define reactive cascade order

[isthisthat]

I have a UI that has 3 components (dropdowns), A, B, C that can each change independently but the values of B are based on A and the values of C are based on B (all of which is defined in a table; see example below). Right now, I have 2 separate @reactive.effect-decorated functions (but without a @reactive.event restriction) that update the options in B when something changes and the options in C when something changes. My issue is that sometimes C updates before B when A changes because there's no strict rule stating otherwise. How do I solve this issue?
If I attached a @reactive.event on A to change the options in B and a @reactive.event on B to change the options in C, then a change in A that happens to have an identical B option will not trigger an update on C. For example consider the following minimal table:

A	B	C
one	0	foo
one	1	fa
two	3	bar

If A has "one" selected and I switch it to "two" I would like C to give "bar" as the only option. This is only possible if C can update either when B changes or A changes but there's no rule stating that B should update before C does.

If I have no @reactive.event restrictions then sometimes C updates before B and I get an erroneous lookup for, e.g. A == "two" and B == 0 and C =="bar"

How do I force reactive order to be A --> B --> C? Hope this makes some sense.

[wch]

@reactive.effect() has a priority parameter which might be what you're looking for.

https://shiny.posit.co/py/api/core/reactive.effect.html

[isthisthat]

Thank you @wch ! I just noticed. However I'm still missing something. When the A drop-down changes and triggers an update on the B drop-down items (via ui.update_select), I cannot get C's @reactive.effect to read the updated B value (via input.B()); instead I get the original B value. Any ideas?

[wch]

I'm having a bit difficulty picturing the whole problem. Can you create a minimal reproducible example and share with a shinylive.io link?

[isthisthat]

Sorry you're right. I was just about to do that but I figured out what the problem was. C's @reactive.effect was being called multiple times, as A cascaded to B and B cascaded to C. As soon as I put an if statement for the table lookup I was doing to be valid, it worked (it essentially skipped "intermediate" look-ups that gave null entries).