very quick flutter test, to draw a frame in foreground of the camera.

camera_frame_test.dart

import 'package:camera/camera.dart';
import 'package:flutter/material.dart';

class CameraFrameTest extends StatefulWidget {
  @override
  _CameraFrameTestState createState() => _CameraFrameTestState();
}

class _CameraFrameTestState extends State<CameraFrameTest> {
  CameraDescription firstCamera;
  CameraController _controller;
  bool cameraLoaded = false;

  @override
  void initState() {
    requestCameras();
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: Colors.white,
      appBar: AppBar(
        elevation: 1,
        backgroundColor: Colors.grey,
        title: Row(
          children: <Widget>[
            Icon(Icons.camera_alt),
            SizedBox(width: 5),
            Text("Camera frame"),
          ],
        ),
      ),
      body: getCameraPreview(),
    );
  }

  final ValueNotifier<bool> onCameraReady = ValueNotifier(false);

  getCameraPreview() {
    return Center(
      child: ValueListenableBuilder<bool>(
        valueListenable: onCameraReady,
        builder: (BuildContext ctx, bool value, Widget _) {
          print("Called event $value");
          return Container(
            decoration: BoxDecoration(
              color: value ? Colors.grey : Colors.red,
            ),
            child: value
                ? getCameraView()
                : Center(
                    child: CircularProgressIndicator(),
                  ),
          );
        },
      ),
    );
  }

  Future requestCameras() async {
    final cameras = await availableCameras();
    if (cameras.isEmpty) {
      onCameraReady.value = false;
      return;
    }
    firstCamera = cameras.first;
    _controller = CameraController(
      firstCamera,
      ResolutionPreset.high,
      enableAudio: false,
    );
    await _controller.initialize();
    onCameraReady.value = true;
  }

  getCameraView() {
    return Stack(
      children: [
        Center(
          child: AspectRatio(
            aspectRatio: _controller.value.aspectRatio,
            child: Transform.scale(scale: 1.5, child: CameraPreview(_controller)),
          ),
        ),
        ClipPath(
          clipper: MyCustomClipper(),
          child: Container(
            color: Colors.black.withOpacity(.7),
          ),
        )
      ],
    );
  }
}

class MyCustomClipper extends CustomClipper<Path> {
  @override
  Path getClip(Size size) {
    final center = Offset(size.width / 2, size.height / 2);
    final innerRect = Rect.fromCircle(center: center, radius: size.width * .45);
    final roundRect = RRect.fromRectAndRadius(innerRect, Radius.circular(20));
    return Path()
      ..addRRect(roundRect)
      ..addOval(Rect.fromCircle(center: Offset(60, 60), radius: 20)) // quasi emoji :P
      ..addOval(Rect.fromCircle(center: Offset(size.width - 60, 60), radius: 20))
      ..addRect(Rect.fromLTWH(0.0, 0.0, size.width, size.height))
      ..fillType = PathFillType.evenOdd;
  }

  @override
  bool shouldReclip(CustomClipper<Path> oldClipper) => true;
}