add talker_listener.launch.py new-style launch file

[wjwwood]

Connects to ros2/launch#76

[wjwwood]

I'd like to expand this to include launch files for everything, but after we get closer to review.

[wjwwood]

I expanded the launch files to include all the topics and service demos in demo_nodes_cpp. If anyone has time to review these quickly, I'd appreciate it!

[dirk-thomas]

Works for me (the launch files themselves that is).

I wasn't able to Ctrl-C though:

[INFO] [launch]: process[talker-1]: started with pid [10880]
[INFO] [launch]: process[listener-2]: started with pid [10881]
[INFO] [talker]: Publishing: 'Hello World: 1'
[INFO] [listener]: I heard: [Hello World: 1]
[INFO] [talker]: Publishing: 'Hello World: 2'
[INFO] [listener]: I heard: [Hello World: 2]
[INFO] [talker]: Publishing: 'Hello World: 3'
[INFO] [listener]: I heard: [Hello World: 3]
^Csignal_handler(2)
signal_handler(2)
[INFO] [launch]: process[talker-1]: process has finished cleanly
[INFO] [launch]: process[listener-2]: process has finished cleanly

There is was hanging indefinitely. That should not block this PR though.

[dirk-thomas]

👍 for the launch files in this PR since my other comment is not related these launch files but launch in general.

[wjwwood]

I'll look into the hang.

[dhood]

think this should add a dependency on launch_ros in the package.xml yeah?

[wjwwood]

think this should add a dependency on launch_ros in the package.xml yeah?

Not sure, I guess you could, but a similar thing comes up where you wonder if every package in ROS 1 that has a launch file should depend on roslaunch.

If we decide to add a dependency, then we might as well add ros2launch in addition to (or instead of) launch_ros. Because those launch files cannot be run on their own.

[dhood]

I think adding a ros2launch dependency is a step further than necessary, because even though they're not usable on their own, it doesn't have to be ros2launch that uses them (could theoretically be anyone).

But I take your point about having all ROS 1 packages depend on roslaunch. In that sense I think it's reasonable that only packages that are actually launching launch files (e.g. in tests) depend on launch_ros themselves.

[wjwwood]

Anyone else have an opinion on that? Should this package (and others that contain launch files) depend on something (either launch_ros or ros2launch)?

[dirk-thomas]

Since the launch files do import launch_ros I think the packages should have an exec_depend on launch_ros.

Regarding the dependency on ros2launch: while I can see the convenience if someone would only install that package how it would help I wouldn't add it since ros2launch is just one way these launch files can be used. As an analogy: if a package provides a component (a node compiled into a shared library) I wouldn't expect a dependency on something like ros2run-component (not that we have that - just as an example).

[wjwwood]

I added a dependency on launch_ros in demo_nodes_cpp, however I don't completely agree with it.

For me, adding launch_ros only doesn't make sense because though it imports launch_ros it will never actually need it unless launched and the only way to do that (currently) is with ros2launch.

So you end up either installing ros2launch (and therefore launch_ros implicitly) to launch it or you don't use the launch files in which case you have a dependency that you're not using.

I understand you could use the launch file without ros2launch some how, but the only case where I can imagine that is by including it from another launch file (in which case you'd need ros2launch again at some point).

Personally, I would have gone for just ros2launch, but I don't feel strongly about it. I think about it from the package developer's perspective when writing tutorials, they'd like to say apt install my-thing and then do ros2launch my_thing my_launch_file.launch.py. Then again I see the arguments not to have a dependency on the command line tool as well (maybe you want to launch it with some future gui based launch tool).

[wjwwood]

I'm going to do CI and merge this while I continue looking into the reason ros2 launch doesn't exit on Linux. I'll make an issue for that.

[wjwwood]

ros2/launch#80