如何使用Swift 3实现AVCam？

import UIKit
import AVFoundation

class AVCamViewController: UIViewController {
    // AVCaptureSession用于捕获音视频数据
    var captureSession: AVCaptureSession?
    // AVCaptureVideoPreviewLayer用于显示摄像头捕获的视频预览
    var videoPreviewLayer: AVCaptureVideoPreviewLayer?
}

override func viewDidLoad() {
    super.viewDidLoad()
    
    // 创建AVCaptureSession实例
    captureSession = AVCaptureSession()
    
    // 获取默认的视频设备
    guard let videoDevice = AVCaptureDevice.default(for: .video) else {
        print("无法获取视频设备")
        return
    }
    
    do {
        // 创建AVCaptureDeviceInput实例
        let videoDeviceInput = try AVCaptureDeviceInput(device: videoDevice)
        
        // 将视频设备输入添加到会话中
        if captureSession?.canAddInput(videoDeviceInput) == true {
            captureSession?.addInput(videoDeviceInput)
        }
    } catch {
        print("无法创建视频设备输入: \(error)")
        return
    }
    
    // 创建AVCaptureVideoPreviewLayer实例并设置其属性
    videoPreviewLayer = AVCaptureVideoPreviewLayer(session: captureSession!)
    videoPreviewLayer?.videoGravity = AVLayerVideoGravity.resizeAspectFill
    videoPreviewLayer?.frame = view.layer.bounds
    
    // 将视频预览图层添加到视图中
    view.layer.addSublayer(videoPreviewLayer!)
    
    // 启动会话
    captureSession?.startRunning()
}