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) shown in the image is no longer required; see TimedTwistState for the latest code. As of version 3.1.0, these calls within individual states can be avoided by making a single call to initialize_flexbe_core in the behavior initialization; that is, initialize is no longer required at the state implementation level. The flexbe_webui writes behaviors with this call. Older behaviors should be updated to make this call, or ensure states are properly initialized by calling initialize_flexbe_core(node) in the behavior's __init__. New states do not require per-proxy initialize() calls if the proxies are initialized by the behavior using initialize_flexbe_core.

As can be seen in the ROS 2 version's constructor, a ProxyPublisher is created to publish TwistStamped messages. The same approach applies to ProxySubscriberCached, ProxyActionClient, and ProxyServiceCaller for subscribers, action clients, and service clients respectively. Note that the explicit initialize() call shown in the image is no longer needed; proxy instances can be created directly in __init__ once initialize_flexbe_core(node) has been called in the behavior. The creation and use of proxy publisher, subscriber, action client, and service client is otherwise the same as 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 uses TimedTwistState._node (a class-level attribute shared across all state instances, set during behavior startup). The node is not available as self._node during __init__ because proxy initialization has not yet occurred at construction time. However, during execution (execute(), on_enter(), on_exit(), etc.) the class-level node is available and TimedTwistState._node can be used 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 class-level node reference.

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