为什么在一个块外部设置，而不是一个块内，`scheduledTimer`会正常触发？ [英] Why would a `scheduledTimer` fire properly when setup outside a block, but not within a block?

问题描述

以下代码片段在完成块外部调用时效果很好，但是当我在块内部设置它时，定时器永远不会被触发。我不明白为什么会有区别：

The following code snippet works perfectly when called outside a completion block, but the timer is never fired when I set it up inside the block. I don't understand why there is a difference:

self.timer = Timer.scheduledTimer(timeInterval: 1,
                                  target: self,
                                  selector: #selector(self.foo),
                                  userInfo: nil,
                                  repeats: true)

我最初在块外调用它时没有使用自引用，但是一旦进入内部，就需要它。然而，我再次在块外测试完全相同的代码，它仍然可以工作。

I was not using the self references when calling it initially outside the block, but then once inside, it was required. However I tested the exact same code outside the block again and it does still work.

该块是一个完成处理程序，在获得 HealthKit 相关信息。

The block is a completion hander that is called after asking permission for HealthKit related information.

推荐答案

问题是有问题的完成块可能没有在主线程上运行，因此没有运行循环。但是定时器需要在运行循环上进行调度，而主线程有一个，大多数后台线程都没有（除非你自己添加一个）。

The issue is that the completion block in question was probably not running on the main thread and therefore didn't have a run loop. But timers need to be scheduled on a run loop, and while the main thread has one, most background threads do not (unless you add one, yourself).

要解决此问题，请在该完成处理程序中将计时器的创建发送回主线程，它应该可以正常工作：

To fix this, in that completion handler, dispatch the creation of the timer back to the main thread and it should work fine:

DispatchQueue.main.async {
    self.timer = Timer.scheduledTimer(timeInterval: 1.0, target: self, selector: #selector(handleTimer(_:)), userInfo: nil, repeats: true)
}

或者使用调度源timer（可以为后台队列调度的计时器，不需要运行循环）。

Or use a dispatch source timer (a timer that can be scheduled for a background queue, and doesn't require a run loop).

var timer: DispatchSourceTimer!

private func startTimer() {
    let queue = DispatchQueue(label: "com.domain.app.timer")
    timer = DispatchSource.makeTimerSource(queue: queue)
    timer.setEventHandler { [weak self] in
        // do something
    }
    timer.schedule(deadline: .now(), repeating: 1.0)
    timer.resume()
}

有关Swift早期版本的语法，请参阅此答案的前一版本。

For syntax for earlier version of Swift, see previous revision of this answer.

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