如何使用AVAssetWriter在ios中写出AAC音频？

import AVFoundation

let assetWriter = try! AVAssetWriter(outputURL: outputURL, fileType: .m4a)

let outputSettings = [
    AVFormatIDKey: kAudioFormatMPEG4AAC,
    AVSampleRateKey: 44100,
    AVNumberOfChannelsKey: 2,
    AVEncoderBitRateKey: 128000,
] as [String : Any]

let assetWriterInput = AVAssetWriterInput(mediaType: .audio, outputSettings: outputSettings)
assetWriterInput.expectsMediaDataInRealTime = true

assetWriter.add(assetWriterInput)

assetWriter.startWriting()
assetWriter.startSession(atSourceTime: kCMTimeZero)

let audioBuffer = AVAudioPCMBuffer(pcmFormat: pcmFormat, frameCapacity: frameCount)
audioBuffer.frameLength = frameCount

while assetWriterInput.isReadyForMoreMediaData {
    // Fill audioBuffer with audio data
    assetWriterInput.append(audioBuffer)
}

assetWriterInput.markAsFinished()
assetWriter.finishWriting {
    // Handle completion
}