purpose of roslaunch_add_file_check()?

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Unfortunately this was never documented, but the documentation for rosbuild_add_roslaunch_check is close:

http://wiki.ros.org/rosbuild/CMakeLis...

The function you are inquiring about is the "rosbuild-less" equivalent function and in the case you are asking about it is operating on the entire launch folder, which I presume contains one or more .launch files. Even the rosbuild version's documentation does not clearly state what exactly it checks:

This macro declares as a unit test a parse check of one or more roslaunch files. The parser (roslaunch/scripts/roslaunch-check) checks a number of aspects of launch files, in a manner similar to roswtf.

Another thing to do is to look at the script it seems to invoke for the test:

https://github.com/ros/ros_comm/blob/...

There should probably be something about that script documented here:

http://wiki.ros.org/roslaunch/Command...

And some mention of the CMake commands provided by roslaunch here:

http://wiki.ros.org/roslaunch#Overview

It seems the reason that documentation doesn't exist in those places is that the function used to be provided by rosbuild but is now provided by roslaunch and that never got documented and at the same time the CMake function was added where roslaunch previously had no documented CMake API either.

Adding this to the wiki would be a great contribution, assuming you have the time and figure out more about what the check actually checks.