dispatch_after时间立即触发 [英] dispatch_after time triggers immediately

问题描述

我承认，这是我第一次使用GCD，很抱歉，如果我很愚蠢.我有一条dispatch_after命令，对我来说这是一个方便的延迟.

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.

我的问题是，我发送邮件时

My problem is that when I send

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

关闭立即触发(例如，我已经对此进行了测试，并立即触发"了打印).应该花更长的时间吧?大概再长500秒.

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

谢谢:)

推荐答案

dispatch_after(_:_:_:)的第一个参数不是延迟，而是时间点.来自文档:

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

何时:dispatch_time或dispatch_walltime返回的时间里程碑.

when: The temporal milestone returned by dispatch_time or dispatch_walltime.

讨论 该函数等待直到指定的时间，然后异步进行 将块添加到指定的队列.

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

您需要使用dispatch_time(_:_:)构造相对于当前时间的延迟:

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()) { ... }

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