[image_tools] Use ROS parameters instead of regular CLI arguments

[jacobperron]

Using of ROS parameters make the code a little tidier, though a little more verbose on the command-line.
IMO, it seems like a good way to demonstrate best practices as well.

Example usage:

$ ros2 run image_tools cam2image --ros-args -p burger_mode:=true -p reliability:=reliable

$ ros2 run image_tools showimage --ros-args -p reliability:=reliable

Connects to #390

[dirk-thomas]

How does a user get a similar help than what was provided by --help before?

[jacobperron]

How does a user get a similar help than what was provided by --help before?

Good question, I guess there's no equivalent to --help.

Maybe this is a good example for a feature/tool that is lacking for ROS parameters.
At the moment, you can list parameters for the node, but only after it is running:

$ ros2 param list
/showimage:
  depth
  history
  reliability
  show_image
  use_sim_time

In addition it would be nice if we could see the types/constraints on these params (ros2/ros2cli#362).

And even better (equivalent to --help) would be to introspect the node before actually running it. I imagine some like

$ ros2 run image_tools showimage --ros-args --help
Node: name_of_node
  Parameters:
    <list of parameters and descriptors>
  Publishers:
    <list of publisher types and topics>
  Subscriptions:
    <list of subscription types and topics>
... etc for services and actions.

Node: another_node
   ... etc

I suppose this kind of introspection feature needs a lot more thought and discussion to make it happen.

[dirk-thomas]

Not relevant in this case where the node is implemented in C++. But in the case of Python nodes using argparse another major advantage is getting free argcomplete functionality. So switching a Python node to parameters would loose that too which I think would be undesired.

[jacobperron]

I've added back an option for help (-h / --help): da736d0

[jacobperron]

• Linux
• Linux-aarch64
• macOS
• Windows

[dirk-thomas]

I was hoping the nodes would declare their parameters and --help would then call a generic function which get those descriptors and renders the usage.

Since the usage text was hard coded before it is certainly good enough for this PR to also hard code them.

The one inline comment doesn't need to be addressed - the same was already the case before this PR.

[dirk-thomas]

Nitpick: each std::endl also flushes the stream which doesn't seem necessary on each line.

[jacobperron]

I was hoping the nodes would declare their parameters and --help would then call a generic function which get those descriptors and renders the usage.

It seemed easier to do something like was done before to ensure formatting looked okay.
But, this is a nice idea. I think that a generic function, like get_node_usage(), should live in the client library or a utility somewhere that node developers can use. I can open up a feature request for this.