Extending a class if it satisfies a specific protocol


I have a "protocol A" that I want to make more convenient when used with NSView.
I like to extend any NSView class that is using protocol A with a calculated variable that is @IBInspectable (or at least @objc).
Is this even possible? And if so how?

I thought I have to do something like
extension NSView where Self: A
or something similar. I do not know if such a syntax exists or what it should look like.

"extension A where Self: NSView" creates this error message:
"@objc can only be used with members of classes, @objc protocols, and concrete extensions of classes"
I did declare A as @objc protocol, with the same problem.

All the best