Glitches when iPhone is locked

[XZB-1248]

Hello Stephen!
I am using this powerful library to implement basic music playback functions, but since I am not familiar with ObjectiveC and the AV frameworks, I am encountering issues with intermittent music playback when the screen is locked.
To be specific, when the screen is unlocked, the playback is smooth and everything works fine; once the iPhone is locked, a few seconds into the play, it starts to become choppy, as if each frame is taking longer than it should, causing the music to not play continuously. (It works fine in simulator.)
I hope to get your help! Thanks in advance!

Here are my logs and code:

Created <AudioPlayerNode: 0x30325aa30> with render block format <AVAudioFormat 0x302b06d00:  2 ch,  44100 Hz, Float32, deinterleaved>
AVAudioEngine output format changed from <AVAudioFormat 0x302b072f0:  2 ch,  44100 Hz, Float32, deinterleaved> to <AVAudioFormat 0x302c84820:  2 ch,  48000 Hz, Float32, deinterleaved>
Enqueuing <SFBMonkeysAudioDecoder 0x3018b7d40: "Faded - Alan Walker.ape"> on <AudioPlayerNode: 0x30325aa30>
Dequeued <SFBMonkeysAudioDecoder 0x3018b7d40: "Faded - Alan Walker.ape">, processing format <AVAudioFormat 0x302e46a80:  2 ch,  44100 Hz, Int16, interleaved>
Decoding started for <SFBMonkeysAudioDecoder 0x3018b7d40: "Faded - Alan Walker.ape">
"isPlaying: true"
Rendering will start in 24.2322 msec for <SFBMonkeysAudioDecoder 0x3018b7d40: "Faded - Alan Walker.ape">

@Perceptible
final class GlobalPlayer: NSObject {
	static let shared = GlobalPlayer()
	private var player: AudioPlayer
	private var isActive = false
	var playList = [AudioPlayable]()
	var current = 0
	var isPlaying = false {
		didSet {
			if isPlaying {
				resume()
			} else {
				pause()
			}
		}
	}
	
	override private init() {
		player = AudioPlayer()
		super.init()
		player.delegate = self
	}
	deinit {
		// MARK: should never be called
		player.stop()
		try? player.currentDecoder?.close()
		player.reset()
		cleanupSession()
	}
}


// MARK: player control
extension GlobalPlayer {
	func play() -> PlayerError? {
		setupSession()
		if let item = playList.safeObject(at: current) {
			do {
				try player.play(item.url)
				return nil
			} catch {
				debugPrint("Failed to play audio: \(item.url) -> \(error)")
			}
		}
		if current + 1 < playList.count {
			current += 1
			return play()
		}
		// TODO: repeat or stop
		return .noPlayableItem
	}
	func pause() {
		player.pause()
		cleanupSession()
	}
	func resume() {
		if player.nowPlaying == nil {
			_ = play()
			return
		}
		setupSession()
		player.resume()
	}
	func seek(to time: TimeInterval) {
		player.seek(time: time)
		updateNowPlayingInfo()
	}
}

// MARK: player lifecycle
extension GlobalPlayer {
	@discardableResult
	func setupSession() -> Result<Bool, Error> {
		guard !isActive else { return .success(true) }
		let session = AVAudioSession.sharedInstance()
		do {
			try session.setCategory(.playback)
			try session.setActive(true)
			setupRemoteCommandCenter()
			isActive = true
			return .success(true)
		} catch {
			debugPrint("Failed to setup audio session: \(error)")
			return .failure(error)
		}
	}
	@discardableResult
	func cleanupSession() -> Result<Bool, Error> {
		guard isActive else { return .success(true) }
		guard isPlaying else { return .success(false) }
		let session = AVAudioSession.sharedInstance()
		do {
			try session.setActive(false, options: .notifyOthersOnDeactivation)
			cleanupRemoteCommandCenter()
			isActive = false
			return .success(true)
		} catch {
			debugPrint("Failed to reset audio session: \(error)")
			return .failure(error)
		}
	}
}


// MARK: player remote command center
extension GlobalPlayer {
	func setupRemoteCommandCenter() {
		let commandCenter = MPRemoteCommandCenter.shared()
		commandCenter.playCommand.addTarget {[weak self](_) in
			guard let self else { return .commandFailed }
			resume()
			return .success
		}
		commandCenter.pauseCommand.addTarget {[weak self](_) in
			guard let self else { return .commandFailed }
			pause()
			return .success
		}
		commandCenter.changePlaybackPositionCommand.addTarget {[weak self](event) in
			guard let self else { return .commandFailed }
			guard let event = event as? MPChangePlaybackPositionCommandEvent else { return .commandFailed }
			seek(to: event.positionTime)
			return .success
		}
	}
	func cleanupRemoteCommandCenter() {
		let commandCenter = MPRemoteCommandCenter.shared()
		commandCenter.playCommand.removeTarget(nil)
		commandCenter.pauseCommand.removeTarget(nil)
		commandCenter.changePlaybackPositionCommand.removeTarget(nil)
	}
}


