CancelToken.swift

var item: DispatchWorkItem!
item = DispatchWorkItem {
    sleep(2)
    guard !item.isCancelled else {
        print("Work has been cancelled")
        return
    }
    print("Work has been done")
}

DispatchQueue.global().async(execute: item)

DispatchQueue.global().async {
    sleep(1)
    item.cancel()
}