https://stackoverflow.com/a/59100039/1271826

CancelDispatchWorkItemViewController.swift

class ViewController: UIViewController {
    @IBOutlet weak var label: UILabel!
    @IBOutlet weak var button: UIButton!

    var isStart = false
    var work: DispatchWorkItem?

    private func getCurrentTime() -> DispatchWorkItem {
        return DispatchWorkItem { [weak self] in
            while self?.work?.isCancelled == false {
                let date = Date()
                let component = Calendar.current.dateComponents([.second], from: date)
                DispatchQueue.main.async {
                    self?.label.text = "\(component.second!)"
                }
                Thread.sleep(forTimeInterval: 1)
            }
        }
    }

    @IBAction func btnPressed(_ sender: Any) {
        isStart = !isStart
        button.setTitle(isStart ? "Stop" : "Start", for: .normal)

        let globalQueue = DispatchQueue.global(qos: .background)
        if isStart {
            let item = getCurrentTime()
            globalQueue.async(execute: item)
            work = item
        } else {
            work?.cancel()
            work = nil
        }
    }
}

This fixes a bunch of issues:

1. You were creating a new DispatchWorkItem every time you click the button;
2. You therefore were never canceling the dispatched work;
3. The if (self?.work.isCancelled)! { ... } pattern is obviously a problem because if self is deallocated, your code would crash;
4. You probably should set work to nil after you cancel it to release the memory associated with it.
5. Also, the idea of while true { ... } loop is a really bad idea, but I’m assuming you did that just for illustrative purposes. But, I’d at least insert a sleep in there to avoid crushing the CPU on tons of unnecessary updates.
6. There’s no benefit of using implicitly unwrapped optional. One small mistake and code could crash. Just make it an ordinary optional.