DispatchWorkItem.swift

import Foundation

// DispatchWorkItem

// DispatchWorkItem API is introduced in iOS 8 to provide more control over dispatch queue
// DispatchWorkItem can be used to cancel the tasks

struct Counter {
    func count() {
        var countWorkItem : DispatchWorkItem?
        
        countWorkItem = DispatchWorkItem {
            for value in 1..<100 {
                guard let _workItem = countWorkItem,
                      !_workItem.isCancelled else {
                    debugPrint("Work item is cancelled")
                    break
                }
                
                debugPrint("\(value)")
                sleep(1)
            }
        }
        
        countWorkItem?.notify(queue: .main, execute: {
            debugPrint("done counting numbers")
        })
        
        let queue = DispatchQueue.global(qos: .utility)
        queue.async(execute: countWorkItem!)
        
        queue.asyncAfter(deadline: .now() + .seconds(4)) {
            countWorkItem?.cancel()
        }
    }
}

let counter = Counter()
counter.count()