Allow for using properties of other widgets on the screen as inputs for rules

[jwlodek]

This may already be possible, but if it is, it is not documented in https://control-system-studio.readthedocs.io/en/latest/app/display/editor/doc/rules.html.

Generally I think giving more concrete rules examples would be useful on that page.

[georgweiss]

@jwlodek, can you please provide a concrete use case?

[jwlodek]

The case that I'm interested in using this for is for a specific widget to be able to adjust it's behavior if any of a subset of other widgets is not marked as visible. I could re-create all the visibility rules on the target widget that I used for the other one, but it would make the rules on this widget much more complex than I need (and it would duplicate the conditionals, necessitating that they be kept in sync as additional conditions are added).

Specifically, I'm working on a nicer representation of PandABox configurations in phoebus. I want to have an LED that is lit if all the connections that are configured on the screen are active. I have each wire's visibility setting tied to the PV that corresponds to the wire being active on the PandA, and I'd like to concatenate them all into a single LED that says if a configuration is loaded or not.

Again, I could do this by copying all the visibility rules into the LED widget, but that then becomes a duplication of the conditionals.