Improve usage of the rules system with macros

[flowln]

The rules system is very interesting for embedding some additional behavior from Qt Designer without going through python code. But I think it could be extended a bit to further improve its flexibility.

In my application, I want to have some part of an interface - possibly embedded in a PyDMFrame - to be visible only on some locations, while it's hidden elsewhere. For that, I wished to use a macro definition to select the visibility of such element when opening the UI. I was able to make it work OK with the following configuration:

The md function (short for macro_defined) I added in a local clone of PyDM in the eval environment:

What I want then is to make this not so hacky-ish, with better native support for PyDM. That would entail IMO:

• Adding such a md function or similar to the eval environment for dealing with macros (you can make-do without it, of course, but it's annoying).
• Possibly making the md-like function automatically get the value of the macro without the ${...} things (so, it would look like md('TEST') in the end).
• Allowing us to not specify a channel every time. I had to add a channel there simply because it required me to, though it doesn't do a thing (except trigger the expression, but we could make it so that, if there's not channels, or no channel is marked as 'Trigger', the expression is only evaluated once at application startup).
• Possibly passing the initial value through eval, so we could only use that. This would also mean we could leave the Expression field empty.

That I think would make using the rules system better, though I appreciate any feedback or comments on this proposal. 🙂

[jbellister-slac]

Hi, sorry for the delay! Caught us during Thanksgiving break. We think that extending the rules engine like this sounds like a good idea too. I think your proposals for not requiring a channel/expression and only evaluating once at application startup sound fine too, the extra flexibility should be a good thing and the documentation can be updated to make it clear what would happen in each case.