Xcode 8 Breakpoint in file not working

In my case breakpoint was not hit in just one callback, so I changed "Swift Compiler" "Optimization Level" to "No optimization" in project target build settings for debug, and it started working in Xcode 9.3 as well as AppCode.

I had the same problem with an old project. Select Product(located at top menu bar) -> Scheme -> EditScheme and I solved this issue by clicking on "Debug Executable", as shown in Picture. Maybe this will help someone else too.