Package for using message path syntax in ROS nodes

[rgov]

The message path syntax that is used to refer to a specific field within a message, or a calculation based on a field, is pretty useful.

I can see a few use cases for employing the same syntax in my ROS nodes, for instance to provide a configuration parameter that tells a node to watch a specific field on a specific topic.

As a concrete example, I have a node that subscribes to a configurable topic and computes statistics (min, max, avg) for a field. It would be great to be able to use the same message path in Studio as in the configuration for this node.

In Python, I could imagine using something like:

sum, count = 0.0, 0

def callback(msg, value):
    sum += value
    count += 1

foxglove.MsgPathSubscriber(path, callback)

Or perhaps

msg_path = foxglove.MsgPath(path)

sum, count = 0.0, 0

def callback(msg):
    sum += msg_path.value_from_msg(msg)
    count += 1

rospy.Subscriber(msg_path.topic, msg_path.msg_type, callback)

[rgov]

If the APIs I propose were seriously considered, I would prefer the former. The latter would have to be able to figure out the message type for the topic which may not be available when the code executes. The former could lazily look up the type once the first message arrives, then dynamically load the definition.