问题描述： 当使用AVAssetWriter同时录制音频和视频时，应用程序崩溃。

解决方法： 可以使用AVCaptureSession来同时录制音频和视频，避免AVAssetWriter无法处理音频和视频的同时写入。

以下是一个示例代码，展示了如何使用AVCaptureSession来同时录制音频和视频：

import AVFoundation

class Recorder {
    var captureSession: AVCaptureSession?
    var videoOutput: AVCaptureMovieFileOutput?
    var audioOutput: AVCaptureAudioDataOutput?
    
    func startRecording() {
        captureSession = AVCaptureSession()
        
        guard let captureSession = captureSession else { return }
        
        // 添加音频输入
        let audioDevice = AVCaptureDevice.default(for: .audio)
        guard let audioInput = try? AVCaptureDeviceInput(device: audioDevice!) else { return }
        if captureSession.canAddInput(audioInput) {
            captureSession.addInput(audioInput)
        }
        
        // 添加视频输入
        let videoDevice = AVCaptureDevice.default(for: .video)
        guard let videoInput = try? AVCaptureDeviceInput(device: videoDevice!) else { return }
        if captureSession.canAddInput(videoInput) {
            captureSession.addInput(videoInput)
        }
        
        // 添加音频输出
        audioOutput = AVCaptureAudioDataOutput()
        if captureSession.canAddOutput(audioOutput!) {
            captureSession.addOutput(audioOutput!)
        }
        
        // 添加视频输出
        videoOutput = AVCaptureMovieFileOutput()
        if captureSession.canAddOutput(videoOutput!) {
            captureSession.addOutput(videoOutput!)
        }
        
        // 设置录制方向
        let connection = videoOutput?.connection(with: .video)
        connection?.videoOrientation = .portrait
        
        // 开始录制
        let outputPath = NSTemporaryDirectory() + "output.mov"
        let outputURL = URL(fileURLWithPath: outputPath)
        videoOutput?.startRecording(to: outputURL, recordingDelegate: self)
        
        captureSession.startRunning()
    }
    
    func stopRecording() {
        videoOutput?.stopRecording()
        captureSession?.stopRunning()
    }
}

extension Recorder: AVCaptureFileOutputRecordingDelegate {
    func fileOutput(_ output: AVCaptureFileOutput, didStartRecordingTo fileURL: URL, from connections: [AVCaptureConnection]) {
        // 录制开始
    }
    
    func fileOutput(_ output: AVCaptureFileOutput, didFinishRecordingTo outputFileURL: URL, from connections: [AVCaptureConnection], error: Error?) {
        // 录制结束
    }
}

使用上述代码，可以在开始录制时调用startRecording()方法，在结束录制时调用stopRecording()方法。同时录制音频和视频，并且避免了AVAssetWriter无法处理同时写入音频和视频的问题。