gcd1.swift

import UIKit
import PlaygroundSupport

//to run serial queue in playground always make the following true
PlaygroundPage.current.needsIndefiniteExecution = true

let mainQueue   = DispatchQueue.main
let globalQueue = DispatchQueue.global()
let serialQueue = DispatchQueue(label: "net.ithinkdiff.app")
let concurQueue = DispatchQueue(label: "net.ithinkdiff.app.concurr", attributes: .concurrent)

func workInConcurrently(_ num: Int, _ sleepTime: Int){
    print("Concurrent Work \(num)")
}

func workInSerially(num:Int){
    print("Serial Work \(num)")
    sleep(1)
}

globalQueue.async {
    workInConcurrently(1, 5)
    workInConcurrently(2, 1)
    workInConcurrently(3, 3)
    workInConcurrently(4, 1)
}

mainQueue.async {
    workInSerially(num: 1)
    workInSerially(num: 2)
    workInSerially(num: 3)
    workInSerially(num: 4)
}