How to break out of a loop that uses async DispatchQueue inside

Question

I am using a for loop coupled with a DispatchQueue with async to incrementally increase playback volume over the course of a 5 or 10-minute duration.

How I am currently implementing it is:

    for i in (0...(numberOfSecondsToFadeOut*timesChangePerSecond)) {
        DispatchQueue.main.asyncAfter(deadline: .now() + Double(i)/Double(timesChangePerSecond)) {
            if self.activityHasEnded {
                NSLog("Activity has ended") //This will keep on printing
            } else {
                let volumeSetTo = originalVolume - (reductionAmount)*Float(i)
                self.setVolume(volumeSetTo)
            }
        }

        if self.activityHasEnded {
            break
        }
    }

My goal is to have activityHasEnded to act as the breaker. The issue as noted in the comment is that despite using break, the NSLog will keep on printing over every period. What would be the better way to fully break out of this for loop that uses DispatchQueue.main.asyncAfter?

Updated: As noted by Rob, it makes more sense to use a Timer. Here is what I did:

    self.fadeOutTimer = Timer.scheduledTimer(withTimeInterval: timerFrequency, repeats: true) { (timer) in
        let currentVolume = self.getCurrentVolume()
        if currentVolume > destinationVolume {
            let volumeSetTo = currentVolume - reductionAmount
            self.setVolume(volumeSetTo)
            print ("Lowered volume to \(volumeSetTo)")
        }
    }

When the timer is no longer needed, I call self.fadeOutTimer?.invalidate()

Rob · Accepted Answer · 2020-05-27T17:13:50.997

3

You don’t want to use asyncAfter: While you could use DispatchWorkItem rendition (which is cancelable), you will end up with a mess trying to keep track of all of the individual work items. Worse, a series of individually dispatch items are going to be subject to “timer coalescing”, where latter tasks will start to clump together, no longer firing off at the desired interval.

The simple solution is to use a repeating Timer, which avoids coalescing and is easily invalidated when you want to stop it.

edited May 27 '20 at 17:13

answered May 27 '20 at 16:19

Rob

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Good point. I am marking this as the answer and updated the question body with the solution. – daspianist May 27 '20 at 16:40
1

@daspianist - I’m glad that worked out for you. FWIW, in the future, if you want to show what your solution is, rather than editing your question to include it, feel free to [post an answer to your own question](https://stackoverflow.com/help/self-answer). – Rob May 27 '20 at 17:01

score 1 · Answer 2 · answered May 27 '20 at 15:54

1

You can utilise DispatchWorkItem, which can be dispatch to a DispatchQueue asynchronously and can also be cancelled even after it was dispatched.

for i in (0...(numberOfSecondsToFadeOut*timesChangePerSecond)) {
    let work = DispatchWorkItem {
        if self.activityHasEnded {
            NSLog("Activity has ended") //This will keep on printing
        } else {
            let volumeSetTo = originalVolume - (reductionAmount)*Float(i)
            self.setVolume(volumeSetTo)
        }
    }
    DispatchQueue.main.asyncAfter(deadline: .now() + Double(i)/Double(timesChangePerSecond), execute: work)
    if self.activityHasEnded {
        work.cancel() // cancel the async work
        break // exit the loop
    }
}

answered May 27 '20 at 15:54

Dávid Pásztor

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Appreciate the solution! I just tried it, and unfortunately the `NSLog("Activity has ended")` still prints continuously. I tested by adding a new log `NSLog("Cancel work")` above `work.cancel()`, which does not get printed. Is it possible that the boolean never reaches inside the queue? – daspianist May 27 '20 at 16:04
@daspianist are you actually setting that boolean anywhere? If your "cancel work" isn't printed either, that means `self.activityHasEnded` is never set to `true`. – Dávid Pásztor May 27 '20 at 16:09
The boolean is set to true so `NSLog("Activity has ended")` can be printed. That line depends on `self.activityHasEnded` being true. – daspianist May 27 '20 at 16:19

How to break out of a loop that uses async DispatchQueue inside

2 Answers2