Render audio file offline using AVAudioEngine - ios

Question

I want to record audio file and save it by applying some effects.
Record is okay and also playing this audio with effect is okay too.
The problem is when I try to save such audio offline it produces empty audio file.
Here is my code:
let effect = AVAudioUnitTimePitch()
effect.pitch = -300
self.addSomeEffect(effect)
func addSomeEffect(_ effect: AVAudioUnit) {
try? AVAudioSession.sharedInstance().setCategory(AVAudioSessionCategoryPlayAndRecord, with: .defaultToSpeaker)
let format = self.audioFile.processingFormat
self.audioEngine.stop()
self.audioEngine.reset()
self.audioEngine = AVAudioEngine()
let audioPlayerNode = AVAudioPlayerNode()
self.audioEngine.attach(audioPlayerNode)
self.audioEngine.attach(effect)
self.audioEngine.connect(audioPlayerNode, to: self.audioEngine.mainMixerNode, format: format)
self.audioEngine.connect(effect, to: self.audioEngine.mainMixerNode, format: format)
audioPlayerNode.scheduleFile(self.audioFile, at: nil)
do {
let maxNumberOfFrames: AVAudioFrameCount = 8096
try self.audioEngine.enableManualRenderingMode(.offline,
format: format,
maximumFrameCount: maxNumberOfFrames)
} catch {
fatalError()
}
do {
try audioEngine.start()
audioPlayerNode.play()
} catch {
}
let outputFile: AVAudioFile
do {
let url = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).first!.appendingPathComponent("temp.m4a")
if FileManager.default.fileExists(atPath: url.path) {
try? FileManager.default.removeItem(at: url)
}
let recordSettings = self.audioFile.fileFormat.settings
outputFile = try AVAudioFile(forWriting: url, settings: recordSettings)
} catch {
fatalError()
}
let buffer = AVAudioPCMBuffer(pcmFormat: self.audioEngine.manualRenderingFormat,
frameCapacity: self.audioEngine.manualRenderingMaximumFrameCount)!
while self.audioEngine.manualRenderingSampleTime < self.audioFile.length {
do {
let framesToRender = min(buffer.frameCapacity,
AVAudioFrameCount(self.audioFile.length - self.audioEngine.manualRenderingSampleTime))
let status = try self.audioEngine.renderOffline(framesToRender, to: buffer)
switch status {
case .success:
print("Write to file")
try outputFile.write(from: buffer)
case .error:
fatalError()
default:
break
}
} catch {
fatalError()
}
}
print("Finish write")
audioPlayerNode.stop()
audioEngine.stop()
self.outputFile = outputFile
self.audioPlayer = try? AVAudioPlayer(contentsOf: outputFile.url)
}
AVAudioPlayer fails to open file with output url. I looked at the file through the file system and it is empty and can't be played.
Picking different categories for AVAudioSession is not working too.
Thanks for help!
UPDATE
I switched to use .caf file extension in my record in output file and it worked. Any idea why is .m4a is not working?

Answer 1

You need to nil outputFile to flush the header and close the m4a file.