DispatchWorkItem+cancellable.swift

import Foundation

extension DispatchWorkItem {
    class func cancellable(_ work: @escaping (() -> Bool) -> Void) -> DispatchWorkItem {
        class WeakBox<T: AnyObject> {
            weak var value: T?
        }
        
        let box = WeakBox<DispatchWorkItem>()
        
        let workItem = DispatchWorkItem {
            work({ box.value?.isCancelled != false })
        }
        box.value = workItem
        
        return workItem
    }
}

Usage.swift

import Foundation

let work = DispatchWorkItem.cancellable { isCancelled in
    var i = 0
    while true {
        if isCancelled() {
            print("cancelled")
            break
        }
        
        i = i + 1
        print(i)
        
        usleep(500_000)
    }
}

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

DispatchQueue.main.asyncAfter(deadline: .now() + .seconds(3)) {
    work.cancel()
}

DispatchQueue.main.asyncAfter(deadline: .now() + .seconds(5)) {
    print("done")
    exit(0)
}

RunLoop.main.run()