Add option to wait for two subscriptions

[DaveCollinsJr]

Since, if following the tutorial to the letter, you will currently have both turtlesim and topic echo subscribed. If you do not do this, sometimes the turtle will move, sometimes it wont - depending on which subscriber the command finds first.

[fujitatomoya]

@DaveCollinsJr lgtm, can you address a nitpick?

[fujitatomoya]

so the prerequisites is that turtle nodes are up and running, but i think that is okay to have -w option here.

[DaveCollinsJr]

Note that in my experience (running all of this in Docker) the -w 2 option is mandatory. If you don't use it, the results are inconsistent: The topic pub begins running as soon as it sees the first subscriber. Therefore: Sometimes the turtle moves (and you see nothing in the topic echo window), sometimes the turtle does not move (and you see something in the topic echo window). The only way I was able to guarantee the behavior the tutorial is trying to demonstrate is to use -w 2

[fujitatomoya]

one sentence per line, missing last period.

Suggested change

	``-w 2`` is an optional argument meaning “wait for two matching subscriptions”. This is needed because we have both turtlesim and the topic echo subscribed
	``-w 2`` is an optional argument meaning “wait for two matching subscriptions”.
	This is needed because we have both turtlesim and the topic echo subscribed.

[DaveCollinsJr]

OK will fix.

[clalancette]

So I kind of think that the use of --once here is a mistake to begin with, and adding in the -w 2 is going in the wrong direction.

This is supposed to be the beginner tutorial. For first-time users using ros2 topic pub, the fact that we immediately hit them with 2 command-line options, the topic name, the topic type, and the args, seems like quite a lot.

I think we should do something entirely different here. I think we should rewrite this whole section to start with:

  ros2 topic pub /turtle1/cmd_vel geometry_msgs/msg/Twist "{linear: {x: 2.0, y: 0.0, z: 0.0}, angular: {x: 0.0, y: 0.0, z: 1.8}}"

That will automatically start publishing at 1 Hz, and will publish no matter how many subscribers are available. Once we've shown the user the simplest invocation of the command, we can then introduce the --once command later. What do you think?

[fujitatomoya]

I am okay with #4674 (review), @DaveCollinsJr can you adjust the PR?

[DaveCollinsJr]

@clalancette, @fujitatomoya : I have updated the PR to rearrange the tutorial as suggested. Now the --rate 1 option is first.

Then it moves into the --once option which, in my experience requires the -w 2 option to work consistently.

[clalancette]

Thanks! This is looking at lot more like I think it should. I've left a few things to improve.

[DaveCollinsJr]

OK @clalancette - I believe I've hit your requested changes. Nice: I didn't realize that the "publish at 1 Hz" was the default.

[clalancette]

Other than one nit, I really like this revamp.