AVSpeechSynthesizer 输出为文件？

AVSpeechSynthesizer 有一个相当简单的 API，不支持保存到内置音频文件。

我想知道是否有办法解决这个问题 - 也许在无声播放时记录输出，以便稍后播放？或者更有效的东西。

这在 iOS 13 中终于成为可能AVSpeechSynthesizer现在有写(_:toBufferCallback:) https://developer.apple.com/documentation/avfoundation/avspeechsynthesizer/3141659-write:

let synthesizer = AVSpeechSynthesizer()
let utterance = AVSpeechUtterance(string: "test 123")
utterance.voice = AVSpeechSynthesisVoice(language: "en")
var output: AVAudioFile?

synthesizer.write(utterance) { (buffer: AVAudioBuffer) in
   guard let pcmBuffer = buffer as? AVAudioPCMBuffer else {
      fatalError("unknown buffer type: \(buffer)")
   }
   if pcmBuffer.frameLength == 0 {
     // done
   } else {
     // append buffer to file
     if output == nil { 
       output = AVAudioFile(
         forWriting: URL(fileURLWithPath: "test.caf"), 
         settings: pcmBuffer.format.settings, 
         commonFormat: .pcmFormatInt16, 
         interleaved: false) 
     }
     output?.write(from: pcmBuffer)
   } 
}