avaudioengine avfoundation ios swift

Export audio file after adding an effect

I have an audio file that I want to process using some effects (like pitch effect) and then write the final result to a file.

Before I process the file and save it to disc I let the user play with the pitch effect and listen to the changes in real time.

this is how I do the real time stuff:

let audioSession = AVAudioSession.sharedInstance()
audioSession.setCategory(AVAudioSessionCategoryPlayback, error: nil)
audioSession.setActive(true, error: nil)
audioEngine = AVAudioEngine()
audioFile = AVAudioFile(forReading: audioUrl!, error: nil)
audioPlayerNode = AVAudioPlayerNode()
changePitchEffect = AVAudioUnitTimePitch()
changePitchEffect.pitch = 1.0 // default
audioEngine.connect(audioPlayerNode, to: changePitchEffect, format: nil)
audioEngine.connect(changePitchEffect, to: audioEngine.outputNode, format: nil)
let frameCapacity = UInt32(audioFile.length)
let buffer = AVAudioPCMBuffer(PCMFormat: audioFile.processingFormat, frameCapacity: frameCapacity)
if audioFile.readIntoBuffer(buffer, error: nil) {
audioPlayerNode.scheduleBuffer(buffer, atTime: nil, options: .Loops, completionHandler: nil) // start playing in a loop

then using a UISlider I let the user change the value of the pitch while listening to the audio in a loop.

So when the user finishes playing with the pitch and taps the next button, I need to save the audio file with the chosen pitch value.

My question is, how do I create a new audio file with the pitch effect?

I don’t want to record while the user is listening to the audio and play with the pitch effect, I only want to export the final result (without playing the file of course)