IOS将AVAudioPCMBuffer的一部分复制到另一个

Question

我有AVAudioPCMBuffer，我想将其中一部分复制到其他缓冲区（从其末尾剪切部分语音）

    let samples = Array(UnsafeBufferPointer(start: buffer.floatChannelData![0], count:Int(buffer.frameLength)))
    let newPcmBuffer = AVAudioPCMBuffer(pcmFormat: FileThatContainsTheBuffer.processingFormat, frameCapacity: AVAudioFrameCount(_playFile.length))

    let count = Int(CGFloat(samples.count) * (/* some value between 0 and 1 */))

    for i in 0 ..<  count
    {
        newPcmBuffer.floatChannelData![0][i] = buffer.floatChannelData![0][i]
    }

    if let url = getFileUrl(fileExtension: "caf")
    {
        let newPlayFile = try AVAudioFile(forWriting: url as URL, settings: [:], commonFormat: self.format!.commonFormat, interleaved: false)
        try newPlayFile.write(from: newPcmBuffer)
    }

当我尝试播放newPlayFile时，它会播放静音（或根本不播放） 我知道问题出在for循环中， 什么是正确的方法呢？