[amelius]

You can add a constraint solver on top of it. So when e.g. C changes, you solve for A and B. You then update the state of A, B, C and redraw. This kind of interaction can be handled by a separate library which is not necessarily part of a GUI system.

[auggierose]

A constraint solver is overkill here, in particular as you cannot solve for A and B uniquely, you might want to add some further rule, like A and B should grow proportionally, when C is modified.

But otherwise, yes, just use a framework that separates State from UI and has bidirectional updates, like SwiftUI.