Restarting xCode does not solve the problem.
Here might be potential solution:
I got into a similar state just today. It was very odd: I could attach any XIB (new or existing) to any already-existing ViewController class in the project, but I could not create a new ViewController and have it attach properly to any XIB. The “assistant” functionality didn’t work, nor did the IB-to-headerfile-connection functionality.
Closing the project and re-opening did not fix it.
Quitting XCode and restarting did not fix it.
Creating a new project and testing the functionality worked fine, which led me to believe there was something corrupt in some cache somewhere.
- Close your project.
- Go to the ~/Library/Developer/XCode/DerivedData folder and REMOVE all subfolders there referencing the project you are working on.
- Open your project in XCode. The problem should be fixed now.
Incidentally, just running a full clean did not seem to clear things up. I had to trash the derived data. I’m certain I got into this position because of git games I was playing, but I had no idea how to get out, because even switching back to earlier git revisions didn’t help. (That was a big clue also that it was something untracked by the project itself.)