视频截图：

public func snapshotPlayer(completion: @escaping (CGImage?) -> Void) {
        guard let currentTime = player?.currentTime() else {
            complete(nil)
            return
        }
        
        guard let asset = player?.currentItem?.asset else {
            completion(nil)
            return
        }
        
        let generator = AVAssetImageGenerator(asset: asset)
        generator.appliesPreferredTrackTransform = true
        
        let time = NSValue(time: currentTime)
        generator.generateCGImagesAsynchronously(forTimes: [time]) { (_, rawImage, _, result, error) in
            guard let rawImage = rawImage, error == nil else {
                DispatchQueue.main.async { completion(nil) }
                return
            }
            
            DispatchQueue.main.async { completion(rawImage) }
        }
    }