DispatchWorkItem 注意

            let time = DispatchTime.now() + XXX/XXX
            workItem = DispatchWorkItem.init(block: {
                doSome()
            })
            DispatchQueue.main.asyncAfter(deadline: time, execute:  workItem!)

time是局部变量，每次都会不一样

workItem.cancel()

work被重新创建，这样是取消不了的