sleepForAsynchronous.swift

import Foundation

var dwi: DispatchWorkItem? = nil
dwi = DispatchWorkItem {
    print("in dispatch work item")
    for i in 0..<1000 {
        if dwi!.isCancelled { break }
        print("dispatch item \(i)")
        usleep(500000)
    }
}

// submit the DWI to a concurrent queue
DispatchQueue.global().async(execute: dwi!)

// sleep() on the main thread
sleep(3)

print("we're done")