GCD `asyncAfter` does not start when app in background/inactive - swift

Question

I'm using AVAudioPlayer for playing records. Between each playback session I have interval from 0 to 10 sec. To make this interval I'm using AVAudioPlayerDelegate and when playing is finished I'm starting new playback after delay:
func audioPlayerDidFinishPlaying(_ player: AVAudioPlayer, successfully flag: Bool) {
guard let session = playbackSessionId,
let audioTrack = audioTrack,
let failureHandler = playingFailure,
let successHandler = playingSuccess else {
playingFinished(flag, error: nil)
return
}
print("audioPlayerDidFinishPlaying fired")
DispatchQueue.global(qos: .utility).asyncAfter(deadline: .now() + LoopInterval.currentInterval) { [weak self] in
print("asyncAfter fired")
guard let strongSelf = self,
let currentSession = strongSelf.playbackSessionId, session == currentSession else { return }
strongSelf.startPlayingRecordInLoop(audioTrack, success: successHandler, failure: failureHandler)
}
}
After app goes to the background (home button), audioPlayerDidFinishPlaying fires, but DispatchQueue.global(qos: .utility).asyncAfter not. So in console I see:
audioPlayerDidFinishPlaying fired
As soon as app become active, asyncAfter fires and I see next log message:
asyncAfter fired
When app is active, all works as expected.

Answer 1

Hope it'll help someone. I found problem: when app goes in background it stops background tasks, and fires them only after becomes active. To avoid need you should keep your app running in background and awaiting for your long-running background task.
backgroundTaskID = UIApplication.shared.beginBackgroundTask(expirationHandler: {
UIApplication.shared.endBackgroundTask(backgroundTaskID)
})
This method lets your app continue to run for a period of time after it transitions to the background. You should call this method at times where leaving a task unfinished might be detrimental to your app’s user experience. For example, your app could call this method to ensure that had enough time to transfer an important file to a remote server or at least attempt to make the transfer and note any errors. You should not use this method simply to keep your app running after it moves to the background.
After task finished you should call endBackgroundTask. If you won't end background task until backgroundTimeRemaining becomes 0, app will be terminated:
UIApplication.shared.endBackgroundTask(backgroundTaskID)
Each call to this method must be balanced by a matching call to the endBackgroundTask(:) method. Apps running background tasks have a finite amount of time in which to run them. (You can find out how much time is available using the backgroundTimeRemaining property.) If you do not call endBackgroundTask(:) for each task before time expires, the system kills the app. If you provide a block object in the handler parameter, the system calls your handler before time expires to give you a chance to end the task.
That's what I did in my case:
func audioPlayerDidFinishPlaying(_ player: AVAudioPlayer, successfully flag: Bool) {
guard let session = playbackSessionId,
let audioTrack = audioTrack,
let failureHandler = playingFailure,
let successHandler = playingSuccess else {
playingFinished(flag, error: nil)
return
}
backgroundTaskID = UIApplication.shared.beginBackgroundTask(expirationHandler: { [weak self] in
guard let taskId = self?.backgroundTaskID else { return }
UIApplication.shared.endBackgroundTask(taskId)
})
DispatchQueue.global(qos: .utility).asyncAfter(deadline: .now() + LoopInterval.currentInterval) { [weak self] in
guard let strongSelf = self,
let currentSession = strongSelf.playbackSessionId, session == currentSession else { return }
strongSelf.startPlayingRecordInLoop(audioTrack, success: successHandler, failure: failureHandler)
if let backgroundTaskID = strongSelf.backgroundTaskID {
UIApplication.shared.endBackgroundTask(backgroundTaskID)
strongSelf.backgroundTaskID = nil
}
}
}