dispatch_after time triggers immediately

问题

This is the first time I've used GCD, I'll admit, so sorry if I've been stupid. I have a dispatch_after command which acts as a handy delay for me.

My problem is that when I send

dispatch_after(500000000000, dispatch_get_main_queue()){
    println("triggered") //or any other code
}

the closure is triggered immediately (e.g. I have tested this and "triggered" prints immediately). It should take longer right? Like 500 seconds longer.

Thanks :)

回答1:

The first parameter of dispatch_after(_:_:_:) is not a delay, but a point in time. From the docs:

when: The temporal milestone returned by dispatch_time or dispatch_walltime.

Discussion This function waits until the specified time and then asynchronously adds block to the specified queue.

You need to construct a delay relative to the current time, using dispatch_time(_:_:):

let delayTime = dispatch_time(DISPATCH_TIME_NOW, Int64(500 * NSEC_PER_SEC))
dispatch_after(delayTime, dispatch_get_main_queue()) { ... }

来源：https://stackoverflow.com/questions/31949034/dispatch-after-time-triggers-immediately