ros2-conversion-best-practices.md

Notes on converting existing FlexBE State Implementations to ROS 2

Most changes to custom FlexBE states from ROS 1 to ROS 2 are very minimal due to FlexBE’s proxies and built-in State classes handling most of the processes for the custom states behind the scenes. As a result, the biggest changes to custom FlexBE states focus on configuring the FlexBE proxies as well as handle any direct calls to ROS 1’s rospy package.

The following image shows the ROS 1 (left) and an early version of the ROS 2 (right) conversion of the TimedTwistState from Flexible Navigation, a FlexBE-based navigation system.

Note: the call to ProxyPublisher.initialize(TimedTwistState._node) is out of date in image; seeTimedTwistState for the latest code. As of version 3.1.0, these calls within individual states can be avoided my making a single call to initialize_flexbe_core in the behavior initialization; that is, initialize is not longer required at the state implementation level. The flexbe_webui will now write behaviors with this call (the older flexbe_app does NOT call initialize_flexbe_core at this time). Older behaviors should be updated to make this call, or be sure states are properly initialized (e.g., ProxyPublisher.initialize(TimedTwistState._node)). For now, most existing states will retain these redundant calls to initialize(self._node) to maintain backward compatibility. New states do not require these if the proxies are initialized by the behavior using initialize_flexbe_core.

As can be seen in the ROS 2 version's constructor, the ProxyPublisher used to create a publisher to publish TwistStamped messages first needs to be initialized using the TimedTwistState's node. This would also be done with a ProxySubscriberCached, ProxyActionClient, and ProxyServiceCaller to create a subscriber, action client, and service client respectively. After initializing, the creation of the proxy publisher, subscriber, action client, and service client is done like in ROS 1. In addition, all of the prior proxy function calls in ROS 1 are supported in ROS 2. Due to this, it is encouraged to use the FlexBE proxies as most of the changes between ROS 1 and ROS 2 are hidden.

With this in mind, the only remaining changes involve direct calls to the rospy package. As shown in the respective ROS 1 and ROS 2 code, they use rospy and the TimedTwistState’s node to set the timestamp of a header message. When setting the timestamp, the ROS 2 version is able to use self._node as opposed to TimedTwistStamped._node in the constructor. The reason is that during the creation of the custom state in the constructor the individual instance of the custom state does not have access to the node. However, during execution the custom state instance has access to its node, where the TimedTwistState can use its node to get the current time. Like getting the time for the timestamp, each instance of directly using the rospy package needs to be changed to use the custom state’s node.

Consult the ROS 2 documentation for more information on using the ROS 2 API.