// MARK: now playing
extension GlobalPlayer {
	func setupNowPlaying(_ metadata: AudioMetadata? = nil, properties: AudioProperties? = nil) {
		let nowPlayingInfoCenter = MPNowPlayingInfoCenter.default()
		var nowPlayingInfo = [String: Any]()
		if let metadata {
			nowPlayingInfo[MPMediaItemPropertyTitle] = metadata.title
			nowPlayingInfo[MPMediaItemPropertyArtist] = metadata.artist
			nowPlayingInfo[MPMediaItemPropertyAlbumTitle] = metadata.albumTitle
			nowPlayingInfo[MPMediaItemPropertyAlbumArtist] = metadata.albumArtist
			nowPlayingInfo[MPMediaItemPropertyLyrics] = metadata.lyrics
			if let artwork = metadata.attachedPictures.first, let image = UIImage(data: artwork.imageData) {
				nowPlayingInfo[MPMediaItemPropertyArtwork] = MPMediaItemArtwork(boundsSize: image.size) {(_) in
					image
				}
			}
		}
		if let properties {
			nowPlayingInfo[MPMediaItemPropertyPlaybackDuration] = properties.duration
		}
		nowPlayingInfo[MPNowPlayingInfoPropertyElapsedPlaybackTime] = player.currentTime
		nowPlayingInfo[MPNowPlayingInfoPropertyPlaybackRate] = isPlaying ? 1.0 : 0.0
		nowPlayingInfoCenter.nowPlayingInfo = nowPlayingInfo
	}
	func updateNowPlayingInfo() {
		let nowPlayingInfoCenter = MPNowPlayingInfoCenter.default()
		var nowPlayingInfo = nowPlayingInfoCenter.nowPlayingInfo ?? [String: Any]()
		nowPlayingInfo[MPMediaItemPropertyPlaybackDuration] = player.totalTime
		nowPlayingInfo[MPNowPlayingInfoPropertyElapsedPlaybackTime] = player.currentTime
		nowPlayingInfo[MPNowPlayingInfoPropertyPlaybackRate] = isPlaying ? 1.0 : 0.0
		nowPlayingInfoCenter.nowPlayingInfo = nowPlayingInfo
	}
	func cleanupNowPlaying() {
		let nowPlayingInfoCenter = MPNowPlayingInfoCenter.default()
		nowPlayingInfoCenter.nowPlayingInfo = nil
		nowPlayingInfoCenter.playbackState = .stopped
	}
}


// MARK: player delegate
extension GlobalPlayer: AudioPlayer.Delegate {
	func audioPlayerNowPlayingChanged(_ audioPlayer: AudioPlayer) {
		guard let nowPlaying = audioPlayer.nowPlaying else {
			audioPlayerPlaybackStateChanged(audioPlayer)
			return
		}
		if let audio = playList.first(where: { $0.url == nowPlaying.inputSource.url }) {
			setupNowPlaying(audio.metadata, properties: audio.properties)
		} else {
			if let fileUrl = nowPlaying.inputSource.url, let audioFile = try? AudioFile(readingPropertiesAndMetadataFrom: fileUrl) {
				setupNowPlaying(audioFile.metadata, properties: audioFile.properties)
			} else {
				audioPlayerPlaybackStateChanged(audioPlayer)
			}
		}
	}
	func audioPlayerPlaybackStateChanged(_ audioPlayer: AudioPlayer) {
		isPlaying = audioPlayer.isPlaying
		updateNowPlayingInfo()
		debugPrint("isPlaying: \(isPlaying)")
	}
}



enum PlayerError: Error {
	case noPlayableItem
}

final class AudioPlayable {
	let url: URL
	var metadata: AudioMetadata?
	var properties: AudioProperties?
	
	init(file: FsListItem) throws {
		self.url = file.absoluteUrl
		if file.downloaded {
			let audioFile = try AudioFile(readingPropertiesAndMetadataFrom: url)
			self.metadata = audioFile.metadata
			self.properties = audioFile.properties
		} else {
			// TODO: start download and set high priority
		}
	}
}

[sbooth]

I haven't had a chance to investigate this yet but my initial reaction is that perhaps kAudioSessionProperty_CurrentHardwareIOBufferDuration changes when the screen is locked and the player isn't reacting appropriately. What appropriately means in this case I'm not sure, but it could be that kAudioUnitProperty_MaximumFramesPerSlice needs to be adjusted. But this is just speculation at this point!

[XZB-1248]

Seems that you are right.
I found this article: https://developer.apple.com/library/archive/qa/qa1606/_index.html, maybe explained the reason?
Hope this gets you some help.

[XZB-1248]

By following this stijnvdb88/Snap.Net#21, I add session.setPreferredIOBufferDuration(0.005) and the issue is resolved now.

[sbooth]

I'm glad that fixed the issue. I'm not sure if there could be implications on power usage by setting that property or not; another option is to set the maximumFramesToRender property on the Audio Units themselves to 4096 or some value based on hardware buffer duration and sample rate.

[XZB-1248]

In fact, I have already tried setting the maximumFramesToRender arbitrarily to 4096, it had no effect but glitched. I also tried setting it to 1024 or other values, but I couldn't even get it to play.

I have no idea about AVAudio, help this information is helpful for you.