spin until future complete for async service client

[Karsten1987]

fixes #316

When hitting ctrl-c, the node exits cleanly now:

[INFO] [add_two_ints_client]: service not available, waiting again...
[INFO] [add_two_ints_client]: service not available, waiting again...
^C[INFO] [rclcpp]: signal_handler(signal_value=2)
[ERROR] [add_two_ints_client]: Interrupted while waiting for the service. Exiting.

Signed-off-by: Karsten Knese karsten@openrobotics.org

[jacobperron]

I think it might make for a more compelling demo if the spinning on the future was encapsulated in the class, although it's a bit more involved. For example, in ros2/examples#230 I have class methods "spin_until_goal_complete" (client) and "spin" (server) for encapsulating the logic related to the futures.

[jacobperron]

Following my previous comment, we'd like to change this (and the other demos) to use composition. I've opened #325 to track this. I'll leave it up to you if you want to merge this as-is.

[nuclearsandwich]

@Karsten1987 with the composition transition underway what is the status of this PR?

[jacobperron]

We can try to address it in #325 (or post) to avoid conflicts.

@skucheria FYI