff downstream when they change. Awareness of the files generated by .action needs to be included here

connects to ros2/ros2#583