我想在Swift中使用AudioKit做一个简单的“记录用户的麦克风->应用效果->保存到wav文件”链。
下面是我尝试使用的代码。我得到的结果是，我通过耳机听到了输出（应用了音高效果），但输出文件是空的。我做错了什么？

import AudioKit
import AVFoundation
import SoundpipeAudioKit

class AudioRecorder: ObservableObject {
  
    let engine = AudioEngine()
    var recoder: NodeRecorder?

    private(set) lazy var outputFileURL: URL = {
        let documentsPath = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask)[0]
        return documentsPath.appendingPathComponent("recording.wav")
    }()

    func startRecording() {
        do {
            try AVAudioSession.sharedInstance().setCategory(.playAndRecord, mode: .default, options: [.defaultToSpeaker])
            try AVAudioSession.sharedInstance().setActive(true)
        } catch {
            print("Failed to set up recording session.")
            return
        }

        guard let input = engine.input else {
            print("No audio input found")
            return
        }

        do {
            recorder = try NodeRecorder(node: input, fileDirectoryURL: outputFileURL)
        } catch let err {
            fatalError("\(err)")
        }
        let pitchShifter = PitchShifter(input)
        engine.output = pitchShifter

        do {
            try engine.start()
        } catch {
            print("Unable to start audio engine: \(error.localizedDescription)")
            return
        }
    }

    func stopRecording() {
        engine.stop()
        do {
            try AVAudioSession.sharedInstance().setActive(false)
        } catch {
            print("Failed to stop recording session.")
        }
    }
}

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