Xcode in new macOS project shows compilation errors like using "Undeclared type NSObject"

I am facing some problems with Xcode. I've searched around and nothing seems to help ... make things worse, I'm learning everything from scratch. swift, xcode, build mac apps ... from ruby ​​/ rails it hurts a lot!

In the meantime, all projects started throwing errors such as using the undeclared type "NSObject", etc.

I am using Xcode Version 6.1 (6A1052d).

The project still gets executed if I clean / build and run it. But as soon as I select a different file and return AppDelegate, the errors appear again. Also cmd + click just displays "Symbol not found" and autocomplete doesn't work.

I created a new empty project (multiple times) and this is still happening. I haven't even touched the code! I have added two images to show my problems.

First line error I managed to fix after UIApplication Routine with Swift :

Basically I removed @NSApplicationMain

from AppDelegate and created a main.swift file with these lines.

import Foundation
import AppKit

NSApplicationMain(C_ARGC, C_ARGV)

      

        
        
        
      

    

it looks like the fix to the NSApplicationMain class should match the NSApplicationDelegate protocol error.

But I can't see how to fix other errors. Any help please? :) Thanks in advance.