gcd2.swift

import UIKit
import PlaygroundSupport
PlaygroundPage.current.needsIndefiniteExecution = true

let workingQueue = DispatchQueue(label: "net.ithinkdiff.app", attributes: .concurrent)
let globalQueue  = DispatchQueue.global()
let defaultGroup = DispatchGroup()    //create a new group

func multiplication(_ num: (Int, Int)) -> Int{
    sleep(1) //to make the method slower
    return num.0 * num.1
}

let groupOfNumbers = [(1, 1), (5, 2), (3, 4)]

for pair in groupOfNumbers{
    //group of task assigning in working queue
    workingQueue.async(group: defaultGroup){
        let result = multiplication(pair)
        print("Result: \(result)")
    }
}

//notification
defaultGroup.notify(queue: globalQueue){
    print("Multiplication Done!")
}