Why do we need the EventHandlers in launch files?

[christophfroehlich]

As @emersonknapp started the discussion in a PR, I'd like move this discussion here.

First of all, I'd say the usage of EventHandlers has some historical reason where the spawners were not that powerful, or even not existing and the ros2cli was used in launchfiles (similar to the test included in example_1).
Nowadays, the spawner can handle multiple controllers at once if they have the same desired lifecylce state. Then this all is quite deterministic. Unless any controller is part of a controller chain, the order of spawning them is not important from the framework side.

But we saw flakiness in the tests, that's why even the order of spawners is important, see #547 and the linked rclpy issue. I'd say we could re-evaluate this, this might not happen if we merge all spawners to a single one where possible.

We also use this pattern to delay the startup of rviz, otherwise the console output is polluted with failing tf lookups unless the joint_state_broadcaster is up. What would be a better and still convenient solution for that?

[emersonknapp]

Thanks for opening this! In launchfiles that I'm dealing with I've noticed this:

1. Most use of Event handlers is for sequential ordered startup (thing, on_exit -> thing, on_exit -> thing)
2. Most ordered startup usage I've seen is for ros2_control

Therefore if I can help remove the need for this here, then I can make most launchfiles become pretty easy to migrate to YAML 😁

Yes, Rviz logging is very noisy on TF lookup issues. Perhaps we ought to put some throttling on the rviz output around those, there really is no reason for it to spew a huge message on every single failed TF lookup. Or disable that logging entirely - I'd consider it "normal operation" and not actually an error or even warning case for there to be tf lookups you try to do that may or may not exist - and the info is shown in the UI elements where needed.

[christophfroehlich]

Fixing this in rviz is the best idea ;) Could you open an issue there please? I'm not very familiar with the codebase of rviz..