how to sync control dispatch Async snippet

.swift

//: Playground - noun: a place where people can play

import UIKit

var str = "Hello, playground"

import Foundation

func test(timeout: Double) {
  let queue = DispatchQueue(label: "test", attributes: .concurrent)
  let group = DispatchGroup()
  var stop = false
  let delay = timeout
  
  queue.async(group: group) {
    var str = [String]()
    var i = 0
    while i < 1000 && !stop{
      str.append(String(i))
      i += 1
    }
    
    print(1, "did", i, "iterations")
  }
  group.wait(wallTimeout: .now() + delay)
  queue.async(group: group) {
    var str = [String]()
    var i = 0
    while i < 2000 && !stop{
      str.append(String(i))
      i += 1
    }
    
    print(2, "did", i, "iterations")
  }
  group.wait(wallTimeout: .now() + delay)
  queue.async(group: group) {
    var str = [String]()
    var i = 0
    while i < 100 && !stop{
      str.append(String(i))
      i += 1
    }
    
    print(3, "did", i, "iterations")
  }
  group.wait(wallTimeout: .now() + delay)
  queue.async(group: group) {
    var str = [String]()
    var i = 0
    while i < 200 && !stop{
      str.append(String(i))
      i += 1
    }
    print(4, "did", i, "iterations")
  }
  
  group.wait(wallTimeout: .now() + delay)
  stop = true
  queue.sync(flags: .barrier) {} // to be sure there are no more jobs in my queue
}

test(timeout: 25.0)
test(timeout: 1.0)