Keep multiple AVAudioPlayers synchronized after user interaction - ios

Question

I'm working on a iOS app which let's users play a song with multiple tracks. Every track is played simultaneously and the volume of each track may be managed independently. The latency between all players is fine, when the song is played for the first time within the lifecycle of the app. However, the latency between all players increases, when the user stops the playback of all tracks and resumes it afterwards.
I create a AVAudioPlayer for every track in my MusicPlayer-Class:
do {
let data = try Data.init(contentsOf: url, options: NSData.ReadingOptions.alwaysMapped )
let player = try AVAudioPlayer(data: data)
player.prepareToPlay()
self.audioPlayers.append(player)
}
After loading the players the playback of the song is started by the playSong-Method:
playerSemaphore.wait()
NotificationCenter.default.post(playerDidReceiveCommand.getNotification())
for item in self.audioPlayers {
item.prepareToPlay()
}
let time = self.audioPlayers[0].deviceCurrentTime + 0.2
for item in self.audioPlayers {
item.play(atTime: time)
}
DispatchQueue.main.async {
NotificationCenter.default.post(playerFinishedCommand.getNotification())
}
playerSemaphore.signal()
When the user decides to pause the Song or the AudioSession is interrupted the pauseSong-Method pauses all the player by setting their volume to 0 , pausing them, synchronizing their playback-times and resetting the proper volume:
func pauseSong() {
playerSemaphore.wait()
NotificationCenter.default.post(playerDidReceiveCommand.getNotification())
DispatchQueue.global().async {
// Set volumes of all audio tracks to 0, so delay on pause is not noticed
for audioTrackPlayer in self.audioPlayers {
audioTrackPlayer.volume = 0
}
// Update Times and reset the volumes now the tracks are paused
for (index, audioTrackPlayer) in self.audioPlayers.enumerated() {
audioTrackPlayer.pause()
audioTrackPlayer.currentTime = self.audioPlayers[0].currentTime
if let currentSongID = self.loadedSongID, let currentVolume = sharedSongManager.getVolumeOfTrackOfSongID(id: currentSongID, track: index), let currentActivation = sharedSongManager.getActivationStatusOfTrackOfSongID(id: currentSongID, track: index) {
if currentActivation == true {
audioTrackPlayer.volume = currentVolume
}
}
}
DispatchQueue.main.async {
NotificationCenter.default.post(playerFinishedCommand.getNotification())
}
}
playerSemaphore.signal()
}
For logging the playertimes I wrote a method which returns all the playertimes and the difference of the max-Value and the min-Value of that array. It works with a time-offset to get exact values for the playertimes:
let startTime = CACurrentMediaTime()
var playerTimes = [TimeInterval]()
var delay: Double?
for item in self.audioPlayers.enumerated() {
let playerTime = item.element.currentTime
let currentTime = CACurrentMediaTime()
let timeOffset = currentTime - startTime
let correctedPlayerTime = playerTime - timeOffset
playerTimes.append(correctedPlayerTime)
}
if let maxTime = playerTimes.max(), let minTime = playerTimes.min() {
delay = Double(maxTime) - Double(minTime)
}
return (delay, playerTimes)
Here is a list of the delay (max-value - minus-value of playertimes) before and after pausing:
And here are the playertimes for one log before and after pausing:
All logs were created on a iPhone X with the most recent iOS.
After loading a new song the lifecycle restarts and I have a low latency until the first time the user pauses the song. I don't like the idea of creating new AVAudioPlayers after every interaction but I tried a lot of things like calculating the delay as I did in the logging function or calling the player instances asynchronously. However, most of the time this even increased the delay. Does anyone know a method to synchronize the players properly after resuming a song?