This started to be pretty common problem for me, because I am making a very complicated project with tons of source files, that’s why I decided to paste a solution here.

If XCode indexing doesn’t work or by other words, if code sense is not working, try this:

1. Open Organizer in XCode by pressing Shift + Cmd + 2
2. Open your project tab
3. Delete derived data

Screenshot:

XCode will reindex your project and code sense should start working.

The second thing, that you can do:

1. Open Build Settings of your project
2. Find Header Search Paths parameter
3. Add this : $(SRCROOT)/**
4. Reopen your project

It helped me once.

Solutions were taken from here.