TipsAndTricks/Creating Recipes for ROS modules

== Introduction ==
Robot Operating System (ROS) isn't an OS but a set of modules for robotics applications that interact via a pub/sub interface. Thanks to meta-ros you can add a wide range of ROS modules to your OS build. But how do you add your own ROS module? This article covers creating a recipe for a ROS module. For this article we'll use the vectornav module as an example. We'll also assume you already have meta-ros in your bblayers.conf.
== Starting with "devtool add" ==
Standard practice for a creating a recipe is use devtool add. The github project doesn't have any releases, so let's use the git repo.
 $ devtool add https://github.com/dawonn/vectornav
This completes OK, let's take a look at the recipe.
 $ devtool edit-recipe vectornav
# Recipe created by recipetool
# This is the basis of a recipe and may need further editing in order to be fully functional.
# (Feel free to remove these comments when editing.)
# WARNING: the following LICENSE and LIC_FILES_CHKSUM values are best guesses - it is
# your responsibility to verify that the values are complete and correct.
LICENSE = "MIT"
LIC_FILES_CHKSUM = "file://vnccpplib/LICENSE.txt;md5=a3e2f266f2adb30b4ae17db437fa9727"
SRC_URI = "git://github.com/dawonn/vectornav;protocol=https"
# Modify these as desired
PV = "1.0+git${SRCPV}"
SRCREV = "497173f3ddbe21216d77d3b2021a5ef48a17ec4a"
S = "${WORKDIR}/git"
# NOTE: unable to map the following CMake package dependencies: catkin
inherit cmake
# Specify any options you want to pass to cmake using EXTRA_OECMAKE:
EXTRA_OECMAKE = ""
Note the comment
 # NOTE: unable to map the following CMake package dependencies: catkin
And sure enough, the build fails as the comment predicted.
$ devtool build vectornav
| CMake Error at CMakeLists.txt:7 (find_package):
| By not providing "Findcatkin.cmake" in CMAKE_MODULE_PATH this project has
| asked CMake to find a package configuration file provided by "catkin", but
| CMake did not find one.
Assuming you've worked with ROS, you'll know that it builds with a cmake based tool called catkin. This is usually installed as a developer package so some Yocto work will need to be done to "build the builder". This seems like a tough task, so let's see how other ROS modules have done this.
== Understanding Existing ROS Recipes ==
=== Recipe Structure ===
Let's look at a an existing meta-ros recipe zeroconf-msgs to see how they support catkin. 
DESCRIPTION = "General ros communications used by the various zeroconf implementations."
SECTION = "devel"
LICENSE = "BSD"
LIC_FILES_CHKSUM = "file://package.xml;beginline=8;endline=8;md5=5ee5b8b046ae48ad94a2037ca953a67b"
DEPENDS = "std-msgs message-generation"
SRC_URI = "https://github.com/stonier/${ROS_SPN}/archive/${PV}.tar.gz;downloadfilename=${ROS_SP}.tar.gz"
SRC_URI[md5sum] = "38e89e637f855c2ea0e8cb65c02dfd08"
SRC_URI[sha256sum] = "a5bfd788bc2e2aefb07cb3a302a25cbeef2ce7e931a3a273cb1ae9669645a696"
S = "${WORKDIR}/${ROS_SP}"
inherit catkin
A few things to note
* The directive inherit catkin includes bitbake logic and it's a reasonable guess that this does all the catkin stuff just as inherit cmake enables cmake.
* As package.xml if referred to by LIC_FILES_CHKSUM must include some licence info. Let's dig into this.
* How are the DEPENDS defined? Let's dig into this as well.
=== Getting meta-data from package.xml ===
Let's take a look at it
<package>
 <name>zeroconf_msgs</name>
 <version>0.2.1</version>
 <description>
 General ros communications used by the various zeroconf implementations. 
 </description>
 <maintainer email="d.stonier@gmail.com">Daniel Stonier</maintainer>
 <license>BSD</license> 
 <url type="website">http://www.ros.org/wiki/zeroconf_msgs</url>
 <url type="repository">https://github.com/stonier/zeroconf_msgs</url>
 <url type="bugtracker">https://github.com/stonier/zeroconf_msgs/issues</url>
 <author email="d.stonier@gmail.com">Daniel Stonier</author>
 <buildtool_depend>catkin</buildtool_depend>
 <build_depend>message_generation</build_depend>
 <build_depend>std_msgs</build_depend>
 <run_depend>message_runtime</run_depend>
 <run_depend>std_msgs</run_depend>
 <export>
 <architecture_independent/>
 </export>in the run_depend that are not also in 
</package>
There is some useful information to create a recipe. Let's create a mapping between package.xml tags and recipe variables.
package.xml Tag 	Recipe directive	Comment
name 	PN 	This is defined by recipe filename
version 	PV 	Usually defined by recipe filename
description 	SUMMARY 	This should map to SUMMARY, not DESCRIPTION which adds detail
license 	LICENSE 	Use package.xml for LIC_FILES_CHKSUM
url 	HOMEPAGE 	type="website". Seems to be an optional tag
url 	SRC_URI 	type="repository". Seems to be an optional tag
buildtool_depend	inherit 	Will always be catkin(?), which inherits cmake
build_depend 	DEPENDS 	1. Append to any values created by devtool
2. If ROS module name contains an underscore, convert to a dash
runs_depend 	RDEPENDS_{PN} 	This should only contain entries that are not already in build_depend
== Putting it all together ==
So let's take a look at the package.xml for our module
<package>
 <name>vectornav</name>
 <version>0.1.0</version>
 <description>ROS interface for the VectorNav INS/GPS</description>
 <maintainer email="dereck@gmail.com">Dereck Wonnacott</maintainer>
 <license>MIT</license>
 <!-- <url type="website">http://ros.org/wiki/vectornav</url> -->
 <author email="dereck@gmail.com">Dereck Wonnacott</author>
 <buildtool_depend>catkin</buildtool_depend>
 
