How to use AVCaptureVideoPreviewLayer to capture a screenshot of iPhone using swift?

Question

Trying to extend AVCaptureVidioPreviewLayer from AVFoundation to capture a screenshot of iPhone with the following swift code always return a blank white UIImage, although with the correct size. Could anyone please help what's the correct way to do this?

extension AVCaptureVideoPreviewLayer {
    func screenshot() -> UIImage? {
        UIGraphicsBeginImageContextWithOptions(self.bounds.size, false, 0.0)
        guard let context = UIGraphicsGetCurrentContext() else { return nil }
        if let connection = self.connection, connection.isVideoOrientationSupported {
            let currentTransform = self.affineTransform()
            context.concatenate(currentTransform)
        }
        self.render(in: context)
        let screenshot = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()
        return screenshot
    }
}