In objective C, a Category will only "be used" in a class if I import it. So if I have a Category NSString+category, I have to #import NSString+category.h in each class I want to use it.
I have such a Category, and some of the classes I want to use it in are written in Swift. If I expose this category to Swift by putting it in the Bridging header, ALL swift classes will use it. How can this be avoided?
Note: My Category is actually on UIViewController and the code I have put there must only be used by SOME ViewControllers. It feels wrong and unneccessary to use this Category on the "other" ViewControllers.
Your base assumption is incorrect:
In objective C, a Category will only "be used" in a class if I import it
The methods in a category are present on the class if the category is compiled. Importing the header makes the method names visible; without that the compiler will give you a warning if you try to send a message using one of those names. (You can call the methods using performSelector: or the runtime library if you're determined.)
The same is true of your Swift class, and because of the way Objective-C headers are brought in to Swift, I don't believe there's a way to limit the methods' visibility in your Swift code.