创建可以取消的子线程任务

  typealias Task = (_ cancel : Bool) -> Void

  func delay(_ time: TimeInterval, task: @escaping ()->()) ->  Task? {

func dispatch_later(_ block: @escaping ()->()) {
    let t = DispatchTime.now() + time
    DispatchQueue.main.asyncAfter(deadline: t, execute: block)
}



var closure: (()->Void)? = task
var result: Task?

let delayedClosure: Task = {
    cancel in
    if let internalClosure = closure {
        if (cancel == false) {
            DispatchQueue.main.async(execute: internalClosure)
        }
    }
    closure = nil
    result = nil
}

result = delayedClosure

dispatch_later {
    if let delayedClosure = result {
        delayedClosure(false)
    }
}

return result;

  }

   func cancel(_ task: Task?) {
task?(true)
  }
  delay(2) { print("2 秒后输出") }
  
  let task = delay(5) { print("拨打 110") }

  // 仔细想一想..
  // 还是取消为妙..
  cancel(task)

一般的应用子线程任务主线程刷新

   // 创建目标队列
  let workingQueue = DispatchQueue(label: "my_queue")


 // 派发到刚创建的队列中，GCD 会负责进行线程调度

  workingQueue.async {
// 在 workingQueue 中异步进行
print("努力工作")
Thread.sleep(forTimeInterval: 2)  // 模拟两秒的执行时间

DispatchQueue.main.async {
    // 返回到主线程更新 UI
    print("结束工作，更新 UI")
    }
  }