 <build_depend>roscpp</build_depend> 
 <build_depend>geometry_msgs</build_depend>
 <build_depend>sensor_msgs</build_depend>
 <build_depend>tf</build_depend>
 
 <run_depend>roscpp</run_depend>
 <run_depend>sensor_msgs</run_depend>
 <run_depend>geometry_msgs</run_depend>
 <run_depend>tf</run_depend>
 
 <!-- Maintainer Note:
 The vnccpplib directory is from http://www.vectornav.com/support/downloads
 I removed the version string from the directory name, sanitized file 
 permissions, and removed all but the src and include directories.
 -->
</package>
And map it to recipe directives
PV 	0.1.0
SUMMARY 	ROS interface for the VectorNav INS/GPS
LICENSE 	MIT
HOMEPAGE	http://ros.org/wiki/vectornav
inherit 	catkin
DEPENDS 	roscpp geometry-msgs sensor-msgs tf
Now let's update the recipe the devtool created with these values
SUMMARY = "ROS interface for the VectorNav INS/GPS"
HOMEPAGE = "http://ros.org/wiki/vectornav"
LICENSE = "MIT"
LIC_FILES_CHKSUM = "file://package.xml;md5=5431059e1a277b5a31f4ad759bc9c76d \
 file://vnccpplib/LICENSE.txt;md5=a3e2f266f2adb30b4ae17db437fa9727"
SRC_URI = "git://github.com/dawonn/vectornav;protocol=https"
PV = "0.1.0"
SRCREV = "497173f3ddbe21216d77d3b2021a5ef48a17ec4a"
DEPENDS = "roscpp geometry-msgs sensor-msgs tf"
S = "${WORKDIR}/git"
inherit catkin
Now let's build it.
 $ devtool build vectornav
Success !
== Recipetool Integration ==
As "devtool add" failed to create a functional recipe but the above steps follow a set process, there is scope for extending recipetool to support ROS modules.
# Detect ROS module by checking for package.xml in root of source with tag buildtool_depend set to catkin
# Create basis of recipe in similar way to cmake (execluding inherit cmake
# Follow mapping in package.xml mapping table
Start with existing recipetool code, get smart with python3 and good luck!