Check who is the current user (uses undocumented function from CoreGraphics)

currentUser.swift

import CoreGraphics

@_silgen_name("CGSSessionCopyAllSessionProperties") private func CGSSessionCopyAllSessionProperties() -> CFArray?

func getCurrentUser() -> String? {
    if let sessions: NSArray = CGSSessionCopyAllSessionProperties() {
        for session in sessions {
            let dict = session as! Dictionary<String, AnyObject>
            if dict["kCGSSessionOnConsoleKey"] as! Bool {
                return dict["kCGSSessionUserNameKey"] as? String
            }
        }
    }
    return nil
}

if let currentUser = getCurrentUser() {
    print(currentUser)
}