Unknown class in interface builder

I just had this happen with a UIViewController subclass (with Xcode 9 beta 2 & Swift 4) and the solution was to tick 'Inherit from Target' where I set the Custom Class in IB.

If your view happens to be in a .bundle file (such as for a framework/static library) that gets copied to a different target, you'll want to set an explicit module target for each xib/storyboard class rather than having it inherit from the target that's hosting it.

For me the problem was that the class was not part of the Target Membership. Just add the class to the target, and you should see it back on the interface builder. This Helped in my case.

After I checked the tick mark called "Inherit Module From Target" in Custom Class in View Controller Section, it works fine. Can Follow the image.

To find this > Select your view controller yellow button > then click "Show Identity Inspector" > then checked the tick mark called "Inherit Module From Target"

What @Anthony Scott mentioned is true until you have Framework_A dependent on Framework_B and class is from Framework_B :) Then you need to deselect checkbox and provide Module which contains given class. Thanks for the question btw. I am using Commons framework for other frameworks and it helped me to realize this ;) For those who develop custom Cocoa Touch Frameworks this can be helpful.