Xcode 6 Swift code completion not working

Question

Using Xcode 6 GM seed my code completion has stopped working. It was working the other day. I was trying to get the unwind segue work around to work. I had made an Objc head

Answer 1

In my case Some File(s) were deleted in File System but were still referenced in the Project i.e. in Red Color. Just delete those from Project and everything was fine. Code Completion started working.

Also Like to point that all the above answers about Clearing/Deleting Derived Data folder seems to work momentarily but whenever there was a new pull or update the problem started again, so the root cause was the unreferenced/moved/deleted files which show up as RED in the project, they needs to be manually cleared.