This method gets sample buffer and converts it to image than draws face rect on shape layer on UIKit

DrawFaceRectFromSampleBuffer.swift

    fileprivate func drawFaceFrame(sampleBuffer: CMSampleBuffer) {

        let pixelBuffer = CMSampleBufferGetImageBuffer(sampleBuffer)
        let cameraImage = CIImage(cvPixelBuffer: pixelBuffer!)

        let faces = faceDetector?.features(in: cameraImage)

        // For converting the Core Image Coordinates to UIView Coordinates
        let ciImageSize = cameraImage.extent.size
        var transform = CGAffineTransform(scaleX: 1, y: -1)
        transform = transform.translatedBy(x: 0, y: -ciImageSize.height)

        for face in faces as! [CIFaceFeature] {

            // Apply the transform to convert the coordinates
            var faceViewBounds = face.bounds.applying(transform)

            // Calculate the actual position and size of the rectangle in the image view
            let viewSize = self.layer.bounds.size
            let scale = min(viewSize.width / ciImageSize.width,
                            viewSize.height / ciImageSize.height)
            let offsetX = (viewSize.width - ciImageSize.width * scale) / 2
            let offsetY = (viewSize.height - ciImageSize.height * scale) / 2

            faceViewBounds = faceViewBounds.applying(CGAffineTransform(scaleX: scale, y: scale))
            faceViewBounds.origin.x += offsetX
            faceViewBounds.origin.y += offsetY

            let faceRect = faceViewBounds.insetBy(dx: -40, dy: -40)

            DispatchQueue.main.async {

                if self.shapeLayer == nil {

                    self.shapeLayer = CAShapeLayer()
                    self.shapeLayer.name = "CornersLayer"
                    self.shapeLayer.strokeColor = UIColor.white.cgColor
                    self.shapeLayer.fillColor = UIColor.clear.cgColor
                    self.shapeLayer.lineWidth = 7.0
                    self.shapeLayer.lineCap = .round
                    self.shapeLayer.lineJoin = .round
                    self.shapeLayer.frame = self.layer.bounds

                    self.layer.addSublayer(self.shapeLayer)
                }

                let cornerLengthToShow = self.layer.bounds.size.height * 0.04

                // Create Paths Using BeizerPath for all four corners
                let topLeftCorner = UIBezierPath()
                topLeftCorner.move(to: CGPoint(x: faceRect.minX, y: faceRect.minY + cornerLengthToShow))
                topLeftCorner.addLine(to: CGPoint(x: faceRect.minX, y: faceRect.minY))
                topLeftCorner.addLine(to: CGPoint(x: faceRect.minX + cornerLengthToShow, y: faceRect.minY))

                let topRightCorner = UIBezierPath()
                topRightCorner.move(to: CGPoint(x: faceRect.maxX - cornerLengthToShow, y: faceRect.minY))
                topRightCorner.addLine(to: CGPoint(x: faceRect.maxX, y: faceRect.minY))
                topRightCorner.addLine(to: CGPoint(x: faceRect.maxX, y: faceRect.minY + cornerLengthToShow))

                let bottomRightCorner = UIBezierPath()
                bottomRightCorner.move(to: CGPoint(x: faceRect.maxX, y: faceRect.maxY - cornerLengthToShow))
                bottomRightCorner.addLine(to: CGPoint(x: faceRect.maxX, y: faceRect.maxY))
                bottomRightCorner.addLine(to: CGPoint(x: faceRect.maxX - cornerLengthToShow, y: faceRect.maxY ))

                let bottomLeftCorner = UIBezierPath()
                bottomLeftCorner.move(to: CGPoint(x: faceRect.minX, y: faceRect.maxY - cornerLengthToShow))
                bottomLeftCorner.addLine(to: CGPoint(x: faceRect.minX, y: faceRect.maxY))
                bottomLeftCorner.addLine(to: CGPoint(x: faceRect.minX + cornerLengthToShow, y: faceRect.maxY))

                let combinedPath = CGMutablePath()
                combinedPath.addPath(topLeftCorner.cgPath)
                combinedPath.addPath(topRightCorner.cgPath)
                combinedPath.addPath(bottomRightCorner.cgPath)
                combinedPath.addPath(bottomLeftCorner.cgPath)

                self.shapeLayer.path = combinedPath
            }
        }
    }