GCD.swift

import Foundation


class UIViewContoller {
    
    func viewDidLoad() {
        
        // 1st case
        print("1")
        
        DispatchQueue.main.async {
            print("2")
            
            DispatchQueue.main.sync {
                print("3")    
            }
            
            print("4")
        }
        
        print("5")
        // end 1st case
        
        // 2nd case
        print("6")
        
        let serialQueue = DispatchQueue(label: "CustomQueue", qos: .background)
        serialQueue.async {
            print("7")
            
            serialQueue.sync {
                print("8")
            }
            
            print("9")
        }
        
        print("10")
        // end 2nd case
        
        // 3rd case
        print("11")
        
        DispatchQueue.global().async {
            print("12")
            
            DispatchQueue.global().sync {
                print("13")    
            }
            
            print("14")
        }
        
        print("15")
        // end 3rd case
    }
}

// Abstract
let vc = UIViewContoller()
print("16")
vc.viewDidLoad()
print("17")


/*
Question: what will be printed in console?
*/