add example of registering custom parameter validation callbacks

[mikaelarguedas]

fixes ros2/examples#164

• Linux
• Linux-aarch64
• macOS
• Windows

[wjwwood]

Seems reasonable to me.

[wjwwood]

Might want to quote the second %s.

[wjwwood]

Or use a : %s

[mikaelarguedas]

Good point, I purposefully avoided the quoting as I was afraid it could confuse users into thinking that a string value was set and not an integer.
I went for the 2nd approach in 6109880

[clalancette]

This is great! The only other thing I might look for is an example that uses std::bind to pass an arbitrary argument into the param_change_callback along with the parameters. It is "just" reusing std::bind, but it is convenient to see the example all put together. However, I won't block this PR for that, it is just a nice-to-have if you feel like adding it in. Thanks.

[mikaelarguedas]

The only other thing I might look for is an example that uses std::bind to pass an arbitrary argument into the param_change_callback along with the parameters.

I think we would need another story to justify passing extra arguments to this callback as currently its only purpose is to check for parity and I have a hard time finding a variable to pass in that would make sense.

[mikaelarguedas]

@clalancette Posting new comment instead of editing to sent notification:

I think we would need another story to justify passing extra arguments to this callback

I'm open to suggestions for this.
If we don't find anything compelling by tomorrow I'm planning on merging this as is.

[clalancette]

If we don't find anything compelling by tomorrow I'm planning on merging this as is.

Nothing really compelling comes to mind, so I'd say go ahead and merge. We can always revisit later.

[mikaelarguedas]

Sounds good, thanks for the review!